JP2001293559A - Soldering method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively mass-produce a printed circuit board without generating defective soldering and with the high reliability of its soldered parts by the use of Sn-Zn solder with which flow soldering has been difficult to perform up to now, by establishing a soldering method with which the flow soldering of the printed circuit board can be performed. SOLUTION: The flow soldering of the printed circuit board 1 is performed by using the Sn-Zn-based solder in an atmosphere of inert gas. An oxygen content at this time is 100 ppm or below in a preheating process 17 and is 500 ppm or below in a soldering process. A temperature control is added to obtain satisfactory wettability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a work to be soldered by bringing a plate-like work to be soldered, such as a printed wiring board on which electronic components are mounted, into contact with a Sn-Zn-based molten solder. The present invention relates to a soldering method for soldering a portion to be soldered.

[0002]

2. Description of the Related Art From printed wiring boards used in discarded electronic equipment, lead (Pb) is dissolved by accelerated acid rain and the like, contaminating groundwater and the like, and the toxicity may affect the human body. It is a problem. Therefore, Sn-Pb, which has been conventionally used for soldering printed wiring boards,
A lead-free solder that does not use lead instead of the (tin-lead) -based solder and a soldering technique using the lead-free solder are being developed.

[0003] Solders that are considered to be promising as lead-free solders include Sn-Ag-Cu (tin-silver-copper) solder and S-Ag-Cu solder.
n-Ag-Bi (tin-silver-bismuth) solder, Sn-
This is a Cu (tin-copper) solder. However, these solders have a problem in that a lift-off phenomenon occurs and soldering failure is likely to occur. Also, melting point (about 210 ° C to 220 ° C)
Therefore, it is necessary to perform soldering at a soldering temperature of about 250 ° C. higher than that of the conventional Sn-Pb-based solder, and the printed wiring board and the electronic components mounted thereon are more thermally stressed than before. There is a problem giving. Furthermore, these solders are generally expensive.

On the other hand, Sn-Zn based solder has a melting point (1
(Approximately 90 ° C. to 200 ° C.), low lift-off phenomenon, high soldering strength, and low cost. (For example, “Soldering materials and processes for full-scale adoption of lead-free solder”
(Refer to Table 2 (p.45 of "Electronic Packaging Technology 1999 Special Issue", pp. 44-53, Technical Research Committee, Inc.))

[0005]

There is an example in which Sn-Zn based solder is used in a reflow soldering method. For example, “Sn—Zn-based lead-free solder paste” (“Electronic Packaging Technology 1999 Special Issue” No. 7
Pages 4 to 77 (Technical Research Committee, Inc.).

However, since Zn forms an active and stable oxide film, the solder wettability of copper (Cu) such as a land to be soldered on a printed wiring board tends to deteriorate.
When mass-producing printed wiring boards, there is no case where flow soldering is performed using Sn—Zn-based solder, and its use has been abandoned.

An object of the present invention is to establish a soldering method capable of performing a flow soldering of a printed wiring board by using an Sn—Zn-based solder, which has been conventionally difficult to flow. Another object of the present invention is to make it possible to mass-produce printed wiring boards at low cost without causing soldering defects and having high reliability of the soldered portions.

[0008]

A feature of the soldering method of the present invention is that each process condition of a soldering process that can obtain good solder wettability using a Sn—Zn-based solder is defined. is there.

(1) A work to be soldered is heated in advance, and thereafter, a jet wave of a molten solder of a Sn—Zn-based solder (tin-zinc-based solder) containing 3 to 12% by weight of Zn (zinc) is applied. This is a soldering method in which the Sn—Zn-based solder is supplied to a soldered portion of the work to be soldered by being brought into contact with each other and soldering is performed under the following conditions.

That is, the work to be soldered is preliminarily heated in an inert gas atmosphere having an oxygen concentration of 1000 ppm or less. Subsequently, the work to be soldered and the Sn- are heated in an inert gas atmosphere having an oxygen concentration of 500 ppm or less. Zn
The molten solder of the system solder is brought into contact with the jet wave.

