JP2010087145A - Electronic component mounting substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a highly reliable electronic component mounting board by suppressing the occurrence of a crack of a fillet for a long period in component mounting by lead-free solder, and a printed wiring board 10 having a metal electrode portion 10a on the surface. And an electronic component 13 joined to the printed wiring board by soldering. A strain suppressing body 20 (ceramic sheet) made of ceramic, which is a member mainly constituting the electronic component, is provided on the surface and / or the inner surface of the printed wiring board facing the electronic component. Then, the difference in expansion and contraction between the electronic component and the printed wiring board accompanying the temperature change in the vicinity of the electronic component is reduced, the stress applied to the solder fillet 11 is reduced, and the life is extended.
[Selection] Figure 6

Description

  The present invention relates to an electronic component mounting board on which electronic components such as chip resistors are mounted on a printed wiring board via solder, and more specifically to improvement of durability against thermal strain.

  As is well known, when an electronic component is mounted on a wiring board, it is fixed by soldering. For example, as shown in FIG. 1, an electronic component 2 is disposed on a wiring substrate 1 having a metal electrode portion 1a on the surface, and the electrode 2a of the electronic component 2 and the metal electrode portion 1a of the wiring substrate 1 are soldered. And fix it. The electrode 2a and the metal electrode portion 1a are electrically connected through the solder fillet 3.

  There are many types of materials constituting the wiring board 1 such as a metal / glass base impregnated with resin and a paper base impregnated with resin. However, in any material, the elastic coefficient is 5 to 25 GPa and the thermal expansion coefficient is within the range of 15 to 25 * 10 −6 / ° C. On the other hand, a ferrite sintered body that is widely used as a material for a chip resistor that is an example of the electronic component 2 exhibits an elastic modulus of about 100 to 200 GPa and a thermal expansion coefficient of about 5 to 10 * 10 −6 / ° C. .

  As described above, the thermal expansion coefficients of the wiring board 1 and the electronic component 2 are greatly different. Therefore, when the temperature environment at the time of manufacture is shifted to a high temperature or a low temperature, a difference in thermal expansion coefficient between the wiring board 1 and the electronic component 2 is obtained. Cause stress. In other words, the degree of expansion / contraction of both varies with temperature change, but the wiring board 1 and the electronic component 2 are connected at the metal electrode portion 1a and the electrode 2a, and between the two 1 and 2 at that portion. Therefore, the difference in the degree of expansion / contraction caused by the difference in the thermal expansion coefficient appears as deformation of the wiring board 1 and the electronic component 2. Since the elastic coefficient of the wiring board 1 is obviously smaller and softer than that of the electronic component 2, the amount of deformation of the wiring board 1 is particularly large, resulting in a warped shape of the wiring board 1.

  When the board is in a state along such a component configuration, stress concentration occurs at a position near the corner portion of the electronic component 2 in the fillet 3, and a large stress is locally generated. However, tin-lead eutectic solder, which is generally used when soldering electronic components, is soft and has a good balance between elongation and strength. It has the merit of being able to absorb the stress generated at the joint and having high durability.

  However, lead contained in eutectic solder is a metal harmful to the human body, and in combination with protection of the global environment, in recent years, there has been an increasing demand for avoiding the use of harmful substances (lead). Therefore, research and development of lead-free solder that does not contain lead has been conducted.

The most promising lead-free solder of this type is a ternary tin-silver-copper alloy. As typical compositions of this alloy, Sn-3.5Ag-0.7Cu, Sn-3.0Ag-0.5Cu, and the like have been developed and put into practical use. In addition to this, there are ternary Sn-Ag-Bi and Sn-Cu-Ni (Ni is a trace additive). There are also lead-free solders such as -Bi and ternary Sn-Ag-Cu-Bi-Ge. As an electronic component mounting technique using this type of lead-free solder, for example, there is one disclosed in Patent Document 1 or the like.
JP 2002-368362 A

  However, the above-described lead-free solder has a small stress absorbing action as described above. For this reason, stress generated due to a difference in thermal expansion coefficient between the substrate and the electronic component due to a temperature change in the usage environment cannot be completely absorbed, and a minute plastic strain is accumulated. The strain accumulated in this way develops into a crack inside the solder fillet, and the crack associated with each temperature change expands and finally reaches the surface of the solder fillet. When high stress is generated even in such a local state, the entire fillet is torn, causing a problem in that the joint is torn and electrical connection cannot be maintained.

