焊锡材料的应变率效应及其材料模型

采用分离式霍普金森压杆和拉杆实验,研究了含铅Sn37Pb、无铅Sn3.5Ag和Sn3.0Ag0.5Cu 3种焊锡材料在600～2 200 s~(-1)应变率下的力学性能,得到了它们在不同应变率下的应力应变曲线.根据实验数据建立了3种焊锡材料的应变率无关弹塑性材料模型和率相关Johnson-Cook材料模型,并用于模拟板级电子封装在跌落冲击载荷下焊锡接点的力学行为.结果表明,高应变率下无铅焊料比含铅焊料对应变率更敏感,其抗拉强度为含铅焊料的1.5倍,其韧性也明显高于含铅焊料;在跌落冲击过程中,焊锡接点经历的应变率可达到1 000 s~(-1)左右;给出的率相关Johnson-Cook材料模型能预测出比率无关的弹塑性模型更合理的应力应变结果.

STRAIN RATE EFFECTS AND MATERIAL MODELS OF SOLDERS

Reliability of solder joints in electronic packages depends on the mechanical properties of solder materials. Under drop impact loadings, the solder joint undergoes high strain rate deformation. Therefore it is important to investigate the effect of strain rate on the mechanical behavior of the solder materials. In this paper, mechanical properties and stress-strain curves of one lead-containing solder, Sn37Pb, and two lead-free solders, Sn3.5Ag and Sn3.0Ag0.5Cu, were investigated at strain rates ranging from 600s^{-1} to 2200s^{-1} by the split Hopkinson pressure and tensile bar techniques. Based on the experimental data of the quasi-static tensile tests and the split Hopkinson pressure bar tests, elastic-plastic material models independent of the strain rate and Johnson-Cook material models dependent of the strain rate of the three solders were developed and employed to predict mechanical behaviors of solder joints in a board level electronic package under drop impact loadings. The results show that at high strain rates, the two lead-free solders are more sensitive to the strain rate, and their tensile strengths are about 1.5 times greater than that of the lead-containing solder, and their ductility is significantly greater than that of the lead-containing solder. Under the drop impact, the solder joints experience a strain rate of 1000s^{-1}, and the proposed material models in Johnson-Cook form are applicable to predict more realistic stress and strain than the elastic-plastic models independent of strain rate.