As a result, oxidation of the soldered portion of the printed wiring board, which is a copper land, is suppressed even in the preheating step, so that the solder wettability of the Sn—Zn-based solder is improved in each step, and the soldering step in the soldering step is improved. The occurrence of poor wetting can be prevented. (2) In the soldering method of (1), the S
The temperature of the molten solder of the n-Zn-based solder is set to 210 ° C to 25 ° C.
Keep at 0 ° C.

That is, when the temperature of the molten solder is 210 ° C. or higher, poor wetting can be eliminated and good wettability can be obtained.

(3) In the soldering method of the above (1) or (2), the work to be soldered is
Heat to <RTIgt;

That is, the temperature of the work to be soldered (printed wiring board), and thus the temperature of the soldered portion is set to 80 ° C.
As described above, non-wetting can be prevented, and good soldering can be performed. (4) In the soldering method of (1) to (3), the jet wave of the molten solder of the Sn—Zn-based solder is brought into contact with the work to be soldered. Immediately after soldering, an oxygen shielding agent for shielding oxygen in the atmosphere is applied to the soldered portion of the work to be soldered.

That is, by applying the oxygen shielding agent, the oxidation of the soldered portion can be prevented, and the state of the soldered portion can be stably maintained.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The soldering method according to the present invention can be carried out in the following embodiments. (1) Configuration An example of an embodiment of a soldering method according to the present invention will be described with reference to FIG.

FIG. 1 is a longitudinal sectional view of a soldering apparatus for explaining an embodiment of a soldering method according to the present invention. The N ₂ gas supply system is shown in a symbol diagram.

That is, the transport conveyor for transporting the printed wiring board 1, which is a work to be soldered, on which a large number of electronic components (not shown) are mounted, includes the first transport conveyor 2 for elevation transport (elevation angle θ _l ) and the depression transport. (A depression angle θ ₂ ), and a tunnel-shaped chamber 4 is provided so as to cover these conveyors 2 and 3. As shown in FIG. 1, the vertical section of the tunnel-shaped chamber 4 is formed in the shape of an "H", so that the height of the carry-in port 5 and the height of the carry-out port 6 are the same from the horizontal plane. It is configured in. Thus, the height of the loading port 5 and the loading port 6
In this case, the soldering device can be easily connected to another device and used in-line.

The first and second conveyors 2, 3 are:
Although not shown, the printed wiring board 1 is provided with holding claws for holding both end portions, and comprises a conveyor frame provided on both end portions and formed in two parallel lines. Usually, one conveyor frame is configured to move and adjust in the width direction of the printed wiring board 1 so as to hold the printed wiring boards 1 having different widths. Arrow A in the figure indicates the direction in which the printed wiring board 1 is transported.

A pre-heater 7 constituting a pre-heating step 17 of the printed wiring board 1 and a solder tank 8 constituting a soldering step 18 are arranged in the tunnel-shaped chamber 4 along the first conveyor 2. I have.

The preheater 7 in the preheating step 17 preheats the printed wiring board 1 to which the flux has been applied in advance, and pre-activates the flux and gives it to the printed wiring board 1 and mounted electronic components (not shown). It is provided to reduce heat shock.

The solder bath 8 in the soldering step 18 is heated by a heater (not shown) to melt the Sn-
A Zn-based solder (Sn-9Zn solder) is housed as a molten solder 9, and the molten solder 9 is supplied to the first pump 1.
0 to the first jet body 12 and the first jet wave 1
Form 3 Further, the second jet wave 15 is sent out to the second outlet body 14 by the second pump 11 to form a second jet wave 15. Then, by bringing these jet waves 13, 15 into contact with the lower surface of the printed wiring board 1, that is, the surface to be soldered where the portion to be soldered is present, the molten solder 9 is supplied to the portion to be soldered. Perform soldering.

The preheater 7 is provided in the tunnel-shaped chamber 4. However, the solder bath 8 is configured such that the opening 4a is provided in the tunnel-shaped chamber 4, and the first jet wave 13 and the second jet wave 15 are located in the tunnel-shaped chamber 4 from the opening 4a. . In order to maintain the sealing of the tunnel-shaped chamber 4, a skirt 4b is provided in the opening 4a provided in the tunnel-shaped chamber 4, and the skirt 4b is immersed in the molten solder 9 of the solder bath 8 to complete the sealing. Has been realized.