  An object of the present invention is to provide a highly reliable electronic component mounting substrate that suppresses the occurrence of cracks and the like of a fillet over a long period of time especially in component mounting using lead-free solder.

  In order to achieve the above-described object, an electronic component mounting board according to the present invention includes (1) a wiring board having a metal electrode portion on the surface, and an electronic component joined to the wiring board by soldering. A strain suppressing structure is provided on the surface and / or the inner surface of the wiring board facing the electronic component to suppress a difference in expansion and contraction due to a temperature change between the wiring substrate and the electronic component.

  For example, in the temperature range of about −60 ° C. to 150 ° C., the cause of fatigue failure at the solder fillet site when a temperature cycle is applied is because the thermal expansion coefficients of the substrate and the electronic component are different. The warping of the substrate that appears. That is, when the substrate is warped, stress concentration occurs at a position near the corner portion of the electronic component in the solder fillet, and a large stress is locally generated. Therefore, in order to increase the fatigue resistance of the solder fillet, the amount of plastic strain accumulated in the stress concentration portion during use may be reduced in order to prevent the fillet from being broken.

  In this case, if the thermal expansion coefficients of the wiring board and the electronic component are equal, the stress on the fillet portion is almost zero. However, the electronic component and the wiring board can exhibit desired characteristics, cost, reliability, etc. It is difficult to change the material and the basic configuration from the viewpoint of the above, and it is difficult to greatly reduce the difference in thermal strain. Therefore, in the present invention, the portion corresponding to and facing the electronic component in the wiring board is configured to have a strain suppressing structure, so that at least a part or all of the installation area of the electronic component is between the electronic component and the wiring board. The difference in the generated thermal strain can be reduced, the stress applied to the solder fillet can be reduced, and the life can be extended. As an example of a specific configuration of the strain suppression structure, the following various structures can be employed.

  (2) The strain suppression structure may be configured by disposing a sheet-like strain suppression body made of a low thermal expansion material lower than the thermal expansion coefficient of the wiring substrate of the wiring substrate on the wiring substrate. (3) And the said low thermal expansion material is good to comprise with the same material as the member for wiring used for the said wiring board. By using the low thermal expansion material, since the difference in expansion and contraction accompanying the temperature change with the electronic component is smaller than in the case of the wiring board, the stress applied to the fillet is also reduced, and the accumulation amount is also reduced. In particular, when the wiring member is formed of the same material as the wiring member, it is possible to easily pattern the strain suppressor on a desired portion corresponding to the electronic component by the same process as the wiring pattern manufacturing process. Further, when viewed from the whole wiring board, the installation area and volume of the strain suppressor are small, so there is almost no influence on the whole board.

  (4) The wiring board is a multilayer board in which wiring layers and base material layers are alternately stacked, and the strain suppressing body made of the low thermal expansion member is formed in the same layer as the wiring layer. Good. When the strain suppressing body is formed in the same layer as the wiring layer, the wiring pattern can be formed by printing or the like, and the pattern of the strain suppressing body can be formed simultaneously or before and after. In particular, when the same material as the wiring member is used as the low thermal expansion member, the wiring pattern and the strain suppressing body can be formed simultaneously, so it is only necessary to change the mask pattern to be used. The case of the provided wiring board can be easily manufactured.

  (5) The strain suppression structure may be configured by disposing a sheet-like strain suppression body made of a member having a thermal expansion coefficient equivalent to the thermal expansion coefficient of the entire electronic component on the wiring board. (6) Further, the strain suppression structure may be configured by disposing a sheet-like strain suppression body made of the same material as a member mainly constituting the electronic component on the wiring board. (7) On the premise of the inventions of (5) and (6), it is preferable that the strain suppressing body is disposed in the vicinity of the surface on the electronic component side. The member mainly constituting the electronic component is ceramic in the embodiment, and is an insulating constituent material (not an internal electrode) of the chip body constituting the main body portion of the electronic component. With such a configuration, the thermal expansion coefficient of a part or all of the electronic component installation area of the wiring board becomes a value close to that of the electronic component, so that the stress applied to the solder fillet is reduced as much as possible. In particular, it is preferable to install near the surface as in (7) because the installation effect is enhanced. Further, when viewed from the whole wiring board, the installation area and volume of the strain suppressor are small, so there is almost no influence on the whole board.

