Tin Whisker Formation Kinetics on Sn-Pb HASL Coating Layer Inside Plated Through Holes With Press-Fit Complaint Pins

Abstract

Summary form only given. It has been long believed that residual stress is the root cause of tin whisker formation on pure tin-plated component leads. However, tin whiskering also happens to a lesser degree on other tin-based alloys under certain conditions. In this study, the tin whisker phenomena were reported on Sn-Pb HASL coating layer inside plated through holes with press-fit complaint pins. Scanning electron microscopy (SEM) was used to monitor the nucleation and growth of whiskers in-situ. In additions, a nonlinear contact finite element analysis was carried out to theoretically calculate the stress/strain distribution on HASL coating inside PTH, which was under compression due to the inserted compliant pins. Experimental and theoretical calculation results show that the whisker initiation and growth to 20 micrometers could be achieved with a matter of 30 minutes at ambient temperature under certain stress conditions. The tin whisker initiation and growth were further studies at higher temperature at about 80 deg. C and at cold around -30 deg. C. Nano-indentation is performed to measure hardness and elastic modulus of the Sn-Pb coating layer where whiskers initiate, then compared to location where there is no whiskers observed. It is believed that establishment of a quantitative relationship between stress level, temperature, and whisker formation/growth could lead to better scientific methods for risk and reliability assessment with tin whiskers to safeguard a smooth Pb-free transition