Intermetallic compounds evolution between lead-free solder and cu-based lead frame alloys during isothermal aging

Abstract

This paper investigated the intermetallic compounds (IMCs) formation between SnAgCu solder and four Cu-based lead frame alloys during reflow soldering and isothermal aging. Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) were used to study the cross-sectional microstructure and stoichiometric information. Optical Microscope (OM) was used to measure the mean thickness of IMCs. It was found that Ni and Sn trace element have important influences on the interfacial reactions. After soldering, for the case of Sn-Ag-Cu solder on Cu-Sn-Cr-Zn, Cu-Sn-P and Cu-Fe-P-Zn-Pb, an island type Cu6Sn5 and a thin Cu3Sn layer were formed at the interface. However, for the case on Cu-Ni-Si-Mg alloy, no Cu3Sn was detected and only a layer of ternary (Cu,Ni)6Sn5 was confirmed and some Cu6Sn5 particles was observed to disperse in the bulk solder. The top morphology of IMCs was also characterized after the solder was selectively etched away. The IMCs on the Cu-Ni-Si-Mg showed long rod-like shape, whilst the IMCs on the other three alloys appeared round. After different duration of aging at 150°C, all the IMCs grew thicker and the grain size became larger. The rod-like IMCs on Cu-Ni-Si-Mg gradually transformed into round shape and it was relatively smaller compared to that on the other three alloys. Moreover, the growth rate of IMCs on Cu-Ni-Si-Mg is the fastest among the four alloys.