  (8) The strain suppressing body may be formed so as to cover a region facing the plurality of the electronic components. If it does in this way, the stress added with respect to the solder fillet part of a some electronic component can be reduced collectively with one distortion suppression body, and manufacture of a distortion suppression body can be performed easily.

  (9) The strain suppressing structure may be a gap extending in the thickness direction of the wiring board. By providing the gap, the expansion and contraction of the wiring board and the transmission of stress stop at that part, so the difference in expansion and contraction due to the temperature change between the electronic component and the wiring board (area where the gap is provided) can be reduced. it can. Therefore, the stress applied to the fillet portion can be reduced and the life can be extended. Further, when viewed from the whole wiring board, the installation area and volume of the strain suppressor are small, so there is almost no influence on the whole board. Various types of voids can be used, and examples include the following.

  (10) The gap may be a plurality of slit-like gaps, and the plurality of slit-like gaps may be arranged in parallel in a direction substantially orthogonal to the longitudinal direction of the electronic component. In addition, when the slit-shaped gap has a length that can traverse the entire electronic component 13, a plurality of slits may be installed instead of a single unit. (11) The gap portion may be composed of a plurality of circular holes. (15) The gap may not be penetrated in the thickness direction of the wiring board, but may be installed in a portion limited to the thickness direction.

  In the present invention, since the difference in deformation amount between the wiring board and the electronic component can be suppressed as the temperature changes, the fracture life of the solder joint can be extended, and a highly reliable electronic component mounting electronic substrate is supplied. be able to.

  FIG. 2 shows a first preferred embodiment of the present invention. As shown in FIG. 2, a rectangular parallelepiped electronic component 13 is mounted at a predetermined position on the printed wiring board 10 by soldering. In other words, the printed circuit board 10 has, for example, a wiring circuit pattern made of copper foil or the like formed on the surface of an insulating base material in which a glass cloth is impregnated with an epoxy resin. Part 10a is formed. In the case of a multilayer printed wiring board, the wiring pattern is also formed inside.

  The electronic component 13 is, for example, a chip resistor or the like, and a rectangular parallelepiped chip body portion is formed of ceramic, and an internal electrode having a predetermined pattern is formed inside the chip body, so that functions and characteristics of desired elements can be obtained. The internal electrode is made of, for example, copper. Further, external electrodes 13a are formed at predetermined positions on the surface of the chip body. In the present embodiment, the external electrodes 13a are formed at both ends in the longitudinal direction so as to extend over the side surface and the upper and lower surfaces, respectively. You may make it prescribe | regulate a site | part (height).

  And the external electrode 13a and the metal electrode part 10a interpose the leadless solder fillet 11, and the metal electrode part 10a and the external electrode 13 are electrically and mechanically connected by the fillet 11. As the lead-free solder, a ternary tin-silver-copper alloy (for example, Sn-3.0Ag-0.5Cu) or a variety of quaternary / quinary alloys can be used.

  Here, in the present embodiment, the strain suppressing body 15 having a low thermal expansion coefficient of the printed wiring board 10 is disposed on a part (in the drawing) of the printed wiring board 10. The strain suppression body 15 is a flat plate shape and is arranged in parallel with the surface of the printed wiring board 10. And this type of printed wiring board 10 is often a multilayer printed wiring board in which a wiring pattern is also provided. This multilayer printed wiring board is formed by alternately laminating the above-mentioned insulating substrate layers made of a predetermined material and wiring layers having a predetermined wiring pattern. Therefore, the strain suppression body 15 is formed in the same layer as the wiring layer. That is, the strain suppressing body 15 can be installed at a desired position by patterning (printing or the like) the low thermal expansion member constituting the strain suppressing body 15 at a predetermined position at the same time in the step of installing the wiring layer.

