JP2015163871A - Printed wiring board and liquid level sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a printed wiring board including at least a pair of electrodes, in which when the occupancy of a liquid to an electrode area is a predetermined value, an electrostatic capacitance between the electrodes is constant and to provide a liquid level sensor including the printed wiring board.SOLUTION: A printed wiring board 1 includes a base material, a wiring pattern 4 which is formed on the base material, and a coverlay which covers the wiring pattern 4. The wiring pattern 4 includes at least a pair of electrodes 4a which detect the electrostatic capacitance between the electrodes. Each of the base material and the coverlay includes at least one of a liquid crystal polymer layer and a fluororesin layer.

Description

  The present invention relates to a printed wiring board used for a capacitive liquid level sensor and a liquid level sensor using the printed wiring board.

A liquid level sensor that detects a liquid level by a change in capacitance is known.
For example, the liquid level sensor described in Patent Document 1 detects capacitance with a pair of electrodes formed on a substrate. This electrode is immersed in a fuel such as gasoline.

  When the liquid level changes, in the region where the electrode is formed (electrode region), the ratio of the region in which the fuel is immersed (fuel occupancy) changes, and the capacitance between the electrodes changes accordingly. Therefore, in this technique, the liquid level is obtained by detecting the capacitance.

JP 2012-225788 A

  This technique assumes that the actual liquid level and the liquid level calculated based on the capacitance are within a predetermined error range, but according to various tests, the error is less than the predetermined range. It turned out that it may become large. For example, even when the fuel occupancy in the electrode region is equal, the capacitance between the electrodes may differ depending on the state of the base material. In this case, the calculation is based on the actual liquid level and capacitance. The liquid level deviates greatly.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an electrostatic capacitance between the electrodes when the occupation ratio of the liquid with respect to the electrode region is a predetermined value in a printed wiring board having at least a pair of electrodes. An object of the present invention is to provide a printed wiring board having a constant capacity and a liquid level sensor including the printed wiring board.

  A printed wiring board according to the present invention includes a base material, a wiring pattern formed on the base material, and a coverlay that covers the wiring pattern, and the wiring pattern detects at least a capacitance between electrodes. Each of the substrate and the coverlay includes a pair of electrodes, and at least one layer of a liquid crystal polymer and a fluororesin.

  According to the present invention, it is possible to provide a printed wiring board in which the capacitance between the electrodes is constant when the liquid occupancy is a predetermined value in the electrode region, and a liquid level sensor including the printed wiring board. it can.

It is a schematic diagram of the printed wiring board which concerns on 1st Embodiment. It is sectional drawing which follows the AA line of FIG. It is a schematic diagram of a liquid level sensor. FIG. 4A is a schematic diagram showing a dipped state before refueling, and FIG. 4B is a schematic diagram showing a dipped state immediately after refueling. FIG. c) is a schematic diagram showing an immersion state after a predetermined time has elapsed since refueling. FIG. 5A is a plan view of the printed wiring board according to the second embodiment, and FIG. 5B is an enlarged view of the vicinity of the electrode pair of the printed wiring board. FIG. 6 is a plan view of a modified example of the printed wiring board according to the second embodiment. FIG. 7A is a cross-sectional view of a printed wiring board according to the third embodiment, and FIG. 7B is a side view of the printed wiring board. It is sectional drawing of the printed wiring board which concerns on 4th Embodiment. It is sectional drawing of the printed wiring board which concerns on 5th Embodiment. It is sectional drawing of the printed wiring board which concerns on 6th Embodiment. It is sectional drawing of the printed wiring board which concerns on 7th Embodiment. It is sectional drawing of the printed wiring board which concerns on 8th Embodiment. It is sectional drawing of the printed wiring board which concerns on 9th Embodiment. It is sectional drawing of the printed wiring board which concerns on 10th Embodiment.

[Description of Embodiment of Present Invention]
First, embodiments of the present invention will be listed and described.
A printed wiring board according to the present invention includes (1) a base material, a wiring pattern formed on the base material, and a coverlay covering the wiring pattern, and the wiring pattern has a capacitance between electrodes. The substrate and the coverlay each include at least one layer of a liquid crystal polymer and a fluororesin. Here, the liquid crystal polymer layer refers to a layer containing a liquid crystal polymer as a main component (a layer containing a liquid crystal polymer having a mass ratio of 50% by mass or more), and the fluororesin layer refers to a fluororesin as a main component. A layer (a layer containing a fluororesin of 50% by mass or more by mass ratio) is shown.

The capacitance between the electrodes varies with the presence of the surrounding material.
When the printed wiring board is immersed in the liquid, the capacitance changes depending on the ratio of the portion occupied by the liquid in the region where the electrode is formed (hereinafter referred to as “electrode region”).

  By the way, even if the ratio of the portion occupied by the liquid in the electrode region is the same in the state where the base material and the cover lay absorb the liquid and the state where the base material and the cover lay do not absorb the liquid, Due to the effect of the absorbed liquid, the capacitance between the electrodes differs in both states. And the difference of the electrostatic capacitance in both states becomes large, so that the absorption factor of the liquid of a base material and a coverlay is high. That is, the capacitance changes depending on the liquid absorption rate.

  In the above configuration, each of the substrate and the coverlay of the printed wiring board has at least one layer of a liquid crystal polymer and a fluororesin. Liquid crystal polymers and fluororesins have a low water and fuel absorption rate compared to resins used for printed wiring boards such as polyimide resins and epoxy resins. Here, the fuel means a combustible liquid such as petroleum such as gasoline, light oil and heavy oil, vegetable oil and alcohol.

  For this reason, the printed wiring board according to the present invention has a smaller characteristic change caused by absorption of water and fuel than a printed wiring board having a conventional structure. That is, when the ratio of the portion occupied by the liquid in the electrode region is the same in the comparison between the state in which the base material and the cover lay absorb the liquid and the state in which the base material and the cover lay do not absorb the liquid Compared with a printed wiring board having a conventional structure, the capacitance difference between the electrodes in both states is small. That is, this printed wiring board has a constant capacitance when the liquid occupation ratio is a predetermined value with respect to the electrode region even when the immersion state or immersion time in the liquid is different.

(2) In the printed wiring board, it is preferable that both the base material and the coverlay are formed of a liquid crystal polymer.
In this configuration, the base material and the coverlay are formed of a liquid crystal polymer. The liquid crystal polymer has a low water and fuel absorption rate compared to resins used for printed wiring boards such as polyimide resin and epoxy resin.

  For this reason, the printed wiring board has a smaller characteristic change caused by absorption of water and fuel than a printed wiring board having a conventional structure. That is, when the ratio of the portion occupied by the liquid in the electrode region is the same in the comparison between the state in which the base material and the cover lay absorb the liquid and the state in which the base material and the cover lay do not absorb the liquid Compared with a printed wiring board having a conventional structure, the capacitance difference between the electrodes in both states is small. That is, this printed wiring board has a constant capacitance when the liquid occupation ratio is a predetermined value with respect to the electrode region even when the immersion state or immersion time in the liquid is different.

  (3) In the printed wiring board, the base material and the coverlay are both formed of a liquid crystal polymer, and the edges of the pair of electrodes of the wiring pattern are separated by a predetermined interval. According to this configuration, since the edges of the pair of electrodes are spaced apart from each other by a predetermined distance, the capacitance of the liquid can be detected by the pair of electrodes.

(4) In the printed wiring board, the coverlay is preferably laminated on the base material through an adhesive layer formed of a liquid crystal polymer.
According to this configuration, since the adhesive is interposed between the cover lay and the base material, bubbles and voids are formed at the periphery of the wiring pattern sandwiched between the cover lay and the base material at the time of manufacture. As a result, there are fewer bubbles and voids between the coverlay and the substrate. As a result, there is little deterioration due to peel or the like due to bubbles or voids, and characteristic changes due to changes over time or temperature changes are reduced.

