JP3963501B2 - Electronic component mounting method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of mounting electronic components on a printed circuit board by using a lead method using lead-free solder .
[0002]
[Prior art]
For mounting electronic components on a blunt circuit board, the flow method (that is, temporarily fixing the electronic parts to the printed circuit board, applying flux, and then contacting (immersing) the soldered portion in the solder bath) And a method of cooling and solidifying the adhered solder).
Conventionally, Sn—Pb eutectic solder has been mainly used as the solder in the above flow method, but Pb is a toxic heavy metal.
In recent years, environmental problems have been taken up on a global scale, and the adverse effects on the ecosystem and contamination of lead have been regarded as problems, and lead-free solder is being studied.
[0003]
[Problems to be solved by the invention]
For example, Sn-7.5Bi-2Ag-0.5Cu, Sn-3.5Ag, Sn-3.4Ag-4.8Bi, etc. are known as lead-free solders used in the above-mentioned flow method. Since it does not contain, the surface tension is large and the melting point is high, so the wetting rate is low under the same soldering temperature (about 240 ° C.) as the flow method using Sn—Pb eutectic solder. Compared to the flow method, it is necessary to increase the soldering temperature or to increase the soldering time (solder contact time or bath immersion time).
However, under such severe flow conditions, there is a concern about the occurrence of cracks and blisters in the resin package of electronic parts due to the heat of the molten solder. As a means for preventing this, it is conceivable to increase the glass transition temperature of the sealing resin material. However, the resin material having a high glass transition temperature increases the moisture absorption rate of the package. Because the above-mentioned blistering is encouraged, it is not an effective means.
[0004]
In flow soldering, even if a soldering point is brought into contact with a solder bath, the soldering point does not get wet until the soldering point is heated above the solidus temperature of the solder. . Thus, in the conventional lead-free solder, the difference between the solidus temperature and the liquidus temperature is small, the solidus temperature is high, and the time t until the soldered portion is heated to the solidus temperature t Since the electronic component is heated only during the time when it is in contact with the solder bath and the electronic component is heated for a long time t, the electronic component must be brought into contact with the solder bath for such a long time. There is a problem that the life of the battery is reduced or broken.
[0005]
Therefore, if the solidus temperature is lowered, the soldering spot of the electronic component is heated to the solidus temperature faster than that, and the initial wetting of the solder is started as soon as that. Thus, the temperature of the solder bath can be set low, and electronic components can be thermally and safely mounted by the flow method.
[0006]
The object of the present invention is to lower the solidus temperature of the solder and start the initial wetting early, thereby shortening the solder bath immersion time of the electronic component, and consequently setting the temperature of the solder bath to be low, It is an object of the present invention to provide a lead-free solder alloy for the flow method that can be thermally and safely mounted.
[0007]
[Means for Solving the Problems]
In the electronic component mounting method according to claim 1, Ag is 0.5 to 1.0% by weight, Bi is 12.0 to 20.0% by weight, Cu is 0.5 to 2.0% by weight, and the balance is Sn. An electronic component is mounted by a flow method using a lead-free solder.
The electronic component mounting method according to claim 2 is such that Ag is 0.5 to 1.0% by weight, Bi is 12.0 to 20.0% by weight, Cu 0.5 to 2.0% by weight, P or Ga is contained. The electronic component is mounted by a flow method using a lead-free solder composed of 0 to 0.5% by weight and the balance being Sn.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
In the lead-free solder alloy used in the present invention, the reason for using Sn as a base material is that it has extremely low toxicity, can impart excellent wettability to a base material, is stable in yield, and is inexpensive.
