Effect of Layer Properties on Stress Evolution, Intermetallic Volume, and Density during Tin Whisker Formation

Compressive stress due to intermetallic (IMC) growth appears to be the main driving force for whisker formation, but many of the underlying mechanisms relating the IMC to stress and whisker formation are not understood. To better understand these fundamental processes, we have measured IMC, stress and whisker evolution in Sn layers deposited on Cu. We present systematic results of the effect of changing the Sn layer structure by modifying the grain size, thickness, Pb content, microstructure, and IMC morphology/growth kinetics. We have also made corresponding measurements of the mechanical properties of the Sn with similar changes in the structure and composition without the growth of IMC. We show that modifications that enhance the stress relaxation in the Sn also lead to reduced whisker growth.