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.
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