Tin Whisker Nucleation and Growth on Sn-Pb Eutectic Coating Layer Inside Plated Through Holes With Press-Fit Pins

It has been long believed that residual stress is the root cause for tin whisker formation on pure tin-plated component leads. However, tin whisker formation could be observed on the surface of other tin-based alloys under certain conditions. In this study, the whisker formation was reported on a coating layer of Sn-Pb eutectic hot air solder leveling (HASL), which was under compression stress conditions due to the inserted compliant pins. In-Situ scanning electron microscopy was used to monitor the nucleation and growth of whiskers. In addition, a mechanical experiment and non-linear contact finite element analysis were used to estimate the magnitude of the stress in the HASL coating layer. It was found that the tin whisker formation with whisker size of more than 10 mum could occur on the surface of 60Sn-60Pb plating within less than 30 min at an ambient temperature under compressing stress conditions. The tin whisker initiation and growth were further studied at an elevated temperature of 70 degC to check if a higher temperature effects Tin whisker formation. It is believed that establishment of a quantitative relationship of whisker formation/growth under compressive stress and elevated temperature conditions could lead to better scientific methods for risk and reliability assessment and smooth transitions to lead-free assemblies.