In the tunnel-shaped chamber 4, a number of plate-shaped members, that is, suppression plates 16 are provided in the longitudinal direction of the tunnel, that is, along the transport direction A of the transport conveyors 2 and 3. The restraining plate 16 is provided so that its plate surface is orthogonal to the transport direction A of the transport conveyors 2 and 3. That is, a labyrinth flow path is formed in the tunnel-shaped chamber 4 by the suppression plate 16 so that unnecessary atmosphere flow does not occur in the tunnel-shaped chamber 4.

The restraining plate 16 is provided downward from the upper wall of the tunnel-shaped chamber 4 toward the conveyors 2 and 3, and is also provided from the lower wall of the tunnel-shaped chamber 4 toward the conveyors 2 and 3. It is provided upward.

A nozzle 20 for supplying N ₂ gas, which is an inert gas, into the tunnel-shaped chamber 4 is provided between the restraining plates 16 on the rear side of the solder bath 8 when viewed from the transport direction A, and a flow control valve 21 is provided. And the target N ₂ by the flow meter 22
It is configured so that it can be adjusted to the gas supply flow rate. The N ₂ gas is supplied from a cylinder or a PSA type N ₂ gas supply device 23, and is supplied to the flow rate control valve via an on-off valve 24, a filter 25 for removing impurities, and a pressure control valve 26 for adjusting a target supply pressure. 21. The pressure gauge 27 is for pressure monitoring.

The N ₂ gas supply flow rate is determined by measuring the oxygen concentration in the tunnel-shaped chamber 4 with an oxygen concentration meter (not shown).
For example, the jet wave 1 of the printed wiring board 1 and the molten solder 9
The atmosphere in the soldering step 18, which is the area where the layers 3 and 15 come into contact with each other, is sampled and measured, and the flow control valve 21 is adjusted and set so that the target oxygen concentration is obtained.

Further, if necessary, as shown by a broken line, a nozzle 31 for similarly supplying N ₂ gas is provided near the preheater 7 in the preheating step 17, and the atmosphere near the preheater 7 is provided. May be configured to measure the oxygen concentration of the sample with an oximeter. In addition, 32 is a flow meter, and 33 is a flow control valve.

Although not shown, if the atmosphere and the N ₂ gas are mixed in advance and the atmosphere having the desired oxygen concentration is supplied to each of the nozzles 20 and 31, the oxygen concentration becomes several thousand pp.
An N ₂ gas atmosphere of about m can be easily formed. (2) Operation The printed wiring board 1 on which a flux is applied in advance to a lower surface having a soldered portion, that is, a soldered surface,
When carried in from the carry-in entrance 5 of the soldering apparatus shown in FIG.
Although not shown held both side edge portions in the first holding pawl of the conveyor 2 of, it is conveyed in the arrow A direction by conveyance elevation theta _l.

Then, the preheater 7 preheats the portion to be soldered to, for example, about 100 ° C. Then, the lower surface of the printed wiring board 1, that is, the surface to be soldered is heated to about 240 ° C. First jet wave 1
3 and the second jet wave 15, and the molten solder 9 is supplied to the portion to be soldered to perform soldering.

Thereafter, the printed wiring board 1 is transferred to the second conveyor 3 at the top of the tunnel-shaped chamber 4, conveyed at a conveyance depression angle θ ₂ , carried out of the exit 6 and the soldering is completed.

This series of soldering steps 18 is performed in a low oxygen concentration N ₂ gas atmosphere. That is, the N ₂ gas supplied from the supply nozzle 20, 31 a N ₂ gas, the tunnel-shaped chamber 4 is N ₂ gas atmosphere of low oxygen concentration.

[0033]

EXAMPLES (1) Relationship between oxygen concentration and the number of non-wettings The present inventors have found that when the oxygen concentration in the soldering step 18 falls below a certain value, the oxidation of the Sn-Zn-based solder is rapidly suppressed. In addition, it has been found that the solder wettability of the printed wiring board 1, which is the portion to be soldered, to the copper land is significantly improved.

