Thermal Reliability Of Low-silver Sn-Ag-Cu Lead-free Solder Joints

High costs and low reliability of lead-free solder limited the lead-free process of electronic products in worldwide. Reduced the silver content of Sn-Ag-Cu lead-free solder become an important way of lead-free for electronic packaging. Therefore, the study of low prices and high-reliability low-silver lead-free solder and solder joints had important scientific significance and application prospects. A low-silver SAC-Ni-P solder were prepared by adding a small amount of Ni and P elements in this study. The performance of low-silver SAC-Ni-P solder and the thermal reliability of its solder joints were system studied. Furthermore, The failure mechanisms of solder joint that applied to the printed circuit board(PCB) was also analysed.The flowing ability, wettability and mechanical properties of low-silver SAC-Ni-P solder were investigated, and comparative with the Sn-0.7Cu, Sn-3.5Ag-0.7Cu and Sn-37 Pb solder. The results showed that the flowing ability and the wettability of four typical solders were both successively:Sn-37 Pb > Sn-3.5Ag-0.6Cu > low-silver SAC-Ni-P > Sn-0.7Cu. In terms of mechanical properties, the tensile strength of low-silver SAC-Ni-P solder was close to Sn-3.5Ag-0.6Cu solder, but its elongation rate was 1.89 times of Sn-3.5Ag-0.6Cu solder.Three thermal reliability acceleration test, thermal aging, thermal shock and thermal cycling, were used to test the Cu / low-silver SAC-Ni-P / Cu solder joints. The test results have showned that IMC growth rate was mainly controlled by the diffusion rate of atoms. The thickness of the interfacial IMC increased with parabolic trend over aging time. The scalloped morphology of IMC became wavy.The IMC growth rate were 0.61×10-14cm2/s, 2.06×10-14cm2/s and 4.83×10-14cm2/s while aging at 75℃,100℃ and 150℃ respectively. The growth activation energy of low-silver SAC-Ni-P solder IMC is about 33.75 kJ/mol. The shear strength of solder joint declined significantly when the aging time increased, and the higher the temperature, the faster the decrease. Due to the second phase and the matrix organization grew, the fracture mode changed from ductile fracture to local brittle fracture with the increasing of aging time. Thermal shock and thermal cycling test have similar results. Because the effect of temperature and thermal stress, the thickness of interfacial IMC of solder joint increased as the cycles increased, and the scalloped morphology of IMC became wavy. Thermal stress accelerated defects aggregation and growth which make the crack initiation. And the crack make the shear strength of solder joint declined with parabolic trend when the cycles time increased. Due to the cycles of thermal cycling test is less than thermal shock test, the shear strength of thermal cycling test joints decreased slowly than thermal shock test joints. The fracture mode changed from ductile fracture to intergranular fracture with the increasing of cycles time. A certain shear load will applied to the solder joint to simulated the thermal stress of solder joints during service, and then do the thermal shock and thermal cycling test. The results have showned that, it was no significant effect on the solder joint microstructure and the thickness of IMC under the experimental conditions, but would further reduce the shear strength of the solder joint.The failure mechanisms of PCB mount solder joint was explored by using ANSYS finite element simulation software. The results showned that, crack initiation occurred in the interface of components and Cu pad at failure solder joints of PCB, where were coincide with the maximum area of stress and strain which knowed by using ANSYS. Then crack propagation along the Cu pad interface. Theinterface brittle phase Cu6Sn5 grew up, and accelerated the failure of solder joints. The low-silver SAC-Ni-P solder joint could meet reliability requirements of PCB by test and simulation.