  Further, in the present embodiment, as described above, the same material as the copper used as the wiring material in the printed wiring board 10 is used as the low thermal expansion material constituting the strain suppressing body 15. That is, copper used as the wiring material has a higher Young's modulus (130 GPa) and a lower thermal expansion coefficient (16 ppm /) than a glass composite substrate such as CEM-3 that constitutes the substrate of the printed wiring board 10. Therefore, the strain suppression body 15 can be configured. By using the same material as the wiring pattern for the strain suppressing body 15, the strain suppressing body 15 can be manufactured simultaneously with the wiring pattern when the wiring layer is formed.

  In addition, the formation position of the strain suppression body 15 is on the lower side of the electronic component 13. As shown in the figure, a pattern / arrangement layout in which a plurality of metal electrode portions 10a and electronic components 13 are located below the metal electrode portion 10a serving as a joint portion of the electronic component 13 and above the strain suppression body 15. And good. Of course, the strain suppression body 15 is located only on a part of the metal electrode portion 10a or only on the lower side of the metal electrode portion 10a, or not on the lower side of the metal electrode portion 10a, but only on the region between the metal electrode portions 10a. Even if it does, although an effect falls rather than the thing of embodiment, durability improves compared with the thing which does not provide the distortion suppression body 15. FIG.

  In practice, a plurality of electronic components are mounted on the surface of the printed wiring board 10. Therefore, the strain suppression body 15 may be provided independently on the lower side of each electronic component 13, or the area is several times larger than that of the electronic component 13, and the strain suppression body 15 is located above one strain suppression body. A plurality of electronic components may be arranged.

  Furthermore, although two layers are provided in the figure, the number of installations is arbitrary and may be one layer or three or more layers. Moreover, it does not restrict to what is provided inside as shown in the figure, and may of course be formed on the surface of the printed wiring board 10. Furthermore, when it says in the height direction of the printed wiring board 10, it is better to provide in the upper side. That is, by disposing the strain suppressing body 15 in the vicinity of the joint portion between the printed wiring board 10 and the electronic component 13, the difference in expansion and contraction (thermal strain) accompanying the temperature change of the printed wiring board 10 and the electronic component 13 in the vicinity of the joint portion. It is because it can be made small.

  In order to reduce the difference in thermal strain between the electronic component 13 and the printed wiring board 10, it suffices if it is only at a position directly below the electronic component 13, and the strain suppressing body 15 of the present embodiment is attached to the printed wiring board 10. Compared with a high Young's modulus, a large effect can be obtained with a small thickness. In addition, since the wiring layers can be formed at the same time in the installation process, there is almost no increase in cost.

  FIG. 3 shows a second embodiment of the present invention. About the structure similar to 1st Embodiment, the same code | symbol is attached | subjected to a corresponding member and the detailed description is abbreviate | omitted. In the present embodiment, the first gap portion 16 penetrating vertically is provided in the installation area of the electronic component 13 of the printed wiring board 10. The first gap 16 is composed of a slit-like elongated rectangular opening. By providing the first gap portion 16 in this way, even if the printed wiring board 10 tries to expand and contract in the longitudinal direction along with a temperature change, the expansion and contraction of the first gap portion 16 stops (no stress is transmitted). Therefore, the amount of change is smaller than that of the conventional one in which the first gap portion 16 is not provided. Therefore, the difference in expansion and contraction accompanying the temperature change between the electronic component 13 and the printed wiring board 10 is reduced, the stress applied to the fillet 11 which is a joining site is reduced, and the accumulation of fatigue is reduced, thereby extending the life. At this time, the longitudinal direction of the slit is preferably directed as perpendicular to the longitudinal direction of the electronic component as possible. In this way, the effect of suppressing the distortion by the slit can be maximized, so that the maximum effect can be obtained with the minimum opening area, and the decrease in rigidity of the entire printed wiring board 10 can be suppressed.

  The shape of the gap to be formed is not limited to the above-described slit, but may be, for example, the second gap 17 formed of a circular opening such as a via as shown in FIG. 4 or any other shape. it can. In this way, the circular opening originally has a technique for creating an opening such as a through hole, and it is possible to divert this to make it in the same process.

