Thermal Shock Resistance And IMC Behavior Of Low-Ag Lead-Free Solder Micro-Joints

Low-Ag Sn-Ag-Cu solder was suggested to have a broad applicationprospect, for its low cost and high shock resistance. But some problems are stillexisted in industry, such as high melting point, poor wettability, poor aging andthermal shock resistance. In order to replace the high Ag solder SAC305by low-Ag solder, adding some alloy elements into low-Ag solder is one of the mostefficient way to improve its comprehensive performance. This paper studied thethermal aging resistance of low-Ag SAC-Bi-Ni/Nisolder joints compared withSAC305, SAC0307and SAC0705. Meanwhile, the thermal shock resistance oflow silver SAC-Bi-Ni/Cu solder joints was studied by Homemade thermal shockexperiment device, compared with SAC305. Additionally, the shear strength,nano mechanical behavior, and micromechanical behavior of the solder jointsbefore and after aging were also investigated.The results showed that with the aging time increases, the IMC thickness offour lead-free solder joints increased. The size of IMC grains increased obviously,and the morphology of IMC significantly changed, but its composition didn’tchange. The shear strength,Young’s modulus, and hardness of four lead-freesolder joints decreased after aging.In the four kinds of solder joints, SAC-Bi-Nihas the largest shear strength, hardness and creep resistance. SAC305was thesecond largest.The IMC growth rate of SAC305solder joint was the fastest, and itsactivation energy was the minimum. The IMC growth rate of SAC-Bi-Ni solderjoint was the slowest, and its activation energy was the maximum. The agingresistance of low-Ag solder joint was significantly improved by adding elementBi and Ni and was higher than SAC305.The thickness of IMC of the two lead-free solder joints increased by layeredgrowth after thermal shock. It was indicated that the IMC growth was mainlyaffected by diffusion mechanism. The hardness and elastic modulus of SAC305 solder joints decreased after thermal shock. Meanwhile, the hardness and elasticmodulus of SAC-Bi-Ni solder joints increased and then decreased. The hardnessand creep performance of SAC-Bi-Ni solder joint was better than SAC305beforeand after thermal shock. The thermal shock resistance of SAC-Bi–Ni was betterthan the SAC305.