Thermomechanical Fatigue Properties Of Lead-Free Solder Joints

As everybody knows, due to the good properties, sufficient supply and low prices, solders, especially Sn-37Pb, have been playing important roles in both mechanical and also electrical connection in surface mount technology. In the past a few years, more and more investigations on the substitutes for Sn-Pb solders have been made by many large electronic manufactures and research institutes world wide due to environmental and economic concerns. Solder joints are still the weakest link in the architecture of the electronic packaging. Therefore, new types of lead-free solders with higher mechanical properties and good reliability performance are extremely demanding.At present, most of the lead-free solders are Sn-Ag based binary or ternary alloys, among which Sn-3.5Ag and Sn-3.0Ag-0.5Cu solder alloys have been considered as the most promising Pb-free substitutes for the Sn-Pb solders. Many researches also showed that rare earth elements can refine the microstructure, so adding rare earth to the Sn-3.0Ag-0.5Cu based solders could improve the properties of the solder joints. Therefore, the eutectic Sn-3.5Ag, Sn-3.0Ag-0.5Cu, and Sn-3.0Ag-0.5Cu-0.1RE solders were selected in the current research as the testing alloys.In this research, thermomechanical fatigue (TMF) and consequent mechanical tests were performed on solder joints made with Sn-3.5Ag, Sn-3.0Ag-0.5Cu and Sn-3.0Ag-0.5Cu-0.1RE lead-free alloys. The temperature profile used in the experiment simulated the real service condition, i.e., from -40oC to 125oC with a dwell time of 10 min and a ramp time of 5 min. After 0, 100, 250, and 500 cycles, surface damage factures were observed and residual shear strengths of the solder joints were evaluated.The results have shown that the morphology of the microstructure and the crack were different after different cycles of thermomechanical fatigue test. For the Sn-3.5Ag solder joints, the crack initiated near the interfacial IMC and propagated into the bulk solder along a 45o line. Crack initiated near the interfacial IMC and propagated parallel along the interfacial IMC of the Sn-3.0Ag-0.5Cu solder joints. In the Sn-3.0Ag-0.5Cu-0.1RE solder joints, crack also initiated near the interfacial IMC and propagated away from the interfacial IMC along approximately a 10°angle.Residual shear strengths of Sn-3.5Ag, Sn-3.0Ag-0.5Cu, and Sn-3.0Ag-0.5Cu-0.1RE solder joints after TMF have shown that the differences of the shear strengths among the three kinds solder joints were not very significant, but the shear strength of Sn-3.0Ag-0.5Cu was better than others. The shear strength of all solder joints decreased with TMF. Sn-3.5Ag solder joints had the fastest decreasing rate and lowest shear strength. The shear strength of Sn-3.0Ag-0.5Cu-0.1RE solder joints decreased with the increase of the TMF cycles and the decreasing rate is the slowest.Features of fracture surfaces of thermomechanically-fatigued Sn-3.5Ag, Sn-3.0Ag-0.5Cu, and Sn-3.0Ag-0.5Cu-0.1RE solder joints were observed and compared. Sn-3.5Ag solder joints exhibited primarily ductile fractured areas. Sn-3.0Ag-0.5Cu solder joints exhibited a mixture of both ductile and brittle fracture surface. Sn-3.0Ag-0.5Cu-0.1RE solder joints contained significantly more brittle areas in the fracture surface with increasing number of TMF cycles.