In the present invention, the reason for adding 0.5 to 1.0% by weight of Ag is that the melting point of the solder is not higher than the melting point of Sn, and Ag ₃ Sn which is an intermetallic compound to be formed is densely dispersed. This is to improve the mechanical strength, particularly the tensile strength (when the Ag amount increases, not only the liquidus temperature becomes too high, but also the crystallization amount of the Ag ₃ Sn intermetallic compound primary crystal increases, the mechanical properties, In particular, the elongation characteristics are lowered and brittle, and since Ag ₃ Sn protrudes from the surface as whisker, there is a possibility of occurrence of a short circuit kit).
[0009]
In the present invention, the reason for adding 12.0 to 20.0% by weight of Bi is to achieve a significant drop in the liquidus temperature of the solder. If it is 20.0% by weight or more, a eutectic point appears in the low temperature part due to a large amount of solid solution with Sn, and the use environment temperature approaches this eutectic point temperature, resulting in coarsening of the structure and deterioration of elongation characteristics. hence, cracking of the soldered joints is concerned.
[0010]
In the present invention, the reason for adding Cu is not only to lower the melting point of the solder but also to further improve the mechanical properties due to a synergistic effect with Ag. The reason why the added amount is 0.3 to 2.0% by weight is that if it is 0.3% by weight or less, it can hardly contribute to the reduction of the melting point and the mechanical strength. This is because not only the phase line temperature becomes too high, but a large amount of Sn—Cu intermetallic compounds are generated, which in turn reduces the mechanical strength.
[0011]
In the present invention, the reason for adding P or Ga is that these elements are preferentially oxidized when the solder is melted to prevent other elements from being oxidized. The reason for this is to eliminate the variation of the composition, and the reason why the additive is made 0.5% by weight or less is that if it is more than this, it becomes expensive, and also weakening of the solder is caused.
[0012]
In the present invention, elements other than those described above may be contained as impurities within the class A range defined in JIS Z-3282. (However, Pb is 0.10% by weight or less)
The liquidus temperature of the lead-free solder according to the present invention is 200 ° C to 220 ° C, the solidus temperature is 140 ° C to 175 ° C, and the solidus temperature is 40 ° C to 60 ° C with respect to the liquidus temperature. Low.
[0013]
In order to mount an electronic component according to the present invention, the solder bath temperature is maintained at about 240 ° C. as in the case of the flow method using conventional eutectic Sn—Pb solder, and the substrate on which the electronic component is temporarily mounted is mounted on the flux bath. And pass through (or dipping) the solder bath.
The soldering part of the board that is in contact with (or immersed in) the solder bath is heated by the solder bath heat, and when the solidus temperature is exceeded, solder wetting and adhesion to the soldering part starts, and the temperature of the soldering part As the temperature rises, alloying at the soldering interface proceeds, and soldering is completed by cooling and solidifying the adhered solder after passing through the solder bath.
[0014]
The solidus temperature is as low as 140 ° C. to 175 ° C., and the soldered portion is heated to this solidus temperature in a sufficiently short time after contact (immersion) of the substrate to the solder bath, and the initial wetting of the solder is accelerated as much Since the solder bath contact (immersion) time of the electronic component can be shortened (when the solidus temperature is high under the same solder bath temperature, the heating to the solidus temperature of the soldering point is much longer. Time is required and the time that does not contribute to solder wetting becomes longer during the solder bath time, so it is necessary to increase the solder bath time accordingly), and in turn, the temperature of the solder bath can be set low, and electronic components Can be mounted thermally and safely by the flow method.
[0015]
The above-mentioned solder baths for the flow method include a flow method, a wave method, a two-stage wave method, a flow dip method, a multi-stage flow method, a cascade method, etc. (A method in which a soldering member is brought into contact with the jet solder) or a vertical bath, a pie method, a flat-bip method, etc. A method of immersing the attachment member) can be used.
[0016]
【Example】
[Examples 1 to 5]
Lead-free solder having the composition shown in Table 1 was prepared.
About each Example goods, it was as Table 1 when the solidus temperature and the liquidus temperature were measured.