(5) In the printed wiring board, the melting point of the adhesive layer is preferably lower than the melting point of the base material and lower than the melting point of the coverlay.
According to this configuration, the deformation of the base material and the cover lay can be suppressed when the base material and the cover lay are bonded by softening the adhesive layer by thermocompression during manufacturing. For this reason, the dimensional accuracy of a printed wiring board is high. Further, since the adhesive layer is softened at the time of thermocompression bonding, bubbles and voids existing between the coverlay and the base material are reduced as compared with the structure without the adhesive layer.

  (6) In the printed wiring board, the base is covered with a first fluororesin layer formed of a fluororesin having a higher melting point than the base, and the cover lay has a higher melting point than the cover lay. It is preferable to cover with a second fluororesin layer formed of a fluororesin.

  According to this configuration, the fluororesin has a lower absorption rate of alcohol, particularly methyl alcohol, than the liquid crystal polymer. For this reason, the printed wiring board has less characteristic change caused by absorption of methyl alcohol.

  (7) In the printed wiring board, it is preferable that at least one of the first fluororesin layer and the second fluororesin layer includes at least one of a filler, a nonwoven fabric, and a fiber fabric sheet.

  According to this configuration, the rigidity of the printed wiring board is increased, and expansion / contraction between the electrodes accompanying deformation of the printed wiring board is suppressed. For this reason, the detection accuracy of a capacitance improves. In the manufacturing stage, a fluororesin sheet including a filler, a nonwoven fabric, and a fiber woven fabric sheet is used as a material for forming the first fluororesin layer or the second fluororesin layer. Since such a sheet has high rigidity and hardly causes wrinkles, the above configuration improves the productivity of the printed wiring board.

  (8) In the printed wiring board, it is preferable that the wiring pattern is formed on the base material via a first adhesive layer, and the coverlay is laminated on the base material via a second adhesive layer. According to this configuration, bubbles and voids are unlikely to enter between the substrate and the coverlay or between the electrodes in the manufacturing process. For this reason, this printed wiring board has few bubbles and voids. As a result, there is little deterioration due to peel or the like caused by bubbles or voids, and characteristic changes due to changes over time or changes in temperature are reduced.

(9) In the printed wiring board, it is preferable that an end portion of the base material and an end portion of the coverlay are sealed so that the first and second adhesive layers are not exposed.
In the state where the first and second adhesive layers absorb the liquid and the state where the first and second adhesive layers do not absorb the liquid, even when the ratio of the portion occupied by the liquid in the electrode region is equal, the absorption is performed. Due to the influence of the liquid, the capacitance between the electrodes becomes different in both states. For this reason, when a printed wiring board has a 1st and 2nd contact bonding layer, it is preferable to comprise a printed wiring board so that a 1st and 2nd contact bonding layer may not absorb a liquid. Therefore, in the above configuration, the end portion of the base material and the end portion of the cover lay are sealed so that the first and second adhesive layers are not exposed. Thereby, it can suppress that a 1st and 2nd contact bonding layer absorbs a liquid.

  (10) In the printed wiring board, it is preferable that an end portion of the base material and an end portion of the coverlay are sealed with a liquid crystal polymer member. According to this structure, the edge part of the said base material and the edge part of the said coverlay can be sealed easily.

  (11) The printed wiring board adopts a configuration in which an end portion of the base material is folded and crimped to the cover lay, or an end portion of the cover lay is folded and crimped to the base material. You can also. Even in this configuration, the end of the base and the end of the coverlay can be easily sealed.

(12) In the printed wiring board, each of the base material and the coverlay is preferably formed of a fluororesin.
In this configuration, the base material and the coverlay are formed of a fluororesin. Fluorine resin has a low water and fuel absorption rate compared to resins used for printed wiring boards such as polyimide resin and epoxy resin.

  For this reason, the printed wiring board has a smaller characteristic change caused by absorption of water and fuel than a printed wiring board having a conventional structure. Thereby, even if this printed wiring board has different immersion states or immersion times in the liquid, the capacitance when the liquid occupancy is a predetermined value with respect to the electrode region is constant.

(13) In the printed wiring board, the coverlay is laminated on the base material through an adhesive layer formed of a liquid crystal polymer having a melting point lower than that of a fluororesin.
According to this configuration, the deformation of the base material and the cover lay can be suppressed when the base material and the cover lay are bonded by softening the adhesive layer by thermocompression during manufacturing. For this reason, the dimensional accuracy of a printed wiring board is high. Further, since a liquid crystal polymer is used as the adhesive layer, the water absorption rate in the adhesive layer is lower than that using an epoxy resin. As a result, the change in capacitance when the liquid occupancy with respect to the electrode region is a predetermined value is reduced.

(14) In the printed wiring board, each of the base material and the cover lay includes at least one of a filler, a nonwoven fabric, and a fiber woven fabric sheet.
According to this configuration, the rigidity of the printed wiring board is increased, and expansion / contraction between the electrodes accompanying deformation of the printed wiring board is suppressed. For this reason, the detection accuracy of a capacitance improves. In the manufacturing stage, a fluororesin sheet including a filler, a nonwoven fabric, and a fiber woven fabric sheet is used as a material for forming the first fluororesin layer or the second fluororesin layer. Since such a sheet has high rigidity and hardly causes wrinkles, the above configuration improves the productivity of the printed wiring board.

  (15) In the printed wiring board, the wiring pattern has a pair of electrodes formed on the same plane, and each of the electrodes is arranged so that a distance is constant from one end to the other end. It is preferable that the extending direction of the liquid crystal polymer and the flow direction of the liquid crystal polymer of the base material and the coverlay coincide.

  A base material or cover lay formed of a liquid crystal polymer has anisotropy with respect to a linear expansion coefficient. That is, the linear expansion coefficient in the flow direction of the liquid crystal polymer is smaller than the linear expansion coefficient in the direction perpendicular to the flow direction.

On the other hand, when a printed wiring board is used for the purpose of detecting a change in liquid level as a change in capacitance, the printed wiring board is arranged so that the liquid level moves along the extending direction of the electrodes.
In order to reduce the variation of the measurement value every time in the measurement of the liquid level, it is preferable that the dimensional change of the dimension along the extending direction of the electrode is small in the substrate and the coverlay. Therefore, in this configuration, the extension direction of the electrodes is the same as the flow direction of the liquid crystal polymer of the base material and the coverlay. Thereby, a base material and a coverlay become difficult to extend in the extension direction of an electrode. As a result, when this printed wiring board is used for a liquid level sensor, it is possible to reduce variations in liquid level measurement values caused by temperature changes.

  (16) In the printed wiring board, the wiring pattern may be configured such that the wiring pattern has a plurality of electrode pairs. In this case, a change in capacitance can be detected in each electrode pair.

When a printed wiring board is used for the purpose of detecting a change in liquid level as a change in capacitance, the printed wiring board is arranged so that the liquid level moves along the arrangement direction of the electrode pairs.
When the printed wiring board is used for such an application, it is possible to detect the presence or absence of a liquid such as fuel in each of the electrode pairs. For this reason, the liquid level can be detected with a certain accuracy regardless of the amount of fuel absorbed by the base material and the coverlay.

  (17) In the printed wiring board, the wiring pattern may include a pair of electrodes arranged to face each other. With such a configuration, it is possible to detect the level of fuel or the like based on the capacitance between the electrodes.

  (18) A liquid level sensor according to the present invention has the printed wiring board. The printed wiring board has a constant capacitance when the liquid occupancy with respect to the electrode region is a predetermined value. For this reason, the detection error of the liquid level can be reduced in the liquid level sensor.

[Details of the embodiment of the present invention]
Specific examples of the printed wiring board according to the embodiment of the present invention will be described below with reference to the drawings. It should be noted that the present invention is not limited to these exemplifications, is shown by the scope of claims, and includes meanings equivalent to the scope of claims and all modifications within the scope of claims. Intended.

