PCB Tech - Pollution of lead-free wave soldering of circuit boards

Pollution of lead-free wave soldering of circuit boards

1. Lead pollution causes solder joints to float
Since the CTE of the PCB in the Z direction is about 55-60ppm/ degree Celsius, plus the less soft solder of SAC305, the CTE is only about 22ppm/ degree Celsius; once the wave soldering, the IMC between the through-hole ring surface and the solder, When the growth is poor due to the hindrance of a small amount of lead, cracking of the solder joint from the ring surface often occurs. When the IMC grows well and is strong, tearing of the solder joint itself may also occur. From the statistics of a large number of solder ring floating cracks, it is found that small holes and small rings (below 14mil) have fewer cracks. Of course, this is due to the amount of solder in the hole, which is the difference effect caused by different heat.

In the lead-free wave soldering or reflow soldering of the circuit board, if the solderable film of the part feet in some solder joints is still using tin-lead or lead-tin-lead-silver treatment layer (Sn36Pb2Ag, 177 degree Celsius) during the transition period, then During the solidification process to form solder joints, a small amount of lead will be squeezed out and moved to the final cooling point of the PCB copper pad. Due to the obstruction of lead, the necessary benign IMC (C u6Sn5) cannot be smoothly produced during soldering, and a ternary alloy with a melting point of Sn/Pb/Ag of 179 degree Celsius is further formed, and the strength is greatly reduced. And often due to the shrinkage of the through-hole ring surface solder joints, the tapered weld body produces Surface Cracks, and even the copper ring will lift from the substrate. With a small amount of lead contamination, cracking of the solder joint and floating of the copper ring are almost inevitable, and the chance of this is more than that of the above-mentioned shrinkage in the Z direction of the sheet and the out of sync with the solder.

At this time, microsection method can be used to further confirm the failure mode (Failure Mode), or the "Differential Scanning Calorimetry; DSC) can be used to measure the melting point of each local solder joint of SAC305? Once the local mP. is lower than 210~C, it can be confirmed that it is affected by a small amount of lead or bismuth, whose unfavorable properties lower the melting point. It is the "tin-zinc-bismuth" low-temperature solder favored by Japanese customers, and it is almost certain Floating cracks will occur.

Another important reason for solder joint cracking is that a small amount of lead in the local area of the solder joint may become the second largest group, and form a localized Sn36Pb2 Ag ternary alloy with tin and silver in 305. The eutectic point (EuteCtiCmp) is only 177°C, which becomes the final solidified area of the weld body, and often becomes a sensitive spot for cracking under insufficient strength. Therefore, it is known that the solder joints are composed of a large amount of lead and tin, and the uniformity and strength of the material are indeed the contribution of lead; however, once the lead becomes a trace amount of pollution, the strength is insufficient due to uneven materials., Engineers must fill in.

2. Bismuth pollution
The possible source of bismuth contamination is the use of Sn8Zn3Bi (mp191-195°C; Japanese household appliances often specify it for use, such as NEC). This wave soldering (or reflow) solder has a low melting point and is cheap, and it can also reduce the Zn content. The tendency to rust in moisture. Another type of solder, SnAgBi (mp215 degree Celsius is also used by the industry, but it is more brittle and prone to whiskers. The source of other bismuth may be the solderable film of lead electroplated tin-bismuth alloy, or another eutectic can be used Alloy 42Sn58Bi (m.P138°C) is a film processed by hot dipping. This film has poor ductility but high brittleness, and is also prone to cracking during subsequent bending. Because the solder contains bismuth, it will move to the copper surface at high temperatures., Causing the trouble of easy cracking in the follow-up, so I had to change the matting surface to ENIG treatment, but it is easy to cause the problem of black matting. What is the cause of the loss?

Once it is suspected that the insufficient solder joint strength may be caused by the lesions of the low melting point alloy, the thermal scan card meter (DCS) method can be used to find the melting point of the solder under the sudden change of heat flow during the heating.

3. Copper pollution in tin ponds
The original copper content in SAC305 or SAC3807 solder is 0.5% and 0.7% bywt, respectively. The copper on the board surface is bound to dissolve into the pool continuously during continuous wave soldering operations. The general experience is that the overall melting point (mp) will also increase after the copper content rises. However, under the set operating welding temperature (260-265°C) and travel speed (for example, 1.0-1.2 m/min), it is certainly not possible in mass production. Any changes will dance. As a result, the drop between the soldering temperature and the melting point becomes smaller (that is, the size of the operating range) and the viscosity increases. As a result, the bridges and short circuits between the tightly spaced lines on the PCB board surface, of course, gradually increase as in response.

And once the copper content exceeds the safe upper limit (0.9% bywt), CuSn, hexagonal needle-like crystals will also be formed in the pool. The needle-shaped IMC has a melting point of 415°C and a specific gravity of 8.28. Therefore, in a SAC305 pool with a specific gravity of 7.44, it will of course become a sinking mud when standing still, so it can be removed. The correct way for the production line is that when the amount of copper increases from 0.5% or 0.7% of the original formula, the added solder should be changed to copper-free SAC300 (the unit price is the same), which is only the addition of tin and silver alloy auxiliary materials, Of course, it can be used to dilute the rise of copper in liquid tin. However, once the CuSn has been formed, the needle-shaped IMC will no longer be able to melt back. It can only be removed from the bottom of the pool after cooling (235 degree Celsius) and standing (2 hours). This is also the best method at present. NS. Otherwise, the fluidity of liquid tin will inevitably deteriorate and it is easy to short-circuit, and the solder joints on the board will inevitably have a needle-like appearance. The mass production line should analyze the copper content every two weeks to be more assured.

Fortunately, the rising star of lead-free solder SCN tin-copper-nickel (such as Nissho NS product SN100C), the degree of melting copper on the board surface is much less than that of SAC alloy, but it cannot exceed 0.9% (the original formula is 0.7%), Otherwise there will be problems with the strength of the solder joints. This kind of SCN not only dissolves copper slowly, but also has a lower price. The solder joint appearance is also much more beautiful than SAC. The disadvantage is that the melting point is slightly higher (227 degree Celsius, but fortunately, the soldering temperature can reach 265-270 degree Celsius, which can be mass-produced). The business is only limited by the patent of the Japanese business NS and cannot be selected.

4. Iron pollution.
When the wave solder pool is constructed of stainless steel, the iron component in it will be attacked by the tin in the long-term high temperature of the liquid SAC to form FeSn, needle-like IMC, and gradually dissolve into the tin pool, resulting in a tin pool pump Damage to important components in Puzhong. The most thorough solution is to change all the tin bath and accessories to titanium alloys to build them, so as to avoid the trouble once and for all. Once the iron contamination of the liquid solder in the tin pool exceeds 0.02% bywt (200ppm), the solder joints will have a sandy appearance.
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