  Further, the present invention is not limited to the one that penetrates the printed wiring board 10 vertically (thickness direction) as in the above-described example. For example, as shown in FIG. You may make it form. In the figure, the third gap portion 18 is formed so as to open downward, but conversely, the gap portion may be formed so as to open upward. In FIG. 5, the shape of the opening is a slit-shaped square opening, but it may be a circular opening or another shape as shown in FIG. If the structure opens to one side as shown in FIG. 5, it is possible to suppress a decrease in rigidity of the printed wiring board 10 and obtain a great effect of improving the life. Further, if the opening is formed wide so as to cross the installation area of the electronic component 13 as shown in FIG. 5, the transmission of stress can be largely eliminated, so the number of slits is small (in some cases, a single unit may be installed). Yes) Even with a configuration, a sufficiently large life improvement effect can be obtained.

  In the present embodiment and the modification, only a gap is provided in a part of the printed wiring board 10, so that only a part of the manufacturing process is added, and the influence on other characteristics is also minimized. Can do. As the area of the opening increases, the strength of the substrate itself is lost. However, in the configuration in which a plurality (small number) of small openings are arranged as in the present embodiment and the modification, the decrease in the rigidity of the substrate is also minimized. be able to.

Next, in order to verify the effects of the present embodiment and the modified example, based on a finite element analysis shape model in which electronic components having the following dimensions and shapes are connected to a printed wiring board with solder, assumed temperature cycle conditions and each part Analysis of the configuration was performed. The analysis conditions including dimensions and shapes are as follows.
(1) Conduct stress analysis under conditions of temperature cycle 125 ° C to -55 ° C x 3 cycles (2) Ceramics for electrodes, copper for electrodes and wiring patterns, Sn-3.0Ag-0.5Cu alloy for solder (3) Print Wiring board is glass composite board (CEM-3)
(4) Electronic component size 3.2 × 1.6 mm, thickness 0.48 mm,
0.08mm distance between electronic components and printed wiring board
0.015mm thickness of external electrode of electronic component,
0.05mm thickness of metal electrode part on printed circuit board
(5) Void pattern a: No gap (comparative example: conventional product)
b: Air gaps (second air gaps) of circular openings (vias) having a diameter of 40 μm are arranged at a pitch of 40 μm in the width direction of the electronic component, and six rows are equally arranged in the length direction of the electronic component (volume of the void portion) (Rate about 2.9%)
c: The air gaps (second air gaps) of the circular openings (vias) having a diameter of 40 μm are arranged at a pitch of 40 μm in the width direction of the electronic component, and 12 rows are equally arranged in the length direction of the electronic component (volume of the air gap portion) (Rate about 5.9%)
d: The air gaps (second air gaps) of the circular openings (vias) having a diameter of 40 μm are arranged at a pitch of 20 μm in the width direction of the electronic component, and 12 rows are equally arranged in the length direction of the electronic component (volume of the air gap portion) (Rate about 7.6%)
e: Six rows of 40 μm wide slit-shaped voids (third voids) formed from the lower surface of the printed circuit board to the center of the substrate are arranged in the length direction of the electronic component (gap portion) Volume ratio of about 3.8%)

  As a result of analysis, strong stress concentration occurred in the solder portion (fillet 11) near the lower corner portion of the electronic component in any case. The degree of stress concentration decreased in the order of a → b → c → d → e. By correlating the amount of plastic strain (nonlinear strain amplitude) accumulated in the solder at the corner obtained from the analysis results with the actual temperature cycle test results, the number of cycles at which fracture occurs in a similar configuration can be predicted. .

  For example, in the configuration of the comparative example shown in a, the number of rupture cycles at a temperature cycle of 125 ° C. to −55 ° C. was about 480 times. Predicting the number of break cycles in the configuration of the present invention from these data, the configuration of b is 505 times (about 1.05 times), the configuration of c is 570 times (about 1.19 times), and the configuration of d is 640 times (about 1.33 times) and 1420 times (about 2.96 times) in the configuration of e. It can be seen that a high life improvement effect can be obtained simply by installing an opening with a small area ratio.

  Further, when a partial gap such as a circular hole is provided, the higher the area ratio, the higher the effect. As is clear from comparison of b, c, and d, when the voids of the same circular hole are used, the effect increases as the volume ratio / area ratio of the voids increases. A remarkable effect is recognized if there is .9%.

  If a gap is provided in the entire component width as in the configuration of e, the stress is not transmitted in the longitudinal direction of the electronic component in the printed wiring board in the first place, and the same effect as the thickness of the printed wiring board is reduced can be obtained. . If the slit penetrates the printed wiring board as in the first gap portion 16, an even greater life improvement effect can be obtained, but the height direction (thickness direction) of the printed wiring board as in the third gap portion (configuration of e). A practically sufficient effect can be obtained only by forming a gap in only a part of the gap.

