Sn-Bi-Ag-Cu钎料波峰焊焊点的剥离现象

采用在波峰焊过程中常用的Sn-Bi-Ag-Cu无铅钎料,进行了通孔波峰焊焊点剥离现象模拟实验.剥离的断面形貌和成分分析表明,凝固延后及铋元素偏析导致焊盘拐角附近的钎料区在结晶后期残存液相,并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极限而发生开裂.开裂机制与结晶裂纹机制相似.强偏析元素铋的存在导致剥离概率急剧提高,铋元素含量增加,剥离的趋势增加.铅污染也使剥离的概率显著增加.冷却速度增加,剥离趋势减小.大的冷却速度能够抑制偏析,却不能完全抑制剥离,且会导致钎焊圆角表面裂纹增加.避免铅污染、降低铋的含量并控制冷却速度可抑制剥离.

Fillet-lifting phenomenon of wave soldering with Sn-Bi-Ag-Cu solder

The lift-off phenomenon in through-hole wave soldering with Sn-Ag-Cu-Bi solder was investigated by the simulation experiment. It can be seen 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 lowest plasticity of the Bi segregation solder in the later crystallization, and then crack comes into being from mechanism similar to that of the crystal crack. The presence of strong segregation element Bi exacerbates the probability of lift-off. The probability of fillet-lifting increases with increasing Bi content and the Pb contamination on the solidification behavior of through-hole solder joint, and decreases with the increasing cooling rate for solder joint. However, the fillet-lifting can not be completely eliminated by the rapid cooling for through-hole solder joint, and the rapid cooling can create cracks at the surface of solder. The fillet-lifting can be suppressed by eliminating Pb contamination, reducing the Bi content in solder and controlling the cooling rate for solder joint.