Development of Lead Free Solder for Microjoining

研究了热循环对无铅钎料Sn-3.5%Ag共晶的降解过程的影响，并将其与传统含铅合金Sn-37%Pb共晶进行了比较。热循环通过使用热电模块来实现，加热和冷却程序在零度以下到373K以上的温度范围内精确控制。然后通过剥离强度评价其耐热疲劳性能。结果表明，Sn-Ag共晶合金的抗热疲劳性能优于常规Sn-Pb共晶合金，但在不受热疲劳作用的情况下，Sn-Ag共晶合金的抗热疲劳性能弱于常规Sn-Pb共晶合金。在热循环过程中Sn-Pb共晶的退化可归因于微观组织的粗化，而在焊点和焊点周围发生的应力集中又加剧了微观组织的粗化。还发现，焊点的剥落强度取决于变形速度，这意味着必须确定变形速度才能评估焊点的强度。

The effect of thermal cycles on the degradation process of lead free solder, Sn-3.5%Ag eutectic, which is regarded as one of the most possible candidates, has been investigated in comparison with that observed in conventional lead bearing alloy (Sn-37%Pb eutectic). Thermal cycles were applied by the use of thermoelectric modules, with which both heating and cooling programs are precisely controlled over the temperature range from subzero to above 373K.Thereafter the thermal fatigue resistance was evaluated through the peeling strength. It is found that Sn-Ag eutectic is superior in the thermal fatigue resistance to conventional Sn-Pb eutectic alloy, though the former is weaker than the latter if not subjected to the thermal faigue. The degradation of Sn-Pb eutectic during thermal cycling can be ascribed to the microstructural coarsening, which in turn is enhanced by the stress concentration occurring at and around the solder joint. It is also found that the peeling strength of solder joint depends on the deformation speed, meaning that the deformation speed must be identified in order to evaluate the strength of joint.