PCB Tech - PCB manufacturer: Causes of poor tin on lead-free spray tin

PCB manufacturer: Causes of poor tin on lead-free spray tin

To be honest, in fact, sometimes I don't recommend using tin spray board for SMT processing, especially double-sided SMT processing, because the defect rate is really high and it is not easy to overcome. I believe that most of the friends who play SMT process will first doubt whether it is the poor solder paste printing (the thickness of the stencil, the opening, the pressure... etc.), Solder paste over-aging oxidation, SMD component foot oxidation, or reflow soldering temperature profile is not adjusted properly.

I don’t know if you have thought about it. Most of the reasons for these poor soldering may be the thickness of the sprayed tin plate. The thickness of the gold layer and the nickel layer of ENIG that everyone is familiar with before should be caused by the thickness. The problem of soldering, but if the tin layer of the sprayed tin plate is too thin, will it cause defective soldering? Below are some book data collected for your reference.

Analyze the causes of poor tin-eating after reflow soldering on the second side of the HASL board. The results of metallographic section analysis show that the copper-tin alloying phenomenon has appeared where the solder pads are not eaten, and this kind of copper-tin alloy can no longer provide solderability.

The same batch of unsoldered PCB blanks were inspected and sliced and analyzed, and it was found that the tin plating of the unsoldered PCB pads had serious alloy exposure. The slicing of the blank PCB can also find the formation of copper-tin alloy. The measured thickness is about 2μm. After considering the alloy will increase the thickness, it is speculated that the original spray tin thickness should be less than 2μm, so it is inferred that the root cause is the original spray The thickness of tin is too thin (below 2μm), so that the copper-tin alloying has been exposed to the surface of the solder pad, and there is not enough tin to be combined with the solder paste, which will affect the effect of the solder pad eating tin.

"How to deal with poor tin on one side of double-sided tin spraying board" (brief)

This should be a collated article that gathered opinions from all parties, but in the end it was mostly attributed to the thickness of the sprayed tin being too thin, which resulted in poor soldering.

The spray tin thickness is recommended to be at least 100u" (2.5μm) or more in the large copper area, 200u" (5.0μm) or more in the QFP and peripheral parts, and 450u" (11.4μm) or more in the BGA pad, which can solve the problem. Reflow soldering on both sides will cause solder rejection, but the thicker the spray tin is, the more disadvantageous it is for the parts with fine pins, and the problem of short circuits is likely to be formed.

In the PCB manufacturing process, the smaller the thickness of the solder pad, the thicker it will be, which will easily cause a short circuit to the fine inter-pin parts.

The longer the storage time of the tin-sprayed board, the thicker the thickness of the IMC (Cu6Sn5) will be, and the more unfavorable the subsequent reflow soldering will be. Generally, the flatter the tin of the tin spray board, the thinner the thickness becomes.