Microstructure evolution and pull strength in Sn-8Zn-3Bi solder and eutectic Sn-37Pb solder for CSP joints

The microstructure of low temperature Sn-8Zn-3Bi and eutectic Sn-37Pb solders and the pull strength of these solders with Au/Ni electrolytic-plated Cu substrate under thermal aging conditions were investigated. The samples were aged isothermally at 100 degrees C and 150 degrees C for 3600 h. Sn-8Zn-3Bi had higher pull strength than eutectic Sn-37Pb. The pull strength decreased with increasing aging times and was independent of the temperature. The microstructure of as-reflowed low temperature Sn-8Zn-3Bi solder mainly consists of the Sn matrix scattered with Zn-rich phase. After aging at 150 degrees C for 600 h, Zn reacted with An and was then transformed to the AuZn compound. With aging, Ni5Zn21 compounds were formed at the Ni layer. Finally, Ni5Zn21 phase, divided into three layers, was formed with column-shaped grains, and the thicknesses of layers were changed. In the eutectic Sn-37Pb solder, irregular-shaped Pb-rich phase was embedded in the Sn matrix. Ni3Sn4 compounds were observed at the interface between the solder and Ni substrate.