Similarly, when the oxygen concentration in the preheating step 17 is equal to or lower than a predetermined value, oxidation is suppressed even during preheating of the soldered portion of the printed wiring board 1 which is a copper land, It has been found that the wettability of the Sn—Zn-based solder is significantly improved.

With reference to FIG. 3 and FIG. 4, an explanation will be given on the results of an experiment in which the number of solder non-wettings is obtained using a test printed wiring board having 790 lands to be soldered on which surface-mounted components are mounted. FIG. 2 will be described later.

FIGS. 3 and 4 both show the transport elevation angle θ _l =
5 °, transport speed V _A = 0.8 m / min, solder temperature T
FIG. 3 shows the number of non-wetting at _H = 240 ° C. and the preheating temperature T _P = 110 ° C. FIG. 3 is a diagram showing the number of non-wetting with respect to the oxygen concentration in the soldering step, and FIG. It is a figure which shows the number of non-wetting with respect to oxygen concentration.

As is apparent from FIG. 3, when the oxygen concentration in the preheating step 17 is 1000 ppm and the oxygen concentration in the soldering step 18 is 500 ppm or less, poor wetting (the number of non-wetting) becomes zero. As is clear from FIG. 4, the oxygen concentration in the soldering step 18 was 500 ppm.
In the preheating step 17, the oxygen concentration is 100
It is understood that poor wetting becomes 0 at 0 ppm or less.

Further, the amount of dross (mainly zinc oxide) generated in the solder bath 8 is reduced to 1/5 or less of the amount generated in the atmosphere by setting the oxygen concentration to 500 ppm in the soldering step 18. , First and second jet waves 1
The flow of 3, 15 is not obstructed.

That is, in the atmosphere, zinc oxide is generated abundantly, and the molten solder 9 having the first and second jet waves 13, 15 formed by forming a hard zinc oxide layer on the surface of the solder bath 8, It will be an obstacle to reflux inside 8.
Then, the wave heights of the first and second jet waves 13 and 15 fluctuate unstablely, and solder balls are scattered and adhere to the printed wiring board 1, so that the soldering quality of the printed wiring board 1 is reduced. Will drop.

The Sn-Zn based solder used in the experiment was Sn-9Zn solder, but Sn-9Zn-5I
Similar results were obtained for n solder. However, when extremely fine so-called micro-soldering in which the size of the soldering land is 1 mm □ or less is performed, the oxygen concentration is desirably 500 pp in the preheating step 17.
It was also found that the number of non-wetting can be reduced to 0 when the amount is not more than m and 100 ppm or less in the soldering step 18. This is because, in particular, by further reducing the oxygen concentration in the soldering step 18, the flowability of the molten solder increases, and the molten solder easily flows into extremely fine soldered portions. (2) Relationship between Temperature of Molten Solder and Number of Unwet As described above, the present inventor has conducted a preliminary heating step 17.
In an inert gas atmosphere having a low oxygen concentration (1000 ppm or less) and an oxygen concentration (500 ppm or less) in the soldering step 18, continuous soldering of the printed wiring board 1 is performed using Sn-Zn-based solder. I found that I could do it.

Further, the present inventor has found that a sufficient solder wettability can be obtained with the Sn-Zn-based solder even at a temperature lower than the conventional Sn-Pb-based solder at a temperature close to the solidus temperature. Was. Moreover, continuous production of the printed wiring board 1 is enabled. Thereby, the thermal stress applied to the printed wiring board 1 and the electronic components mounted thereon can be significantly reduced.

FIG. 5 shows non-wetting of molten solder against temperature.
It is a figure showing the relation with a number. Note that the trial for obtaining FIG.
Printed circuit board 1 used for the test has surface mounted components
Test print with 790 soldered lands
This is the wiring board 1. In this case, the printed wiring board 1
Transfer speed V_A = 0.8 m / mim and the transport elevation angle θ ₁ 
= 5 °, preheating temperature T_P = 110 ° C, preheating
Oxygen concentration D in step 17 _P = 1000ppm, soldering
Oxygen concentration D in step 18_H = 500 ppm.