[Printed wiring board]
With reference to FIG.1 and FIG.2, the printed wiring board 1 which concerns on 1st Embodiment is demonstrated. As shown in FIGS. 1 and 2, the printed wiring board 1 includes a base material 2, a wiring pattern 4, and a cover lay 3.

Both the base material 2 and the coverlay 3 are formed of a rectangular resin sheet.
The wiring pattern 4 is formed on the substrate 2, and the wiring pattern 4 (excluding the terminal portion 4 b) is covered with the cover lay 3.

  Both the base material 2 and the coverlay 3 are made of a resin having a low absorption rate of the fuel 30. In addition to this configuration, it is preferable that both the base material 2 and the coverlay 3 have high heat resistance. Furthermore, it is preferable that both the base material 2 and the coverlay 3 have a small linear expansion coefficient.

  For example, both the base material 2 and the coverlay 3 are formed of a resin whose main component is a liquid crystal polymer. Here, the main component indicates that the ratio of the liquid crystal polymer to the resin is 50% by mass or more. Preferably, the substrate 2 and the coverlay 3 are formed of a resin having a liquid crystal polymer of 90% by mass or more. Liquid crystal polymers suitable for forming the substrate 2 and the coverlay 3 include polycondensates of ethylene terephthalate and parahydroxybenzoic acid, polycondensates of phthalic acid, biphenol and parahydroxybenzoic acid, 6-hydroxy Examples include polycondensates of 2-naphthoic acid and parahydroxybenzoic acid.

  The thickness of the substrate 2 is preferably 20 μm or more and 100 μm or less. More preferably, the thickness of the base material 2 is 25 μm or more and 50 μm or less. The thickness of the coverlay 3 is set similarly to the thickness of the base material 2. That is, the thickness of the coverlay 3 is preferably 20 μm or more and 100 μm or less, and more preferably 25 μm or more and 50 μm or less.

  Such a setting is because if the thickness of the substrate 2 or the coverlay 3 becomes too large, the capacitance between the electrodes 4a becomes too small and the detection accuracy of the fuel 30 is too low. Moreover, it is because the detection precision of the fuel 30 falls because the amount of fuel absorption of the base material 2 or the coverlay 3 increases. On the other hand, if the thickness of the base material 2 or the cover lay 3 becomes too small, the sealing function of the base material 2 or the cover lay 3 is lowered. For example, if the base material 2 or the cover lay 3 is damaged, fuel or air may enter from this portion and the wiring pattern 4 may be deteriorated.

  Further, in these liquid crystal polymers, when the mixture of isooctane and toluene in a volume ratio of 1: 1 is used as the fuel 30, the absorption rate of the fuel 30 is preferably 2% by mass or less. Is preferably 1.0% by mass or less. Thereby, the change of the electrostatic capacitance between the electrodes 4a due to the absorption of the fuel 30 can be suppressed.

  Here, the absorptance indicates a value (percentage display) obtained by dividing the mass when the mass of the liquid crystal polymer is immersed in the fuel 30 and the mass does not change by the mass of the liquid crystal polymer at the time of drying. The reason for defining the properties of the liquid crystal polymer using the fuel 30 in which isooctane and toluene are mixed at a volume ratio of 1: 1 is that there are a variety of common fuels 30 on the market. This is because even the fuel 30 may have different components, and characteristics cannot be defined using such a general fuel 30.

Further, in these liquid crystal polymers, in the range of −20 ° C. to 240 ° C., the linear expansion coefficient in the flow direction DF is 10 × 10 ⁻⁵ or less, and the linear expansion coefficient in the perpendicular direction DR is 20 × 10 ⁻⁵ or less. preferable. More preferably, in the range of −20 ° C. to 240 ° C., the linear expansion coefficient in the flow direction DF is 2 × 10 ⁻⁵ or less, and the linear expansion coefficient in the perpendicular direction DR is 10 × 10 ⁻⁵ or less. Here, the flow direction DF indicates the flow direction of the liquid crystal polymer formed during molding (that is, the alignment direction of the liquid crystal polymer), and the perpendicular direction DR indicates a direction perpendicular to the flow direction. .

The wiring pattern 4 has a pair of electrodes 4a. These electrodes 4 a are arranged on the substrate 2. That is, the electrodes 4a are arranged on the same plane.
Each electrode 4a is provided with a terminal portion 4b for connection to an electric circuit of the measuring portion 11 (described later). Each of the terminal portions 4 b is disposed on one end side of the base material 2.

  The two electrodes 4a are formed in parallel to each other so that the distance Sa is a constant width from one end to the other end. The constant width means that the distance Sa between the electrodes 4a from one end to the other end (see FIG. 1; the distance Sa between the edge of one electrode 4a and the edge of the other electrode 4a) is within a predetermined distance range. Indicates that there is. For example, the interval Sa is set to 200 μm ± 100 μm.

  The maximum value of the interval Sa (the maximum value of the set dimension of the interval Sa) is preferably 500 μm, and the minimum value (the minimum value of the set dimension of the interval Sa) is preferably 50 μm. If the distance Sa exceeds 500 μm, the capacitance between the electrodes 4a becomes too small, and the liquid level detection accuracy of the fuel 30 is too low. Further, when the distance Sa is smaller than 50 μm, it becomes difficult for the liquid crystal polymer to flow between the electrodes 4a, so that the reliability as a sensor is lowered, for example, detection variation increases. Each of the pair of electrodes 4a is preferably a rectangle having the same shape and the same area.

In addition, each of the electrodes 4a is preferably formed such that its extension direction DE (longitudinal direction) and the flow direction DF of the liquid crystal polymer coincide with each other.
This is due to the following reason. The resin sheet of the liquid crystal polymer (base material 2 and coverlay 3) has anisotropy with respect to the coefficient of linear expansion. That is, in the resin sheet, the linear expansion coefficient in the flow direction DF is smaller than the linear expansion coefficient in the perpendicular direction DR. On the other hand, when the printed wiring board 1 as a constituent element of the liquid level sensor 10 is disposed in the fuel tank 20 (see FIG. 3), the printed wiring board 1 is disposed such that the electrode 4a is along the vertical direction DL. . With such an arrangement, the liquid level of the fuel 30 moves along the extending direction DE of the electrode 4a of the printed wiring board 1. However, if the dimension of the electrode 4a in the extending direction DE changes, the liquid level detection accuracy decreases. For this reason, the printed wiring board 1 of this type preferably has a small dimensional change in the dimension along the electrode 4a (extension direction DE). For this reason, it is preferable that the flow direction DF and the extension direction DE of the electrode 4a are made to coincide with each other for the base material 2 and the coverlay 3.

  The wiring pattern 4 is formed of copper, gold, silver, nickel, or a laminate thereof. Such a wiring pattern 4 is formed by a subtractive method or a semi-additive method. The thickness of the wiring pattern 4 is preferably 12 μm or more and 18 μm or less.

The wiring pattern 4 is sealed with the base material 2 and the coverlay 3.
For example, the base material 2 on which the wiring pattern 4 is formed and the cover lay 3 are pressure-bonded in a vacuum chamber. Thereby, the wiring pattern 4 is sealed so that air bubbles and voids do not remain between the base material 2 and the coverlay 3.

  Further, in order to ensure the sealing of the wiring pattern 4, the distance L (see FIG. 1) between the edge of the electrode 4a of the wiring pattern 4 and the edge of the substrate 2 and the coverlay 3 is set to 0. It is preferable to set it to 5 mm or more. When this distance is smaller than 0.5 mm, the width of the bonding portion between the base material 2 and the cover lay 3 becomes too narrow, and the possibility that a gap is generated between the base material 2 and the cover lay 3 is increased. .

[Liquid level sensor]
FIG. 3 shows a schematic diagram of the liquid level sensor 10. FIG. 3 shows an example in which the liquid level sensor 10 is disposed in the fuel tank 20.

  The liquid level sensor 10 includes the printed wiring board 1 according to the above embodiment and a measurement unit 11 that measures the capacitance between the electrodes 4a. The measurement unit 11 and the printed wiring board 1 are connected by a conducting wire 12.