  FIG. 6 shows a third embodiment of the present invention. In the present embodiment, the same member as the member (ceramic) that mainly constitutes the electronic component 13 is disposed in the vicinity of the substrate surface immediately below the electronic component as the strain suppressing body 20. That is, the strain suppression body 20 of this embodiment is comprised from a ceramic sheet.

  This strain suppressor 20 is made of the same material as the main component of the electronic component, and a ceramic sheet layer is formed at a desired position on the printed wiring board 10 at an appropriate timing in the manufacturing process of the printed wiring board 10 by the same manufacturing method as the electronic component. Good to create. Thereby, the distortion suppression body 20 in which the behavior of thermal expansion is substantially equal to the electronic component 13 can be obtained. Of course, a ceramic sheet may be manufactured by a separate method, and may be mounted at a predetermined position on the printed wiring board 10 by being attached at an appropriate timing.

  The electronic component 13 has a larger elastic coefficient than the printed wiring board 10. Although it depends on the material used, in many cases there is a difference of 10 times or more. Therefore, since the thermal expansion of the printed wiring board 10 can be stopped even with a small volume of member, the difference in expansion and contraction accompanying the temperature change between the electronic component 13 and the printed wiring board 10 at that portion can be reduced. Since the stress related to the connection portion (fillet 11) can be reduced, a great effect can be obtained as a whole. And if the insertion position of the strain suppression body 20 is arranged at a position directly below the electronic component and in the vicinity of the surface layer portion (including the case where it is exposed to the surface), a great effect is obtained. Of course, you may arrange | position the distortion suppression body 20 in the predetermined position inside the printed wiring board 10 instead of surface layer vicinity.

  Furthermore, the installation position of the strain suppression body 20 may include a portion close to the solder joint (fillet 11) to which stress is applied. In this case, as shown in FIG. 6B, it is better to include the formation part of the fillet 11 and arrange it over the entire length of the electronic component 13 in the longitudinal direction. However, as shown in FIG. Even if the strain suppression body 20 does not exist in the region facing the formation site, the strain suppression body 20 may be disposed up to the vicinity thereof. Even in such a case, since the difference in expansion and contraction with the electronic component 13 accompanying the temperature change in the portion of the printed wiring board 10 where the strain suppressing body 20 is disposed is reduced, the stress in the fillet 11 can be reduced. it can.

  Further, as shown in FIG. 6B, when the strain suppression body 20 is disposed over the entire length of the electronic component 13 in the longitudinal direction, the whole of the electronic component 13 also in the width direction (short direction). Although it is most preferable to arrange so as to cover, it may be configured so as to face partly.

  Moreover, although the member which mainly comprises the electronic component 13 was used as a material constituting the strain suppression body 20 in this embodiment, a member having a thermal expansion coefficient equivalent to the thermal expansion coefficient of the entire electronic component may be used. good.

Next, in order to verify the effects of the present embodiment and the modified example, based on a finite element analysis shape model in which electronic components having the following dimensions and shapes are connected to a printed wiring board with solder, assumed temperature cycle conditions and each part Analysis of the configuration was performed. The analysis conditions including dimensions and shapes are as follows.
(1) Conduct stress analysis under conditions of temperature cycle 125 ° C to -55 ° C x 3 cycles (2) Ceramics for electrodes, copper for electrodes and wiring patterns, Sn-3.0Ag-0.5Cu alloy for solder (3) Print Wiring board is glass composite board (CEM-3)
(4) Electronic component size 3.2 × 1.6 mm, thickness 0.481 mm,
0.08mm distance between electronic components and printed wiring board
0.015mm thickness of external electrode of electronic component,
0.05mm thickness of metal electrode part on printed circuit board
(5) Strain suppressor (ceramic sheet) installation status pattern a: No gap (Comparative example: Conventional product)
b: A strain suppressor is installed at a position directly below the electronic component 13 on the substrate surface in a range that does not overlap the metal electrode portion 10a (thickness 40 μm): Pattern c in FIG. 6A: The strain suppressor is mounted on the substrate surface layer 6 (thickness: 40 μm) in a position directly below the electronic component 13 in a range overlapping with the metal electrode portion 10a: pattern shown in FIG.

