Whisker nucleation in indentation residual stress field on tin plated component leads

Author： Liang Jin Xu Zhi-Hui Li Xiaodong

ISSN： 0957-4522

Source： Journal of Materials Science: Materials in Electronics, Vol.18, Iss.6, 2007-06, pp. : 599-604

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Abstract

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 introduce a stress field and a scanning electron microscope was used to monitor the nucleation of whiskers in-situ. Nanoindentation was also performed to measure hardness and elastic modulus of the tin coating and substrate. The stress/strain field around the micro-indent was calculated using finite element method. Experimental and theoretical calculation results show that stress gradient plays an important role in whiskers nucleation.