SnZn Solder Alternative For Low-Cost Pb-Free Surface Mount Assemblies
Authors: Gavin J. Jackson, Ian J. Wilding, Richard Boyle, Maurice N. Collins, Eric Dalton, Jeff Punch, Puwei Liu and Matthew Holloway Company: Henkel and Stokes Institute, University of Limerick Date Published: 10/14/2012
Abstract: SnZn alloys are an attractive economical consideration for use in lead free solder materials due to the lower cost and lower melting points relative to SnAgCu (SAC) alloys. However, SnZn solder materials are significantly different both chemically and metallurgically from conventional SAC based Pb-free solder materials. One consequence of this is that SnZn powder reacts strongly with solder paste flux mediums resulting in paste shelf life measured in hours rather than weeks. There are also some question marks on the long term reliability of SnZn solder joints. These factors have previously prevented the development of a fully viable solder paste and soldering process. However, attempts to overcome these drawbacks by novel developments in fluxing technology have been performed. The characteristics and performance of a model SnZn solder paste including viscosity, solderability, reflow performance on Pb-free printed wiring boards (PWBs), viscosity and shelf life have been compared with standard SAC solder pastes in a surface mount assembly process. In addition the long term reliability of SnZn solder joints under accelerated temperature cycling (ATC), 0ºC-100ºC, has been assessed. The results show that significant increases in shelf life can be achieved by developments in flux technology whilst also providing acceptable soldering performance and favourable long term reliability characteristics. These encouraging observations with SnZn alloys in solder pastes may potentially lead to a viable lower cost alternative to conventional SAC solder pastes.