Tin Whisker Growth Research On Component Plating Finish And Its Process Mitigation

Tin whiskers growth on high-tin content lead finishes creating reliability problems with electronic components and its products. Since 1950's, lots of theories about whisker growth have been developed. It is widely accepted that SnPb alloys have been the most successful in controlling whiskers. However, with the RoHS lead-free finishes carrying out, pure Sn and Sn-based finishes faced serious reliability proplems again.The Sn whisker accelerated test on several common packages carried out according to the JEDEC standard. Sn whisker growth phenomenon and mechanism with pure Sn plated on Cu alloy lead-frames were studied. And 3 contrasitive experiments carried out to study the influence factors in process, too. Some efficency menthods on whisker mitigation during process were developed.Experiment results found that different whisker growth characters under 3 different accelerated conditions which were temperature cycling, room temperature storage and high temperature and humidity storage. The whisker incubation time was short under temperature cycling and gowth in a high rate, a high density and a uniform length. And a large amount of collapses were found on the finish relating to Sn whisker growth in high density. Under room temperature storage and high temperature and humidity storage conditions, 3-4 month is the turning point of the whisker growth rate. Sn whisker growth rate was faster after 3-4 month.Deeper research about the uS-XRD stress test on finish surface and AFM test on Sn/Cu IMC growth et al showed that different whisker growth mechanisms under 3 different accelerated conditions. Under room temperature storage condition, the driving force of whisker growth is the irregular growth of Cu6Sn5 which are caused by the un-balance diffusion of Cu and Sn at the Cu/Sn interface. Under high temperature and humidity storage condition, the high temperature and moisture leads to oxygen diffusion and/or thicker oxide formation on the surface which increased extra stress inside. Corrosion become an complicated factor. Under temperature cycling condition, the CTE mismatch of the substance induced thermo-mechanic stress is another important source.And the different factors'influences on whisker growth in process such as expocy molding compound (EMC) in molding and trim/from were studied by contrastive experiments. It turned out that no significant effect found about the EMC's influence on whisker. There was not evidence change about the residual stress in plating finish with and without mechanic deformation during trim/form process. The clamping force caused grain distortion and multiplied the orientation, so stress released in the finish and less whisker growed. The soft Sn finish surface was easily destroyed by trim/form tools and caused whisker growth by corrosion.Finaly,a series of whisker mitigation method was developed according to the experiments and conclusions above.