Stress and Fatigue Analysis of Solder Joints Subjected to Extreme Aging for Normal and Over-molded PCBs

Electronic device applications in the automotive industry have expanded significantly during the past two decades. These electronic devices usually experience extreme temperature ranges (between -40°C to 140°C) from typical weather and day-to-night cycles or generated heat from the engine of the car. The electronic components inside these devices use solder alloys in their interconnection assembly. Thus, it is essential the interconnection assemblies won’t fail prematurely from the extreme temperature cycles. Therefore, it is critical to understand the material properties evolution of the solder alloys in the form of solder joints from thermal-cycle loads. Engineers and scientists have been conducting research in discovering methods to improve the lifespan of the solder joints exposed to thermal-cycle aging. In this study, overmolding was used as a potential method to improve the lifecycle of solder joints. Overmolding is an epoxy resin that can be injected molded onto the electronic components and solder joint by using a special mold. The performance of these samples was compared to the normal samples (without overmolding) and quantitatively measured. The prepared samples went under accelerated life test to mimic the extreme weather conditions that an automobile will experience in real life and to quicken the event. Multiple experimental techniques such as Knoop hardness, polarized light microscopy (PLM), x-ray diffraction (XRD), scanning electron microscopy- energy dispersive x-ray (SEM-EDS,) and high energy x-ray diffraction (HEXRD) were used to characterize the material properties and investigate the failure mechanism in the solder alloys exposed to extreme temperature ranges. Meanwhile, electrical impedance measurements are conducted on all the electronic modules to assess their reliability during the environmental test (thermal-mechanical loads). xiii In the results sections, the physics of fatigue and failure in solder alloys will be explained. This will include the dynamic recrystallization, grain coarsening of β-Sn and increase in the thickness of the primary intermetallic layer. Additionally, the mechanical property degradation of the solder joint from thermal cycle loads has been quantitatively measured in terms of hardness and elastic modulus. Furthermore, the improvement of the overmolding on the solder joint life cycle has been demonstrated using the characteristic lifetime from Weibull analysis. The results for an FEA model simulating a solder joint under extreme temperatures will be discussed, as well.