Formation and Growth of Tin Whiskers on Aluminum–Tin Alloys

Abstract

In order to clarify the mechanism of formation of nodules and whiskers on the conducting wires used in aluminum electrolytic capacitors, aluminum–tin binary alloys were subjected to an investigation as the model alloys for the joints in the conducting wires. The concentration of tin in the binary alloys was 1, 5 or 10 at%. The 10 at% tin alloy showed the highest number of nodules or whiskers on its polished surface after storing under ambient conditions for 7.8 Ms. Many whiskers whose length were greater than several tens of micrometers were observed for 5 at% tin alloy. The 1 at% tin alloy showed few nodules or whiskers. Growth of the nodules and whiskers is caused by diffusion of the tin atoms from the strained or high-energy areas into the low-energy ones or the root grains. The extrusion toward the surface at the root grains then develops nodules and whiskers. As a preventive measure of whisker formation, selective etching of the aluminum phase using a solution of sodium hydroxide was confirmed to be successful. Thus the aluminum phase was thought to form a non-uniform distribution of strain in the tin phase. This acts as the driving force for diffusion of the tin atoms.