  The measurement part 11 measures the electrostatic capacitance between the electrodes 4a periodically. The capacitance is measured by, for example, a resonance method or a method of inputting a frequency signal. In the resonance method, a coil is arranged in series or in parallel with the pair of electrodes 4a. In addition, this coil is arrange | positioned inside the measurement part 11 in FIG.

  The printed wiring board 1 is disposed in the fuel tank 20 so that the extending direction DE of the electrode 4a coincides with the vertical direction. For this reason, when the liquid level of the fuel 30 changes, the liquid level moves along the extending direction DE of the electrode 4a.

  Further, when the liquid level of the fuel 30 changes, a region corresponding to the pair of electrodes 4a (a region corresponding to a portion where the wiring pattern 4 is formed. A region surrounded by a broken line in FIG. , This is referred to as “electrode region AE”), and the ratio of the occupied portion SA of air (the occupied ratio of air) and the ratio of the occupied portion SG of the fuel 30 (that is, the occupied ratio of the fuel 30) change. This ratio correlates with the liquid level. That is, when the liquid level rises, the ratio of the occupied portion SG of the fuel 30 increases and the ratio of the occupied portion SA of the air decreases.

  The capacitance between the electrodes 4a is the sum of the capacitance of the air occupation portion SA and the capacitance of the occupation portion SG of the fuel 30 in the electrode region AE. For this reason, the liquid level of the fuel 30 and the electrostatic capacity between the electrodes 4a are generally correlated by a linear expression.

FIG. 4 shows an immersion state of the printed wiring board 1 before the liquid level is changed, after the liquid level is changed, and after a predetermined time has elapsed after the liquid level is changed.
Fig.4 (a) shows the immersion state of the printed wiring board 1 in the state where the liquid level before refueling is low. FIG.4 (b) shows the immersion state of the printed wiring board 1 in a state where a liquid level is high by refueling. FIG. 4C shows a dipped state of the printed wiring board 1 when a predetermined time elapses without fuel 30 being used after refueling.

  When the liquid level in the fuel tank 20 rises due to refueling, the occupied portion SG of the fuel 30 increases in the electrode region AE. For this reason, the electrostatic capacitance between the electrodes 4a changes. Thereafter, if the liquid level does not change, the capacitance between the electrodes 4a should not change.

  However, in a conventional liquid level sensor having a printed wiring board, time elapses after the liquid level in the fuel tank 20 is increased by refueling even when the fuel 30 is not used (that is, even when the liquid level does not change). When it does, the electrostatic capacitance between the electrodes 4a may change. For this reason, the measurement part 11 detects the electrostatic capacitance between the electrodes 4a as a different value at both time points even when the liquid level is equal between the time point immediately after refueling and the time point after the elapse of a predetermined time. As a result, the liquid level sensor 10 outputs different liquid level levels even though the actual liquid level is the same at both time points.

The cause of such an event is caused by the base material 2 and the coverlay 3 absorbing the fuel 30.
This point will be described below.

  Before the fuel 30 is supplied, when the fuel 30 is gradually reduced, the air occupying portion SA is dried in the electrode region AE, so that the portion of the fuel 30 in this portion (portion indicated by the symbol SA in FIG. 4A) is reduced. Absorption is low.

  As shown in FIG. 4B, when the fuel 30 is refueled and the liquid level of the fuel 30 rapidly rises, the air occupied portion SA immediately before refueling is changed to the occupied portion SG of the fuel 30. In such a case, the fuel 30 is not absorbed by the base material 2 and the coverlay 3 of this portion (that is, the air occupying portion SA immediately before refueling, hereinafter referred to as “the air occupying portion SAB”). In FIG. 4B, the portion below the immediately preceding air occupying portion SAB in the electrode region AE is shown as a portion SGB immersed in the fuel 30 before and after refueling.

After fueling, when the fuel 30 is not used and a predetermined time has elapsed, the base material 2 and the coverlay 3 in the immediately preceding air occupying part SAB are in a state of absorbing the fuel 30.
That is, there is no change in the liquid level between the time when the fuel 30 is supplied and the time after the predetermined time has elapsed since the fuel 30 is supplied, but the fuel absorption state of the immediately preceding air occupation part SAB is different. At the time when the fuel 30 is supplied, the fuel 30 exists from a position away from the surface of the electrode 4 a by the thickness of the base material 2 or the coverlay 3.

  On the other hand, when the fuel 30 is supplied and the base material 2 or the cover lay 3 absorbs the fuel 30 after a predetermined time has elapsed, it is separated from the surface of the electrode 4a by the thickness of the base material 2 or the cover lay 3 In other words, the fuel 30 (substance constituting the fuel 30) exists from a position closer to the electrode 4a side.

  As described above, the fuel 30 is supplied in the direction away from the electrode 4a in the base material thickness direction (or cover lay thickness direction) between the time when the fuel 30 is supplied and the time after the fuel 30 is supplied for a predetermined time. The difference is that the (substance constituting the fuel 30) begins to exist. For this reason, even if the liquid level is equal at both time points, the electrostatic capacities become different.

  A situation in which the capacitance becomes a different value even though the liquid levels are equal does not occur only in such a situation. For example, such a situation also occurs due to a difference in temperature conditions around the printed wiring board 1. Since the substrate 2 and the cover lay 3 have different absorption rates of the fuel 30 depending on the temperature, the capacitance detected in winter and the capacitance detected in summer even when the liquid level is at a predetermined position Is different.

The following can be understood from these events.
The electrostatic capacity between the electrodes 4a when the fuel 30 is not absorbed by the base material 2 and the cover lay 3, and the electrostatic capacity between the electrodes 4a when the fuel 30 is absorbed by the base material 2 and the cover lay 3 Different from capacity. For this reason, it is preferable that the base material 2 and the coverlay 3 have a low absorption rate of the fuel 30.

  Therefore, in the present embodiment, the base material 2 and the cover lay 3 that do not easily absorb the fuel 30 are employed. That is, the influence of the increase or decrease in the amount of fuel contained in the base material 2 and the coverlay 3 is reduced with respect to the capacitance between the electrodes 4a. Accordingly, the capacitance can be made constant when the liquid occupation ratio with respect to the electrode region AE is a predetermined value. That is, according to this configuration, it is possible to reduce variation in capacitance when the liquid occupancy is a predetermined value with respect to the electrode region AE, as compared with the conventional printed wiring board.

  “Constant” means that the error rate of capacitance is 1% or less, preferably 0.5% or less. Here, the error rate is defined as “(actual measurement value−reference value) / reference value × 100”. The reference value is measured immediately after the printed wiring board 1 is placed so that the substrate 2 and the coverlay 3 are sufficiently dried and the electrode area AE is immersed 100% in area ratio. Shows the measured capacitance. The electrostatic capacity measured after the printed circuit board 1 was placed so as to be immersed 100% in the electrode area AE in the area ratio until the fuel absorption of the base material 2 and the coverlay 3 reached the saturated state. Indicates capacity.

The printed wiring board 1 has the following effects.
(1) The substrate 2 and the coverlay 3 of the printed wiring board 1 are both formed of a liquid crystal polymer. The liquid crystal polymer has a lower absorption rate of water and fuel 30 than a resin used for a conventional printed wiring board such as a polyimide resin or an epoxy resin.

  For this reason, the printed wiring board 1 has a smaller characteristic change caused by absorption of water and fuel 30 than a printed wiring board having a conventional structure. That is, in the comparison between the state in which the base material 2 and the cover lay 3 absorb water and fuel 30 and the state in which the base material 2 and the cover lay 3 do not absorb water and fuel 30, the fuel in the electrode region AE When the ratio of the portion occupied by 30 is the same value (predetermined value), the capacitance difference between the electrodes 4a in both states is smaller than that of a printed wiring board having a conventional structure. For this reason, this printed wiring board 1 has a constant capacitance when the occupancy of water or fuel 30 is a predetermined value with respect to the electrode region AE regardless of the immersion time, immersion state, or temperature condition.

