Indentation Induced Tin Whisker Formation on Tin Plated Component Leads

Tin whisker formation has been a serious concern for application of pure Tin as a Pb-free component lead finish. It has been long believed that residual stress is the root cause of whisker formation. A fundamental question is if stress produced by other than the plating processing and post-plating metallurgical reactions can induce whisker formation. In this study, micro indents were made on pure Tin plated component leads to induce stress for studying stress induced whisker formation. Nano-indentation was performed to measure hardness and elastic modulus of the Tin coating layer where whiskers initiate. Scanning electron microscopy (SEM) was used to study indentation deformation mechanisms and to monitor the nucleation and growth of whiskers in-situ. In additions, finite element analysis was carried out to theoretically calculate the stress/strain distribution around the indentations. Experimental and theoretical calculation results show that whiskers form at a certain stress level. This suggests that there might exit a critical stress threshold that governs the whisker formation. It is believed that establishment of a quantitative relationship between stress level and whisker formation/growth could lead to a breakthrough in risk and reliability assessment with pure Tin application in the electronic industry and in safeguard for smooth Pb-free transition.