Tin whisker growth under cycling current pulse

A new test method, called as cycling current pulse method, was developed to evaluate the trend of whisker growth, which the cycling current pulse was first applied for cyclical heating and cooling on the tin coating sample in order to accelerate the whisker growth. The loading current in samples was controlled by the device composed of alpha-simple applied controller and a constant current source, and the working current density varied from 5 times 10⁴A/dm² to 10 times 10⁴A/dm². It resulted in temperature of the samples varied from 40 degC to 75 degC, and both heating and cooling dwell time per cycle were 5 min, 0-2200 cycles (about 15 days) were carried out in present work. Tin whisker growth on surface finish is observed by SEM after different cycles; the results showed that the longest fluted whisker about 120 microns in length was observed after 2200 cycles current pulse test. As a comparison with the isothermal condition, there was a relative short incubation period in cycling current pulse test at current density 5 times 10⁴A/dm². However, at current density of 10 times 10⁴A/dm², whisker growth was slowdown, the reason is not clear now. Present work shows that the cycling current pulse method is effective to evaluate the trend of whisker growth