  (2) In the printed wiring board 1, the wiring pattern 4 has a pair of electrodes 4a formed on the same plane, and the distance between the electrodes 4a is a constant width from one end to the other end of the electrodes 4a. Preferably there is. According to this configuration, the capacitance of each electrode 4a due to an increase in the change in the occupancy ratio of water and fuel 30 to the electrode region AE, compared to a configuration in which the distance between the electrodes 4a is not constant from one end to the other. The rate of change is constant, and the relationship between the two is approximated by a linear expression.

(3) In the printed wiring board 1, it is preferable that the extension direction DE of the electrode 4a and the flow direction DF of the liquid crystal polymer of the base material 2 and the coverlay 3 coincide.
According to this configuration, the base material 2 and the coverlay 3 are difficult to extend in the extending direction DE of the electrode 4a. As a result, when this printed wiring board 1 is used for the liquid level sensor 10, it is possible to reduce variations in the liquid level measurement values caused by temperature changes.

  (4) The liquid level sensor 10 includes the printed wiring board 1 of the present embodiment. The printed wiring board 1 has a constant capacitance when the occupation ratio of water and fuel 30 is a predetermined value with respect to the electrode region AE. Thereby, the detection error of the liquid level in the liquid level sensor 10 is reduced, and the detection accuracy of the liquid level is improved.

[Other Embodiments]
The above embodiment is an example of the present technology.
Hereinafter, other embodiments will be described.

A printed wiring board 101 according to the second embodiment will be described with reference to FIG.
The printed wiring board 101 is obtained by changing the structure of the wiring pattern 4 in the printed wiring board 1 according to the first embodiment. FIG. 5A shows a plan view of the printed wiring board 101. FIG. 5B is an enlarged view near the electrode pair 42 of the printed wiring board 101. In FIG. 5A, some wirings 41 are indicated by dots, and the description thereof is omitted.

The wiring pattern 4 includes a plurality of wiring pairs 40. The wiring pair 40 indicates two wirings 41 that are energized at the same time when measuring capacitance.
As shown in FIG. 5A, the wiring 41 includes an electrode 41a, a wiring part 41b, and a terminal part 41c. The electrode 41a is disposed at the tip of the wiring part 41b. The terminal portion 41c is disposed at the end of the wiring portion 41b. In the wiring pair 40, the edge of the electrode 41a of one wiring 41 (the part closest to the other wiring 41) and the edge of the electrode 41a of the other wiring 41 (the part closest to the one wiring 41) are a predetermined distance. Are arranged apart from each other. The predetermined interval is a distance at which the liquid can be detected by the pair of electrodes 41a. The form of the interval is not particularly limited. The interval between the pair of electrodes 41a may be enlarged in the middle from one end to the other end, may be reduced in the middle, or may have a structure in which the enlarged portion and the reduced portion are alternately repeated. It may be constant from one end to the other end. For example, in the wiring pair 40, the electrode 41a of one wiring 41 and the electrode 41a of the other wiring 41 are arranged in the perpendicular direction DR of the liquid crystal polymer so that the interval Sb (see FIG. 5B) is a constant width. It is arranged along. The electrodes 41a constitute an electrode pair 42. These electrode pairs 42 are arranged in a line along the flow direction DF of the liquid crystal polymer of the substrate 2 and the coverlay 3. The maximum value of the interval Sb is preferably 500 μm, and the minimum value is preferably 50 μm. If the distance Sb exceeds 500 μm, the capacitance between the electrodes 41a becomes too small, and the liquid level detection accuracy of the fuel 30 is too low. Further, when the interval Sb is smaller than 50 μm, it becomes difficult to flow the liquid crystal polymer between the electrodes 41a, so that the reliability as a sensor is lowered, for example, detection variation increases. Here, the interval Sb is an average of the plurality of electrode pairs 42. Further, it is desirable that the variation in the distance Sb between the plurality of electrode pairs 42 provided on the printed wiring board 101 is small. For example, for these electrode pairs 42, the minimum value of the interval Sb is 0.5 times the average value of the interval Sb, and the maximum value of the interval Sb is 1.5 times the average value of the interval Sb. desirable.

  In FIG. 5, each wiring portion 41b is composed of a portion along the perpendicular direction DR of the liquid crystal polymer and a portion along the flow direction DF of the liquid crystal polymer. The structure of the wiring portion 41b is as described above. The form is not limited. For example, each wiring part 41b may be configured to extend obliquely with respect to the flow direction DF of the liquid crystal polymer, or may be configured with a curve, due to limitations on the substrate size or the like.

According to such a configuration, the liquid level can be detected with a certain accuracy regardless of the amount of fuel 30 absorbed by the base material 2 and the coverlay 3. This point will be described below.
Since each electrode pair 42 is electrically independent, the capacitance between the electrodes 41a can be measured independently. For example, a predetermined voltage is applied between the wiring pairs 40 in the order from the outermost wiring pair 40 to the inward direction or in the reverse order. Thereby, the electrostatic capacitance between the electrodes 41a of the wiring pair 40 is measured. And it is determined whether the fuel 30 exists in the part between the electrodes 41a based on whether the electrostatic capacitance is more than a threshold value. In this determination, a determination value indicating whether the fuel is “present” or “not present” is output.

  Then, this determination is made for all of the wiring pairs 40, until the fuel 30 is determined from the “present” state to the “not present” state, or until the fuel 30 is determined from the “not present” state to the “present” state. Done. With such a determination operation, the wiring pair 40 in which the fuel 30 is switched from the “not present” state to the “present” state or the fuel 30 is switched from the “present” state to the “not present” state in the row of the wiring pairs 40. Is detected. The position of the wiring pair 40 detected in this way indicates the liquid level of the fuel 30.

  By the way, in the printed wiring board 1 which concerns on 1st Embodiment, a liquid level is estimated based on the magnitude | size of the electrostatic capacitance between the electrodes 4a. However, in the case of this configuration, both the concentrations of the substance (for example, the fuel 30) existing between or near the electrodes 4a (changes in the concentration gradient of the substance with respect to the thickness direction of the base material 2 or the coverlay 3) both. The capacitance between the electrodes 4a changes. Even if the base material 2 and the cover lay 3 are formed of a member having a low absorption rate of the fuel 30, there is some absorption of the fuel 30 as long as the resin is used as this member. It is considered that there is a limit to further increasing the value.

On the other hand, according to the structure of the printed wiring board 101 shown in FIG. 5, it is possible to detect a change in capacitance in each electrode pair 42.
For this reason, if an appropriate threshold value is set, the portion between the electrodes 41a regardless of the change in the capacitance between the electrodes 41a accompanying the change in the concentration of the substance (substance constituting the fuel 30) existing between the electrodes 41a. Thus, the presence or absence of the fuel 30 can be accurately determined. As a result, the liquid level can be detected with a constant accuracy regardless of the amount of fuel 30 absorbed by the base material 2 and the coverlay 3.

  Further, since the capacitance between the electrodes becomes constant because the base material 2 and the coverlay 3 are formed of a liquid crystal polymer, the accuracy of determining whether or not there is fuel in each wiring pair 40 Will increase. Thereby, the detection accuracy of a liquid level is improved.

A modification of the printed wiring board 101 according to the second embodiment will be described with reference to FIG.
In the printed wiring board 101 according to the second embodiment, the wiring pattern 4 is formed only on one surface of the substrate 2. However, in the printed wiring board 102 according to this modification, the wiring pattern 4 is formed on both surfaces of the substrate 2. Is formed. The wiring part 41b of the wiring pattern 4 is connected to the first wiring part 41bx arranged on the same surface side as the electrode 41a, and the first wiring part 41bx via the through hole or via hole, and the arrangement surface of the electrode 41a. The second wiring portion 41by is disposed on the opposite surface. The terminal portion 41c is provided at the end of the second wiring portion 41by.

