Tin whisker formation - Results, test methods and countermeasures

Electroplated tin layers as used as lead-free solderable finish on the terminations of semiconductor devices are known to form whiskers. These whiskers are monocrystals of tin and grow within weeks to years with a diameter of some microns up to a length of several hundreds of microns or even millimetres. Thus they can cause shorts and the failure of a whole electronic circuit. This paper shows that these whiskers grow under the force of compressive stress in the layer, which is usually developed by the growth of an irregular intermetallic layer at the substrate / plating interface. We show that none of the actually known test methods can accelerate whisker growth or that growth rates can be correlated to ambient conditions. However, we show the effectivity of barrier layers to prevent whisker growth and propose a test method to judge on the whisker safety by extrapolation of the initial growth rate within the first weeks after electroplating.