Effects of postbake on the microstructure and whisker growth of matte Sn finish

Abstract

The effects of postbake treatment on a pure matte Sn-plated copper leadframes under high temperature and humidity conditions were investigated. The samples were divided into two categories: those without postbake treatment (WOPB) and those with postbake treatment (WPB), which exposed at 125□ for 1 hour. The X-ray intensity decreased through all crystal orientations after the postbake treatment. For matte Sn-plated Cu leadframes stored at 55°C/85%RH for 1800 hours, nodule-shaped whiskers were observed on samples WOPB, while none were on samples WPB. The WPB samples have a regular layer of IMCs approximately 27% narrower than the WOPB samples. The IMCs had two distinct layers divided into large-grains and small-grains. The large-grain layers located on the Sn side grew before the small-grain layers. The IMCs in the WOPB and WPB samples were identified as Cu6Sn5 and η-Cu 6.26Sn5. IMC precipitates were also observed at the grain boundaries and some IMC precipitates were embedded at the Sn matrix. No Cu 3Sn or Kirkendall voids were observed at the interface between the Sn and Cu. © 2007 IEEE.