Furthermore, the structure of the wiring pattern 4 is different from the wiring pattern 4 of the printed wiring board 101 according to the second embodiment in the following points.
In the plurality of electrode pairs 42 arranged in a line, the one-side electrode 41a group is divided into a predetermined number of groups, and each electrode 41a in the same group is connected to one second wiring portion 41by. With such a configuration, the number of second wiring portions 41by is reduced, and the area of the printed wiring board 102 is reduced.

A printed wiring board 201 according to the third embodiment will be described with reference to FIG.
FIG. 7A is a cross-sectional view of the printed wiring board 201, and FIG. 7B is a side view of the printed wiring board 201. FIG.

  This printed wiring board 201 differs from the printed wiring board 1 according to the first embodiment in the electrode arrangement structure. In the printed wiring board 1 shown in FIG. 1, the two electrodes 4a are arranged on the same plane, but in the printed wiring board 201, the two electrodes 51a are arranged so as to face each other, and the respective facing surfaces are arranged. Are set in parallel. Such a configuration is realized by bending the printed wiring board 201 at an intermediate portion of each electrode 51a. The bending position is not limited to the position described in FIG. 7, and the bending position may be determined so that the pair of electrodes 51a face each other depending on the individual design. In order to obtain a structure that requires such bending, the printed wiring board 201 preferably has flexibility. 3 to 6, the printed wiring boards 1 and 101 are flexible, so that the printed wiring along the wall of the non-planar structure, for example, the cylindrical fuel tank 20, can be obtained. Since the plates 1 and 101 can be closely attached and firmly fixed, the liquid level can be detected with high accuracy. For this reason, it is preferable that the printed wiring boards 1, 101, 102 have flexibility.

  Each of the electrodes 51a is formed in a rectangular shape. Each of the electrodes 51 a is configured such that the extension direction DE (longitudinal direction) thereof coincides with the flow direction DF of the base material 2 and the coverlay 3.

  The two electrodes 51a are arranged so that the distance Sc is constant from one end to the other. The constant width indicates that the distance Sc between the two electrodes 51a is within a predetermined distance range from one end to the other end. In order to realize this, spacers 52 are arranged between the two electrodes 51a around both ends of the electrode 51a. The fuel 30 freely circulates in the space between the electrodes 51a.

The distance Sc between the two electrodes 51a is set in consideration of the following.
When the distance Sc between the two electrodes 51a is too narrow, the fuel 30 spreads between the two electrodes 51a due to capillary action. Considering this, the interval Sc is set so that the fluctuation of the liquid level due to the capillary phenomenon does not become too large. On the other hand, when the distance Sc between the electrodes 51a is too large, the measured capacitance becomes small, and the measurement accuracy of the liquid level decreases. The electrode area may be increased in order to compensate for the decrease in the liquid level measurement accuracy.

According to the printed wiring board 201, the liquid level of the fuel 30 and the like can be detected based on the capacitance between the electrodes 51a, as in the printed wiring board 1 according to the first embodiment.
Moreover, since the printed wiring board 201 has a pair of electrodes 51a facing each other on the facing surface, the maximum capacitance between the electrodes 51a can be changed by changing the electrode area. For this reason, in this configuration, it is possible to change the capacitance measurement accuracy relatively easily.

Note that the printed wiring board 201 according to the third embodiment can also be configured as follows.
Each of the electrodes 51a is formed on a separate substrate, and these substrates are arranged in parallel to each other. This configuration also has the same effect as the printed wiring board 201 shown in FIG.

A printed wiring board 301 according to the fourth embodiment will be described with reference to FIG.
The printed wiring board 301 differs from the printed wiring board 1 described above in the following points. That is, the printed wiring board 1 according to the first embodiment has a structure in which the base material 2 and the cover lay 3 are pressure-bonded, but the printed wiring board 301 has a first structure between the base material 2 and the wiring pattern 4. An adhesive layer 5 a is provided, and a second adhesive layer 5 b is provided between the cover lay 3 and the wiring pattern 4. Since the first and second adhesive layers 5a and 5b are bonded to each other, moisture hardly enters the inside from the bonding surfaces of the first and second adhesive layers 5a and 5b. The first and second adhesive layers 5 a and 5 b are formed of a resin having a low absorption rate of the fuel 30.

  According to such a configuration, bubbles and voids are unlikely to enter between the substrate 2 and the cover lay 3 or between the electrodes 4a in the manufacturing process. This is because the first and second adhesive layers 5a and 5b flow in the pressure bonding between the base material 2 and the cover lay 3 to spread between the base material 2 and the cover lay 3 and between the electrodes 4a. This is to pour in. For this reason, this printed wiring board 301 has fewer bubbles and voids than those in which the base material 2 and the coverlay 3 are laminated without using an adhesive layer. As a result, the printed wiring board 301 has little deterioration due to peel or the like caused by bubbles or voids, and little characteristic change due to time-dependent characteristic changes or temperature changes caused by bubbles or voids.

  In addition, the thickness of the first adhesive layer 5 a and the thickness of the second adhesive layer 5 b are preferably larger than the thickness of the wiring pattern 4. According to this configuration, it is possible to further suppress the generation of bubbles and voids between the edge of the wiring pattern 4 and the two electrodes 4a in the manufacturing process. For example, when the thickness of the wiring pattern 4 is 18 μm, both the thickness of the first adhesive layer 5a and the thickness of the second adhesive layer 5b are set to 25 μm.

  In addition, the structure which adhere | attaches the base material 2 and the coverlay 3 via the 1st contact bonding layer 5a and the 2nd contact bonding layer 5b is the printed wiring board 101 which concerns on 2nd Embodiment, or the printed wiring board which concerns on 3rd Embodiment. 201 also applies.

  Moreover, the structure about the thickness of the 1st contact bonding layer 5a and the 2nd contact bonding layer 5b is applied also to the printed wiring board 401 which concerns on 5th Embodiment shown below, and the printed wiring board 501 which concerns on 6th Embodiment. Is preferred.

A printed wiring board 401 according to the fifth embodiment will be described with reference to FIG.
This printed wiring board 401 differs from the printed wiring board 1 according to the first embodiment in the following points. That is, the printed wiring board 1 according to the first embodiment has a structure in which the base material 2 and the cover lay 3 are pressure-bonded. However, the printed wiring board 401 according to the present embodiment includes the base material 2 and the wiring pattern 4. A first adhesive layer 5 a is provided therebetween, and a second adhesive layer 5 b is provided between the cover lay 3 and the wiring pattern 4. The first and second adhesive layers 5a and 5b are sealed so as not to be exposed. The structure of the base material 2 and the coverlay 3 is the same structure as the printed wiring board 1 which concerns on 1st Embodiment.

  The first and second adhesive layers 5a and 5b are formed of an epoxy resin or the like. Since the first and second adhesive layers 5a and 5b have a high absorption rate of the fuel 30, the end portions of the base material 2 and the cover lay 3 are liquid crystal polymer members 6 so that the first and second adhesive layers 5a and 5b are not exposed. It is sealed with. The liquid crystal polymer member 6 is formed of a liquid crystal polymer having an absorptance of the fuel 30 of 2% by mass or less, preferably 1.0% by mass or less, like the substrate 2 and the coverlay 3.

  In the state where the first and second adhesive layers 5a, 5b absorb the fuel 30, and the state where the first and second adhesive layers 5a, 5b do not absorb the fuel 30, the fuel 30 in the electrode region AE. Even when the proportions of the occupied portions are equal, the electrostatic capacity between the electrodes 4a is different in both states due to the influence of the absorbed fuel 30.

  Therefore, in the above configuration, the end portion of the substrate 2 and the end portion of the cover lay 3 are sealed with the liquid crystal polymer member 6 so that the first and second adhesive layers 5a and 5b are not exposed. With this configuration, the first and second adhesive layers 5a and 5b can be prevented from absorbing the fuel 30.

