Grain structure evolution of tin-based lead-free solder joints in electronic packaging assembly and its impacts on the fatigue reliability

Solder is one of the main joints materials used in the electronics for decades of years. It is not only the mechanical support but also conducts the electric signals in the electronics. However, the regulation of lead content in electronic devices has driven the transition from lead-tin solder to the lead-free solder that requires lots of research to fill the knowledge gap in the lead-free solder property. At the same time, the reliability issues due to solder failure are of great interest as the miniaturization and mobilization trend of the electronic devices continues. A lot of works have been done to understand the fatigue failure mechanism and its affecting factors with some degree of inconsistence. In this research, the grain structure evolution of tin-based lead free solder in electronic packaging and its relation to the fatigue reliability are studied.;First of all, to evaluate the fatigue reliability and understand the fatigue behavior of the tin-based lead-free solder joints in BGA packaging assembly, isothermal shear fatigue tester was designed. Solder joints under cyclic shear load is found to fail by crack opening mode and crack is fixed at the solder neck area in the package side regardless of the testing conditions.;It is found that the failure of the solder joint under shear follows the Coffin-Manson model and a frequency-modified Coffin-Manson Model was developed for fatigue lifetime prediction. Fatigue property parameters that dictate the fatigue behavior of the solder joints were extracted from this model, namely fatigue ductility coefficient, cyclic strain hardening exponent/cyclic ductility exponent, and frequency exponent. The strain/work hardening rate is found to influence the fatigue resistance of the solder joints more than other parameters.;In the second part of this research, the grain structure evolution on tin-based lead-free solder under one-time thermal exposure (cooling to room temperature) was investigated indicating that recrystallization and mechanical twinning can occur far easier than previously thought. First, it reveals that either the activation energy for recrystallization or the thermal stress in the packaging assembly is higher than expected. Second, it is found that deformation twinning in tin is promoted by hydrostatic pressure stress while dislocation glide is prevented in tin in the packaging assembly. Grain structure sensitivity on the cooling temperature, cooling rate, thermal history, solder composition, and assembly configuration are investigated.;Finally, fatigue tests on solder joints with polygrain and twin structure were carried out showing that recrystallization and twinning improve the fatigue resistance of the solder joints by more than 50%. In summary, first, this research is scientifically important that it gives a new understanding on the mechanism of deformation twinning promoted by hydrostatic pressure stress in tin and tin-alloys.