The zero cross time and the maximum wetting force measured by the surface tension method at the same molten solder temperature of 240 ° C. as in the flow method using eutectic Sn—Pb solder were as shown in Table 1. When one end of the piece is immersed, a force f that is pulled down by the adhesion tension of the solder and a force f ′ that is pulled up by the buoyancy act on the test piece, and the wetting force (f′−f) is determined by the contact angle θ. It fluctuates in accordance with the change with time, is negative between time 0 and t, increases positively after elapse of time, and reaches a saturation value. t is the zero crossing time, which is the time when the contact angle θ is approximately 90 °]. However, the test piece used was a 30 × 30 × 0.3 mm phosphorous deoxidized copper plate coated with a resin flux, the immersion speed was 2 mm / second, the immersion depth was 4 mm, and the immersion time was 10 seconds. did.
[0017]
Table 1
Example 1 Example 2 Example 3 Example 4
Ag (% by weight) 0.5 0.5 0.5 1.0
Bi (% by weight) 12.0 15.0 20.0 12.0
Cu (% by weight) 0.5 0.5 0.5 0.5
Sn (wt%) Remainder Remainder Remainder Remainder Solidus temperature (° C) 170 162 145 169
Liquidus temperature (° C.) 215 212 205 213
Zero cross time (seconds) 1.2 1.1 1.0 0.9
Maximum wettability
(dyne / cm) 268 265 264 272
[0018]
Example 5
Ag (% by weight) 1.0
Bi (% by weight) 15.0
Cu (% by weight) 0.5
Sn (wt%) remainder Solidus temperature (° C) 158
Liquidus temperature (° C) 209
Zero cross time (seconds) 0.8
Maximum wettability
(dyne / cm) 272
[0019]
[Comparative Examples 1-3]
Lead-free solders having the compositions shown in Table 2 were prepared. As in the examples, the solidus temperature, liquidus temperature, zero cross time, and maximum wetting force were measured and as shown in Table 2.
[0020]
Table 2
Comparative Example 1 Comparative Example 2 Comparative Example 3
Ag (% by weight) 2.0 3.5 3.4
Bi (% by weight) 7.5 0 4.8
Cu (% by weight) 0.5 0 0
Sn (% by weight) Remainder Remainder Remainder Solidus temperature (° C.) 185 221 209
Liquidus temperature (° C.) 215 221 222
Zero cross time (seconds) 1.5 2.7 2.0
Maximum wettability
(dyne / cm) 280 300 265
[0021]
Further, for each of the examples, P was added at 100 ppm, or Ga was added at 0.3 wt%, and the zero cross time and the maximum wetting force were measured. The same results were obtained, and the flow method was used. When solder was used, the generation of oxides was extremely light, so that the effect of suppressing oxidation by addition of P and Ga could be confirmed.
[0022]
【The invention's effect】
In the flow mounting method of the electronic component according to the present invention, since lead-free solder having a solidus wire is used in a low temperature range of 140 ° C. to 175 ° C., contact or immersion with molten solder during flow soldering The soldering part can be wetted with solder from the initial stage of the solder, and the flow soldering can be performed under mild heating conditions as compared with the conventional lead-free solder alloy having a solidus wire in a high temperature range, and the liquidus line is higher than the Sn-Pb eutectic solder. Despite the high temperature, it is possible to perform flow soldering with lead-free solder under the conventional flow soldering temperature (240 ° C.).

Claims (2)

Flow method of electronic parts using lead-free solder consisting of Ag 0.5-1.0 wt%, Bi 12.0-20.0 wt%, Cu 0.5-2.0 wt%, the balance Sn An electronic component mounting method, characterized in that the electronic component is mounted by the method described above. Ag is 0.5 to 1.0 wt%, Bi is 12.0 to 20.0 wt%, Cu is 0.5 to 2.0 wt%, P or Ga is 0 to 0.5 wt%, and the balance is Sn An electronic component mounting method comprising mounting the electronic component by a flow method using lead-free solder.