Microstructural characteristics and mechanical performance of Bi-Sb-Ni added Sn 3.0Ag-0.5Cu multicomponent lead-free solder alloy / Zhou Ding

• Main Author: Zhou , Ding
• Format: Thesis
• Published: 2024
• Subjects: TJ Mechanical engineering and machinery
• Online Access: http://studentsrepo.um.edu.my/15495/2/Zhou_Ding.pdf; http://studentsrepo.um.edu.my/15495/1/Zhou_Ding.pdf; http://studentsrepo.um.edu.my/15495/

Sn-Ag-Cu (SAC) alloys have been identified as promising replacements for standard 63Sn-37Pb eutectic solder after the restriction in legislation of lead (Pb) in electronics has been actively pursued. However, the reliability of Pb-free solders, used in harsh conditions such as in automotive, is still a serious concern. Simultaneous additions of multiple alloying elements to conventional SAC based solders have recently been investigated to improve their reliability. The present study contains two parts. In the first part of the study, the effects of isothermal aging on the microstructure and mechanical properties were investigated for Sn-3.0Ag-0.5Cu (SAC305) and a SAC305 based alloy with simultaneously addition Bi, Sb and Ni added (noted as solder M with composition of Sn-3.0Ag-0.5Cu-3.2Bi-2.95Sb-0.037Ni). The solder alloys were thermally aged at 125℃ for up to 1008 hours. Subsequently, tensile tests under different strain rates and creep tests at various applied tensile stresses for up to 5x104 s were conducted. In addition, microhardness tests of thermally aged solder alloys after creep tests were studied, the changes of hardness from fracture tips to clamping side were discussed. The findings showed that aging time led to the global increase of grain size significantly. The average grain diameter of as-received solder M was 2.5 times smaller than SAC305. After 1008h aging at 125℃, the increase of grain diameter of solder M was also lesser than SAC305. Overall, the addition of Bi, Sb, and Ni improved microstructural refinement, suppressed the grain growth of solder M, and reduced the grain growth rate in comparison with SAC305. Tensile tests showed that the ultimate tensile strength (UTS) of the solder M was higher than SAC305 during 125℃ aging. In addition, thermal aging resulted in a decreased creep rate for solder M, but the opposite performance of SAC305 was found. Different performances of two aged solder alloys revealed that Sb and Ni increased the mechanical strength of SAC305 by forming intermetallic phases, while Bi provided both solid solution and precipitation strengthening. In the second part of this work, simulations of SAC305 in a novel hybrid quad flat package (QFP) under thermal cyclic loading were conducted by using finite element method (FEM). Due to the variation of temperature during service, solder alloy applied in the novel QFP undergoes high thermal stress, which can lead to large deformation and failure. Thus, the prediction of thermal stress and strain distributions of solder is a prerequisite for reliability analysis of electronic package. Simulations of SAC305 in a novel hybrid QFP under thermal cyclic loading were conducted by using ANSYS. The stress distribution and accumulated creep strain were found in the critical location of solder in package, and lifetime was predicted. Furthermore, the simulation results were compared with experimental results from industry.