无铅波峰焊Sn-Bi-Ag-Cu钎料焊点剥离机制

对波峰焊常用的Sn-Bi-Ag-Cu无铅钎料进行了通孔波峰焊焊点剥离现象的模拟实验。分析表明,凝固延迟及铋元素偏析导致焊盘拐角附近钎料区在结晶后期残存液相最终成为缩孔的聚集区,结晶后期该区的低塑性使收缩应变容易超过材料的塑性而发生开裂,且发生机制与结晶裂纹发生的机制相同,强偏析元素铋的存在导致剥离概率极高。通孔拐角附近是应力应变的集中区,沿界面至焊盘外缘,应力应变逐渐降低。Sn-Bi-Ag-Cu系钎料焊点剥离的发生机制是由于Bi元素在焊盘/钎料界面富集使拐角附近钎料的液相线温度降低,结晶后期该区因固液共存而成为相对低塑性区,在应力作用下开裂并向焊盘外缘扩展。

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-BiAg-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.