As is clear from FIG. 5, the molten solder 9
It can be seen that when the temperature is 210 ° C. or more, poor wetting is eliminated and good wettability can be obtained. On the other hand, the conventional S
In the case of n-Pb solder, the temperature of the molten solder 9 is set to 240 ° C.
It is known that poor wetting easily occurs unless the temperature is set to about 250 ° C. However, in the case of Sn-Zn based solder, 21
Good soldering can be performed at a temperature lower than the conventional temperature of 0 ° C to 240 ° C. (3) Relationship between preheating temperature and number of non-wettings The present inventor has determined that, in addition to the oxygen concentration condition of (1) and the temperature condition of molten solder of (2), the soldered portion of the printed wiring board 1 That is, the temperature of the soldering land of the printed wiring board 1 and the temperature of the soldered portion of the mounted electronic component, that is, the preheating temperature of these soldered portions, are important in reducing the number of non-wetting. I found something.

FIG. 6 is a diagram showing the relationship between the preheating temperature of the printed wiring board and the number of non-wetting. FIG. 6 uses the same printed wiring board 1 as in FIG. 5, and the transport elevation angle θ _l of this printed wiring board 1 is 5 ° and the transport speed V _{A is} 0.8 m / mi.
n, oxygen concentration D _P = 1000 pp in preheating step 17
m, oxygen concentration D _H = 500 pp in soldering process 18
m, the data when the temperature T _H of the molten solder 9 is 240 ° C.

As can be seen from the results shown in FIG. 6, when the temperature of the printed wiring board 1 and thus the temperature of the portion to be soldered is set to 80 ° C. or higher, the number of non-wetting is reduced to zero, and good soldering can be performed. . The preheating temperature is 150
Exceeding ° C is not preferable because the thermal stress applied to the printed wiring board 1 and the electronic components mounted on the printed wiring board 1 increases.

As described above, the Sn-Zn based solder is characterized in that it has good wettability even at a temperature close to the melting point (210 ° C. to 240 ° C.). Also, the correlation with the preheating temperature is low. Therefore, even if the temperature of the molten solder 9 is changed, good wettability can be obtained by setting the preheating temperature of the printed wiring board 1 in the range of 80 ° C to 120 ° C. (4) Application of Oxygen Shielding Agent With reference to FIG. 2, a method of applying an oxygen shielding agent to a soldered portion of a printed wiring board will be described.

FIG. 2 is a sectional view of a printed wiring board to which an oxygen shielding agent has been applied. That is, a printed wiring board in which an oxygen shielding agent is applied to a surface to be soldered of a printed wiring board to which soldering has been performed using an Sn—Zn-based solder is shown.

In FIG. 2, the chip-type electronic component 36 and the lead-type electronic component 37 are soldered to the printed wiring board 1, and an oxygen shielding agent 34 is applied so as to cover the soldered portion 35.

The application of the oxygen shielding agent 34 can be performed by a general coating technique such as brush coating, spray coating, or application in contact with a liquid flow wave. The better the result, the sooner it is applied after the molten solder is supplied to the soldered portion 35, the better the result will be obtained. Further, as the oxygen shielding agent 34, a resin material can be used, and an ultraviolet curable resin, a silicon varnish, and the like are known.

By applying the oxygen shielding agent 34 to the surface to be soldered (at least the soldered portion 35) of the printed wiring board 1 to which the molten solder has been supplied to the soldered portion 35, The oxidation of the Sn—Zn-based solder supplied to the attachment portion 35 can be prevented. That is, oxidation of Zn (zinc), which is easily oxidized, can be prevented, and a stable portion to be soldered over time can be maintained. Also, a Sn-Zn-based solder easily generates a whis force. Wishing force is a cause of short-circuit failure in a high-density mounting printed wiring board in which the space between adjacent soldered portions 35 is narrow, and therefore it is necessary to prevent the occurrence thereof. Then, the application of the oxygen shielding agent 34 can suppress the growth of the wiping force.

The whis force is easily generated in a lead-free solder having a large Sn composition ratio. For other lead-free solders, for example, Sn-Ag-Cu (tin-silver-copper) -based solder, the application of the oxygen shielding agent 34 is effective.

