High temperature gold/tin eutectic (80 Au/ 20 Sn) solder is used in manufacturing for multiple reasons. These motives may include the ability to post solder a part/device without reflow, high temperature field applications, and allow soldering to thick Au layers without the possibility of precipitating AuSn4 brittle intermetallics. In the following military case, Au/Sn eutectic was employed because of high temperature service and the guarantee of no occurrence of gold embrittlement when soldering to the thick Au outer plating. The Au was plated over an electroplated nickel (Ni) layer on a Kovar (iron/nickel/cobalt) housing. The soldering resulted in an extremely poor bond strength of a duroid circuit to the Kovar housing. The results showed contamination in the supplier’s electroplated Ni bath caused the plating to have poor bond strength. The failure occurred within the Ni plating layer.

We investigated the cause of the whisker/discoloration which were found in the transistor lead of stocks (package type TO-18, low power use). In process of the investigation, we estimate two corrosion models that the first model is the remnant of sulfuric acid in cracks of the nickel-phosphorus plating layer in the transistor lead, the second model is the out-gassing or the dissolved ions from the stock container and conductive mat. As the results of the investigation which includes analyses of the whisker/discoloration cross section made by FIB (Focused Ion Beam), a reproductive experiment and so on, the whisker/discoloration were the corrosion reacted between the solder (Pb-Sn) on the transistor lead and SO 4 2- ions of the stock container. We estimate that the new corrosion will not occur and grow in mounted devices because of rejecting the source of corrosion (stock containers). Further, in the worst case of the corrosion occurrence, protective coatings were applied to the mounted transistor lead, as the measure against falling away from the transistor lead.

At the surface of the tin-plated brass substrate placed at 50°C, the tin whiskers grew evidently within a short time, due to the formation of zinc oxide on the surface and alloying between plated tin and brass substrate as supposed. While at the surface of the brass substrate plated with tin on nickel, there was no trace of the tin whisker at all. Nickel greatly represses the diffusion of base metal materials into the tin layer. Nickel and tin plated monolithic chip capacitors placed at the same condition for 18 years were also observed and the tin whisker growth phenomenon has never taken place either. As a result, the tin plated film on the nickel over silver thick film does not provide the tin whisker growth. Nickel underplating plays an important role in tin plated capacitors for not only the solder leaching but also the tin whisker growth problems.