  In addition, the structure which seals the edge part of the base material 2 and the coverlay 3 with the liquid-crystal polymer member 6 is the print which concerns on 2nd Embodiment about what has an adhesive layer between the base material 2 and the coverlay 3. The present invention is also applied to the wiring boards 101 and 102 and the printed wiring board 201 according to the third embodiment.

A printed wiring board 501 according to the sixth embodiment will be described with reference to FIG.
This printed wiring board 501 is obtained by changing the printed wiring board 401 according to the fifth embodiment as follows. In the printed wiring board 401 according to the fifth embodiment, the ends of the base material 2 and the coverlay 3 are covered with the liquid crystal polymer member 6 so that the first and second adhesive layers 5a and 5b are not exposed. Instead of the configuration, in the printed wiring board 301 according to the sixth embodiment, the end portion 2a of the base material 2 is folded and crimped to the cover lay 3. In addition, the structure which folds the edge part of the coverlay 3 and crimps | bonds to the base material 2 is also employable. Also by this structure, the edge part of the base material 2 and the edge part of the coverlay 3 can be sealed easily.

  The configuration in which the end portion of the printed wiring board 501 is sealed by folding back the end portion of the base material 2 or the cover lay 3 is a second embodiment in which an adhesive layer is interposed between the base material 2 and the cover lay 3. The present invention is also applied to the printed wiring board 101 according to the embodiment and the printed wiring board 201 according to the third embodiment.

A printed wiring board 601 according to the seventh embodiment will be described with reference to FIG.
The printed wiring board 601 is different from the printed wiring board 1 according to the first embodiment in the following points. That is, the printed wiring board 1 according to the first embodiment has a structure (see FIG. 2) in which the base material 2 and the coverlay 3 are directly bonded to each other.

  On the other hand, in this printed wiring board 601, the cover lay 3 is laminated on the base material 2 through the adhesive layer 5. The adhesive layer 5 is made of a liquid crystal polymer. Preferably, the liquid crystal polymer constituting the adhesive layer 5 is lower than the melting point of the substrate 2 and lower than the melting point of the coverlay 3.

  With such a configuration, the water absorption rate of the printed wiring board 601 decreases. In addition, when the base material 2 and the coverlay 3 are bonded by thermocompression during manufacturing, the heating temperature can be made lower than the melting point of the base material 2 and the melting point of the coverlay 3. And even in such temperature setting, the adhesive layer 5 can be sufficiently softened, and the adhesive (material of the adhesive layer 5) can be filled in the gaps between the wiring portions 41b and the gaps between the electrodes 41a. Become. Thereby, formation of a bubble and a void between the base material 2 and the coverlay 3 or between the electrodes 4a is suppressed.

  For this reason, the printed wiring board 601 has an improved characteristic that the electrostatic capacity is constant when the liquid occupancy is a predetermined value with respect to the electrode region, and is less deteriorated due to peel or the like caused by bubbles or voids. In addition, the characteristic change with time or the characteristic change due to temperature change becomes small.

A printed wiring board 701 according to the eighth embodiment will be described with reference to FIG.
The printed wiring board 701 is different from the printed wiring board 1 according to the first embodiment in the following points. That is, the printed wiring board 701 according to the eighth embodiment includes a first fluororesin layer 7 that covers the substrate 2 and a coverlay in addition to the configuration of the printed wiring board 1 according to the first embodiment (see FIG. 2). 2 and a second fluororesin layer 8 that covers 3. The first fluororesin layer 7 and the second fluororesin layer 8 are layers mainly composed of a fluororesin. Here, the main component indicates that the ratio of the fluororesin to the resin is 50% by mass or more. Preferably, the 1st fluororesin layer 7 and the 2nd fluororesin layer 8 are formed with resin whose fluororesin is 90 mass% or more.

  The fluororesin has a lower absorption rate of alcohol, particularly methyl alcohol, than the liquid crystal polymer. For this reason, the printed wiring board 701 having the above configuration has a small characteristic change caused by absorption of methyl alcohol, and the printed wiring board 701 has a capacitance when the liquid occupancy is a predetermined value with respect to the electrode region. The property of being constant is improved.

  The thickness of the first fluororesin layer 7 is preferably 25 μm or more and 200 μm or less. More preferably, the thickness of the first fluororesin layer 7 is not less than 50 μm and not more than 100 μm. The thickness of the second fluororesin layer 8 is set similarly to the thickness of the first fluororesin layer 7.

  This is because if the thickness of the first fluororesin layer 7 or the second fluororesin layer 8 becomes too large, the capacitance between the electrodes 4a becomes too small and the detection accuracy of the fuel 30 is too low. On the other hand, if the thickness of the first fluororesin layer 7 or the second fluororesin layer 8 becomes too small, the function of blocking the methyl alcohol of the first fluororesin layer 7 or the second fluororesin layer 8 is lowered. . For example, if the first fluororesin layer 7 or the second fluororesin layer 8 is damaged, methyl alcohol may enter from this portion and the base material 2 or the coverlay 3 may absorb the methyl alcohol.

  Examples of the fluororesin include PTFE (polytetrafluoroethylene), PFA (polytetrafluoroethylene / perfluoroalkyl vinyl ether copolymer), FEP (tetrafluoroethylene / hexafluoropropylene copolymer), tetrafluoroethylene / six Three types of monomers: fluorinated ethylene copolymer, polyvinylidene fluoride, polychlorotrifluoroethylene, chlorotrifluoroethylene / ethylene copolymer, polyvinyl fluoride, THV (tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride) A thermoplastic fluororesin), a fluoroelastomer, and the like. Further, a mixture or copolymer containing these compounds is used as a material constituting the fluororesin.

  Further, preferably, at least one of the first fluororesin layer 7 and the second fluororesin layer 8 includes a fiber woven sheet. Examples of the fiber fabric sheet include a glass cloth sheet and a carbon sheet. Instead of the fiber woven sheet or in addition to the fiber woven sheet, the first fluororesin layer 7 and the second fluororesin layer 8 may include a filler and a non-woven fabric. Examples of the filler include a glass filler and a carbon filler. Examples of the nonwoven fabric include a glass nonwoven fabric and a carbon fiber nonwoven fabric. The fluororesin sheet as a member for forming the first fluororesin layer 7 and the second fluororesin layer 8 including the fiber woven sheet or non-woven fabric is impregnated with the fluororesin for the fiber woven fabric sheet or non-woven fabric, or the fiber woven fabric sheet or non-woven fabric and It is formed by hot pressing with a fluororesin sheet. Thereby, the rigidity of the printed wiring board 701 becomes high, and expansion / contraction between the electrodes accompanying the deformation of the printed wiring board 701 is suppressed. In addition, the productivity of the printed wiring board 701 is improved for the reasons described above.

A printed wiring board 801 according to the ninth embodiment will be described with reference to FIG.
The printed wiring board 801 is different from the printed wiring board 1 according to the first embodiment in the following points. In the printed wiring board 1 according to the first embodiment, the base 2 and the coverlay 3 are formed of a resin mainly composed of a liquid crystal polymer, whereas in the printed wiring board 801 according to the ninth embodiment. The base material 2 and the cover lay 3 are formed of a resin whose main component is a fluororesin. Here, the main component indicates that the ratio of the fluororesin to the resin is 50% by mass or more. Preferably, the base material 2 and the coverlay 3 are formed of a resin having a fluororesin of 90% by mass or more.

  The thickness of the substrate 2 is preferably 25 μm or more and 200 μm or less. More preferably, the thickness of the base material 2 is 50 μm or more and 100 μm or less. The thickness of the coverlay 3 is set similarly to the thickness of the base material 2. That is, the thickness of the coverlay 3 is preferably 25 μm or more and 200 μm or less, and more preferably 50 μm or more and 100 μm or less.

  Such a setting is because if the thickness of the substrate 2 or the coverlay 3 becomes too large, the capacitance between the electrodes 4a becomes too small and the detection accuracy of the fuel 30 is too low. Moreover, it is because the detection precision of the fuel 30 falls because the amount of fuel absorption of the base material 2 or the coverlay 3 increases. On the other hand, if the thickness of the base material 2 or the cover lay 3 becomes too small, the sealing function of the base material 2 or the cover lay 3 is lowered. For example, if the base material 2 or the cover lay 3 is damaged, fuel or air may enter from this portion and the wiring pattern 4 may be deteriorated.