  As a result of analysis, strong stress concentration occurred in the solder portion (fillet 11) near the lower corner portion of the electronic component in any case. The degree of stress concentration decreased in the order of a → b → c. By correlating the amount of plastic strain (nonlinear strain amplitude) accumulated in the solder at the corner obtained from the analysis results with the actual temperature cycle test results, the number of cycles at which fracture occurs in a similar configuration can be predicted. .

  For example, in the configuration of the comparative example shown in a, the number of rupture cycles at a temperature cycle of 125 ° C. to −55 ° C. was about 480 times. Predicting the number of break cycles in the configuration of the present invention from these data, it was 640 times (about 1.3 times) in the configuration of b and 1740 times (about 3.6 times) in the configuration of c.

  As described above, when the strain suppression body 20 is arranged in the entire lower part of the electronic component 13 (configuration c), the fatigue resistance of the solder is dramatically improved, but it is partially installed compared to the size of the electronic component. A large improvement effect can be obtained just by doing (configuration b), which is often sufficient in practice. From this result, the hardness of the strain suppression body (Young's modulus 10 times higher than that of the substrate) works effectively, and even if the strain suppression body is thin (for example, 40 μm), a great effect of increasing the life can be obtained. And if the strain suppression body with larger thickness is installed, the effect will further increase.

It is a figure which shows a prior art example. It is a figure showing a suitable 1st embodiment of the present invention. It is a figure which shows suitable 2nd Embodiment of this invention. It is a figure which shows the modification of suitable 2nd Embodiment of this invention. It is a figure which shows the modification of suitable 2nd Embodiment of this invention. It is a figure which shows suitable 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Printed wiring board 10a Metal electrode part 11 Solder paste 11 Fillet 13 Electronic component 13a External electrode 15, 20 Strain suppression body 16 1st space | gap part 17 2nd space | gap part 18 3rd space | gap part

Claims (12)

A wiring board having a metal electrode portion on the surface;
Electronic components to be joined on the wiring board by soldering;
With
An electronic component mounting board, wherein a strain suppressing structure for suppressing a difference in expansion and contraction due to a temperature change between the wiring board and the electronic component is provided on a surface and / or an inner surface of the wiring substrate facing the electronic component. .   2. The strain suppression structure is configured by disposing a sheet-like strain suppression body made of a low thermal expansion material lower than a thermal expansion coefficient of the wiring substrate of the wiring substrate on the wiring substrate. Electronic component mounting board described in 1.   The electronic component mounting board according to claim 2, wherein the low thermal expansion material is the same material as a wiring member used for the wiring board. The wiring board is a multilayer board formed by alternately laminating wiring layers and base material layers,
The electronic component mounting board according to claim 1, wherein the strain suppressing body made of the low thermal expansion member is formed in the same layer as the wiring layer.   2. The strain suppression structure is configured by disposing a sheet-like strain suppression body made of a member having a thermal expansion coefficient equivalent to the thermal expansion coefficient of the entire electronic component on the wiring board. Electronic component mounting board described in 1.   2. The electronic component according to claim 1, wherein the strain suppression structure is configured by disposing a sheet-like strain suppression body made of the same material as a member mainly constituting the electronic component on the wiring board. Mounting board.   The electronic component mounting board according to claim 5, wherein the strain suppressing body is disposed in the vicinity of the surface on the electronic component side.   The electronic component mounting board according to claim 2, wherein the strain suppressing body is formed so as to cover a region facing the plurality of electronic components.   The electronic component mounting board according to claim 1, wherein the strain suppressing structure is a gap extending in a thickness direction of the wiring board.   The said space | gap part is a some slit-shaped space | gap, The some slit-shaped space | gap is juxtaposed in the direction orthogonal to the longitudinal direction of the said electronic component. Electronic component mounting board.   The electronic component mounting board according to claim 9 or 10, wherein the gap portion includes a plurality of circular holes.   The said space | gap part is installed in the part limited with respect to the thickness direction, without penetrating in the thickness direction of the said wiring board, The any one of Claim 9 to 11 characterized by the above-mentioned. Electronic component mounting board.

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