Behavior of whisker growth on tin plated Fe-42%Ni substrates

Recently, development of lead-free solder is promoted. As plating materials, Sn metal is thought to be the best lead-free solder. However Sn is known to have a serious problem of whiskers growing spontaneously on the surface. The whiskers are liable to shorten electronic circuits. The mechanism of whisker growth has not yet been resolved. To solve the problem of whisker generation, it is important that we solve growth mechanism of the whisker. In this study, the authors studied the effect of atmosphere in which the whisker grows. Sn was electroplated to a thickness of approximately 10 mum on a plate of Fe-42wt%Ni after the normal degreasing operation. The tinned plate was heat-treated in a thermal cycle as follows changing atmosphere: The lowest and highest temperature in thermal cycle were 233K and 403K respectively, and these temperature were maintained for 10 min. Assuming that the heating and cooling rates were constant for 50 min, the time for one cycle was 2h. The maximum number of cycles was 1000. The surface morphology was examined by SEM and FIB. There were differences in the whisker growth by atmosphere. This shows that oxidation of tin surface affects the whisker growth. Now these results, growth mechanism of tin whisker is discussed. Precipitates were detected on the boundaries of Sn grain. The precipitates observed at the grain boundary in FIB images are also considered to be Ni₃Sn₄