PCB manufacturer's advice on controlling tin whisker growth

The pure tin solution still has no results at present. The more effective methods include annealing after electroplating to make the tin layer grain larger, Cu-based tin plating layer using Ni base, optimizing the plating solution formulation and tin-free plating to make tin whisker decrease. At present, the most common method for inhibiting the growth of tin whiskers by PCB manufacturers is to plate the Sn layer with Ni and heat it at 150 ° C for 1~2 h for annealing to maintain the proper size of the tin grains to reduce tin fluidity in the grain boundaries of the material.

1, Nickel barrier layer whether it is pure tin or tin alloy coating, nickel should be plated with a thickness of 1um or thicker as a barrier layer before plating to reduce the diffusion of Cu and Sn on the substrate.

2. Increasing the thickness of the tin plating layer is mainly for the tin plating layer of the component electrode. Because the component pins are not like the PCB pad, the soldering surface cannot ensure that the surface of the pin is covered by the non-pure tin solder with less tendency to grow tin whisker. Therefore, it must have its own ability to inhibit whisker growth.

Tin whisker growth on copper substrate research results: when the thickness of tin plating is less than 3.5um, the length of tin whisker exposed to air for 50 days exceeds 120um. When the thickness of tin plating is greater than 5.35um, it is exposed to air for 75 days tin whisker growth length is maintained at 40um; when the tin plating thickness is greater than 10.1um, exposed to the air for 80 days before tin whisker produced, exposed to air for 750 days tin whisker growth length is maintained within 10um. Therefore, the PCB manufacturer increases the thickness of the tin-plated layer to help release the stress of the tin layer. As long as the thickness of the tin-plated layer is greater than 10 μm, the risk of tin whisker can be greatly reduced, which is reason it is recommended that the thickness of the tin-plated layer should be greater than 10 μm.

3, Annealing treatment

Tin whiskers are usually produced after thousands of hours of incubation after plating. The length of this incubation period depends on the thickness of the tin plating layer, the lattice structure of the tin plating layer, and the lattice structure of the base metal. According to the PCB manufacturer's practice, the tin alloy plating product is annealed, that is, 24H after the completion of the plating.Heat treatment was performed at a temperature of 150 ° C for 1 to 1.5 hours. This is currently the main measure to control the occurrence of tin whiskers.

4, Using matte tin plating

At present, some lead-free pure tin electroplating additives have been developed on the market which can effectively prevent the formation of tin whiskers. The additives have the advantages of fine crystal, good solderability, low energy consumption and simple use, thereby establishing useful ways of a tin whisker inhibition.

5, Using lead-free alternative materials

The compatibility of lead-free solder coatings with tin-lead (SnPb) and lead-free solders is the first consideration for PCB manufacturers when selecting lead-free coatings for electronic component electrodes. NiPdAu and tin-lead (SnPb) and industry standards  lead-free solder alloy SnAgCu is soldered together and it has forward and backward compatibility. Secondly, considering the tendency of tin whiskers, NiPAu solder coatings do not have tin whiskers due to the presence of tin-free components. This is a complete solution, but the cost is slightly higher, but in the application of high reliability requirements such as aviation and aerospace. It is worth considering.