Development of Lead Free Solder for Microjoining

研究了热循环对无铅焊料(锡-3.5%银共晶)退化过程的影响，并与传统含铅合金(锡-37%铅共晶)的退化过程进行了比较。通过使用热电模块进行热循环，在零下到373K以上的温度范围内精确控制加热和冷却程序。然后通过剥离强度来评估热疲劳性能。结果表明，在热疲劳条件下，锡银共晶合金的抗热疲劳性能优于传统的锡铅共晶合金，但在不受热疲劳影响的情况下，锡银共晶合金的抗热疲劳性能弱于传统的锡铅共晶合金。锡铅共晶在热循环过程中的退化可归因于组织的粗化，而组织粗化又因焊点及其周围的应力集中而增强。研究还发现，焊点的剥离强度取决于变形速度，这意味着为了评价焊点的强度，必须确定变形速度。

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.