Fillet-lifting mechanism for Sn-Bi-Ag-Cu solder joint in lead-free wave soldering

The mechanism of fillet-lifting for the Sn-Bi-Ag-Cu solder was studied, which is one of the mainly used lead-free solder in wave-soldering. The analysis results show that the segregation of Bi becomes more remarkable at the corner of the solder body/PCB pad. The local irregularity of Bi delays the solidification of the solder at the above area, and thus the residual liquid solder at that place becomes the aggregation area of shrinkage void during the following crystallization. The shrinkage strain exceeds the low plasticity of the Bi segregation solder in the later crystallization, then the crack is similar to that of the crystal crack. The presence of strong segregation element Bi exacerbates the probability of lift-off, which could be reduced by the rapidly cooling only in a limited extent. The stress-strain concentration is detected at the corner of Cu pad, the lower stress for the outer of Cu pad. Based on the above two leading reasons for fillet-lifting, the stress-strain concentration at the corner of Cu pad and the solidification delay, the mechanisms of fillet-lifting for Sn-Bi-Ag-Cu through-hole lead-free solder joint were given. Near the interface region of solder and Cu-pad, the liquidus of solder is lowered by the enriched Bi element, and the low plasticized zone produces at this region due to the existence of residual liquid phase. With the stress-strain concentration, the cracks produces near the corner of Cu pad and then propagated to its outer.