As the fluororesin, the fluororesin mentioned in the description of the printed wiring board 701 according to the eighth embodiment can be used.
Furthermore, at least one of the base material 2 and the coverlay 3 preferably includes both (see FIG. 13) a fiber fabric sheet 9. Examples of the fiber fabric sheet 9 include a glass cloth sheet and a carbon sheet. Instead of the fiber fabric sheet 9 or in addition to the fiber fabric sheet 9, the base material 2 and the coverlay 3 may include the same filler as described above and the same nonwoven fabric as described above. Thereby, the rigidity of the printed wiring board 801 is increased, and the expansion and contraction between the electrodes accompanying the deformation of the printed wiring board 801 is suppressed. In addition, the productivity of the printed wiring board 801 is improved for the reasons described above.

  According to the said structure, the base material 2 and the coverlay 3 are both formed of the fluororesin. The fluororesin has a lower absorption rate of water and the fuel 30 than a resin used for a conventional printed wiring board such as a polyimide resin or an epoxy resin.

  For this reason, the effect similar to 1st Embodiment is acquired. That is, the printed wiring board 801 has a constant capacitance when the occupation ratio of the water and the fuel 30 is a predetermined value with respect to the electrode region AE regardless of the immersion time, the immersion state, or the temperature condition.

A printed wiring board 901 according to the tenth embodiment will be described with reference to FIG.
The printed wiring board 901 is different from the printed wiring board 801 according to the ninth embodiment in the following points. A printed wiring board 801 according to the ninth embodiment has a structure in which the base material 2 and the coverlay 3 are directly bonded to each other (see FIG. 13). On the other hand, in this printed wiring board 901, the cover lay 3 is laminated on the base material 2 through the adhesive layer 5. The adhesive layer 5 is made of a liquid crystal polymer. The thickness of the adhesive layer 5 is preferably larger than the thickness of the wiring pattern 4. Preferably, the liquid crystal polymer constituting the adhesive layer 5 is lower than the melting point of the substrate 2 and lower than the melting point of the coverlay 3. With such a configuration, the water absorption rate of the printed wiring board 601 decreases. In addition, when the base material 2 and the cover lay 3 are bonded by thermocompression during manufacturing, the heating temperature can be made lower than the melting point of the base material 2 and the melting point of the cover lay 3. Thereby, the same effect as in the eighth embodiment can be obtained.

[Other Embodiments]
-About each said embodiment and modification, the structure of the electrode 41a of the wiring pattern 4 is not limited. In the first and second embodiments, the two electrodes 41a constituting the electrode pair 42 are set so that the distance Sb is a constant width. However, the two electrodes 41a are close to each other. The edges need not be parallel to each other and may be enlarged or reduced in part.

  That is, the two electrodes 41a constituting the electrode pair 42 need only be arranged so as to be close to each other to the extent that the electrostatic capacitance between them can be detected by applying a predetermined voltage. The shape of the two electrodes 41a is not limited to a rectangle. For example, the electrode 41a is formed in a comb shape, and the two electrodes 41a are arranged so that the teeth of one electrode 41a and the teeth of the other electrode 41a are alternately arranged. According to this configuration, in the electrode pair 42, the portion where the two electrodes 41a are close to each other becomes longer, so the capacitance between the electrodes 41a increases.

-About the liquid level sensor 10, the following structure is also employable.
The liquid level sensor 10 includes the printed wiring board 1 according to the first embodiment. Instead of the printed wiring board 1, any one of the printed wiring boards 101 to 901, 102 of the second to tenth embodiments is used. The liquid level sensor 10 may be configured to include. Such a liquid level sensor 10 also has the same effect as the liquid level sensor 10 described above.

The printed wiring boards 1, 101 to 901, 102 are useful when used in the liquid level sensor 10 that measures electrostatic capacitance.
The liquid level sensor 10 is useful when used as a measuring device that measures the liquid level of the fuel 30.

DESCRIPTION OF SYMBOLS 1,101,102,201,301,401,501,601,701,801,901 ... Printed wiring board 2 ... Base material 2a ... End part 3 ... Coverlay 4 ... Wiring pattern 4a ... Electrode 4b ... Terminal part 5 ... Adhesive layer 5a ... first adhesive layer 5b ... second adhesive layer 6 ... liquid crystal polymer member 7 ... first fluororesin layer 8 ... second fluororesin layer 9 ... textile fabric sheet 10 ... liquid level sensor 11 ... measuring section 12 ... conducting wire 20 ... Fuel tank 30 ... Fuel 40 ... Wire pair 41 ... Wire 41a ... Electrode 41b ... Wire portion 41bx ... First wire portion 41by ... Second wire portion 41c ... Terminal portion 42 ... Electrode pair 51a ... Electrode 52 ... Spacer AE ... Electrode Area DF ... Flow direction DR ... Right angle direction DE ... Extension direction

Claims (18)

A substrate, a wiring pattern formed on the substrate, and a coverlay covering the wiring pattern;
The wiring pattern includes at least a pair of electrodes for detecting capacitance between the electrodes,
Each of the base material and the coverlay has at least one layer of a liquid crystal polymer and a fluororesin. The printed wiring board according to claim 1, wherein both the base material and the coverlay are formed of a liquid crystal polymer. Both the base material and the coverlay are formed of a liquid crystal polymer,
The printed wiring board according to claim 1, wherein end edges of the pair of electrodes of the wiring pattern are separated by a predetermined interval. The printed wiring board according to claim 2, wherein the coverlay is laminated on the base material via an adhesive layer formed of a liquid crystal polymer. The printed wiring board according to claim 4, wherein a melting point of the adhesive layer is lower than a melting point of the base material and lower than a melting point of the coverlay. The base material is covered with a first fluororesin layer formed of a fluororesin having a melting point higher than that of the base material,
The printed wiring board according to any one of claims 2 to 5, wherein the cover lay is covered with a second fluororesin layer formed of a fluororesin having a melting point higher than that of the cover lay. The printed wiring board according to claim 6, wherein at least one of the first fluororesin layer and the second fluororesin layer includes at least one of a filler, a nonwoven fabric, and a fiber fabric sheet. The printed wiring board according to claim 2, wherein the wiring pattern is formed on the base material via a first adhesive layer, and the coverlay is laminated on the base material via a second adhesive layer. . The printed wiring board according to claim 8, wherein an end portion of the base material and an end portion of the cover lay are sealed so that the first and second adhesive layers are not exposed. The printed wiring board of Claim 9. The edge part of the said base material and the edge part of the said coverlay are sealed with the liquid crystal polymer member. The printed wiring board according to claim 9, wherein an end portion of the base material is folded and crimped to the coverlay, or an end portion of the coverlay is folded and crimped to the base material. The printed wiring board according to claim 1, wherein each of the base material and the cover lay is made of a fluororesin. The printed wiring board according to claim 12, wherein the coverlay is laminated on the base material through an adhesive layer formed of a liquid crystal polymer having a melting point lower than that of a fluororesin. The printed wiring board according to claim 12 or 13, wherein each of the base material and the cover lay includes at least one of a filler, a nonwoven fabric, and a fiber woven fabric sheet. The wiring pattern has a pair of electrodes formed on the same plane, and an interval between the electrodes is a constant width from one end to the other end,
The printed wiring board according to any one of claims 2 to 11, wherein an extending direction of the electrodes coincides with a flow direction of the liquid crystal polymer of the base material and the coverlay. The printed wiring board according to claim 1, wherein the wiring pattern has a plurality of electrode pairs. The printed wiring board according to any one of claims 1 to 14, wherein the wiring pattern includes a pair of electrodes arranged to face each other.   The liquid level sensor which has a printed wiring board of any one of Claims 1-17.