[0052]

According to the soldering method of the present invention, the flow of a printed wiring board (work to be soldered) using an Sn-Zn-based solder, which has conventionally been determined to be unsuitable for flow soldering, is described. Soldering can be performed continuously. In addition, it is possible to perform soldering at a lower soldering temperature than the conventional Sn-Pn solder, and it is possible to reduce the thermal stress on the printed wiring board and the electronic components mounted thereon as compared with the conventional case. Also, there is no occurrence of a lift-off phenomenon in the soldered portion.

Therefore, a printed wiring board having high soldering quality and high reliability can be manufactured at low cost, and electronic equipment, which is an essential infrastructure for daily life, can be obtained at low cost and used with confidence. Will be able to

Immediately after soldering, by applying an oxygen shielding agent to the soldered portion of the printed wiring board,
Oxidation of the portion to be soldered is prevented, the portion to be soldered stable over time is maintained, and the growth of the whis force can be suppressed. That is, a highly reliable printed wiring board can be manufactured.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a soldering apparatus for explaining an embodiment of a soldering method according to the present invention.

FIG. 2 is a longitudinal sectional view of a printed wiring board for explaining a method of applying an oxygen shielding agent in the soldering method of the present invention.

FIG. 3 is a diagram showing the number of non-wettings with respect to the oxygen concentration in a soldering step.

FIG. 4 is a diagram showing the number of non-wettings with respect to the oxygen concentration in a preheating step.

FIG. 5 is a diagram showing the relationship between the temperature of molten solder and the number of non-wettings.

FIG. 6 is a diagram illustrating a relationship between a preheating temperature of a printed wiring board and the number of non-wettings.

[Explanation of symbols]

Reference Signs List 1 work to be soldered (printed wiring board) 2 first transport conveyor 3 second transport conveyor 4 tunnel-shaped chamber 4a opening 4b skirt 5 carry-in 6 carry-out 7 preheater 8 solder layer 9 molten solder 10 first pump 11 2nd pump 12 1st blow-off body 13 1st jet wave 14 2nd blow-off body 15 2nd jet wave 16 Suppression board 17 Preheating step 18 Soldering step 20 Nozzle (gas supply port) 21 Flow control valve 22 Flow meter 23 N ₂ gas supply device 24 On-off valve 25 Filter 26 Pressure control valve 27 Pressure gauge 31 Nozzle 32 Flow meter 33 Flow control valve 34 Oxygen shielding agent 35 Solderable part 36 Chip type electronic component 37 Lead type Electronic components

Continuation of the front page (72) Inventor Shoichi Moriya 2-27-1, Shimomaruko, Ota-ku, Tokyo F-term (reference) in Japan Electric Heating Instruments Co., Ltd. 4E080 AA01 AB03 BA07 BA08 CB04 CB10 DA04 5E319 AA01 AC01 BB01 CC25 CC28 CC58 CD28 CD31 CD35 CD60 GG03 GG05 GG11

Claims (4)

[Claims] 1. A work to be soldered which has been heated in advance and is then brought into contact with a jet wave of a molten solder of a Sn—Zn-based solder containing 3 to 12% by weight of zinc to be soldered to the work to be soldered. In the soldering method in which the Sn—Zn-based solder is supplied to a soldering portion and soldering is performed, the work to be soldered is heated in advance in an inert gas atmosphere having an oxygen concentration of 1000 ppm or less, and subsequently, the oxygen concentration is 500 ppm. A soldering method, comprising: bringing the work to be soldered into contact with a jet wave of a molten solder of the Sn-Zn-based solder in an inert gas atmosphere described below. 2. The soldering method according to claim 1, wherein the temperature of the molten solder of the Sn—Zn-based solder is set at 210 ° C. to 250 ° C. to perform the soldering. 3. The method according to claim 1, wherein the work to be soldered is previously heated to 80 ° C.
3. The soldering method according to claim 1, wherein the soldering is performed at 150 [deg.] C. 4. The method according to claim 1, wherein the work to be soldered is Sn-Z.
The oxygen shielding agent for shielding oxygen in the atmosphere is applied to a portion to be soldered of the work to be soldered immediately after the jet wave of the molten solder of the n-type solder is brought into contact. 3. The soldering method according to any one of 3.