Lead free packaging and Sn-whiskers

Abstract

Replacement of Pb/Sn terminations on electronic devices with pure Sn has proven to be much more difficult than expected. The main problem is Sn whisker formation. Sn whiskers are single crystal, mechanically strong, metallic filaments that can nucleate and grow over time in such a way as to lead to device failure in the field. In this study, we present the results of Sn-whisker formation and growth studies for plated matte-Sn on copper lead frames employing stress tests as outlined in the proposed JEDEC Sn-whisker specification. The results indicate that the propensity for whisker formation decreases with increasing Sn-thickness, and with a post plate heat treatment. The propensity for whisker growth is found to dramatically change once the devices are subjected to a melting of the matte-Sn plating. A reflow preconditioning is found to be a major variable influencing the formation and growth of Sn-whiskers on bare Cu lead frames which even negates the potential benefits for thicker Sn-plates and a post-plate anneal. Only the application of a Ni-underplate was found to produce a truly robust matte-Sn whisker solution resistant to excessive whisker growth in both the as-made and post-reflowed states.