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

The effects of postbake treatment on a pure matte Snplated 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 Cu
6
Sn
5
and η-Cu
6.26
Sn
5
. IMC precipitates were also observed at the grain boundaries and some IMC precipitates were embedded at the Sn matrix. No Cu
3
Sn or Kirkendall voids were observed at the interface between the Sn and Cu.