Experimental SAC305 shear stress-strain hysteresis loop construction using Hall's one-dimensional model based on strain gages measurements

International audienceTemperature-induced solder joint fatigue is a main reliability concern for aerospace and military industries whose electronic equipment used in the field is required to remain functional under harsh loadings. Due to the RoHS directive which eventually will prevent lead from being utilized in electronic systems, there is a need for a better understanding of lead-free thermomechanical behavior when subjected to temperature variations. Characterizing solder joints properties remains a challenge as viscoplastic behavior during thermal cycling is complex, and their small dimensions prevent direct measures from being performed. This paper reports the experimentation based on strain gage measurements, allowing the construction of the shear stress-strain hysteresis loop corresponding to Sn3.0Ag0.5Cu (SAC305) solder joints behavior during thermomechanical loading. This methodology, initially developed in 1984 by P. Hall for Sn60Pb40 interconnects, allows the measurement of the strain energy density dissipated during temperature cycles. The approach developed in this study has two objectives: 1) conduct thermal cycles until failure in order to get the number of cycles to failure (N1%), 2) instrument a specific assembly with strain gages to plot the corresponding shear stress-strain hysteresis loop and allow experimental characterization of SAC305 solder joints. Custom daisy-chained 76 I/O Ceramic Ball Grid Array (CBGA76) components were designed and assembled on flame retardant (FR-4) multi-layered (8 copper ground planes) Printed Circuit Boards (PCB). The component and the PCB were optically characterized to measure their corresponding Coefficient of Thermal Expansion (CTE). Four strain gages were specifically placed at the center of the assembly on top and bottom faces of both PCB and CBGA76 component. As-reflowed solder joints were also investigated to ensure that the resulting microstructure and β-Sn grain morphology are representative of those usually observed in SAC305 solder joints after reflow. A slow rate thermal cycle profile was then considered to allow viscoplastic phenomena to occur and the corresponding SAC305 solder joints shear stress-strain hysteresis loop was plotted. Failure analysis revealed that Ag3Sn coarsening and recrystallization occurred which ensure the representativeness of the microstructural changes usually observed on failed SAC305 interconnections during thermomechanical damage. The correlation between the measured strain energy density and measured lifetime corresponds to one point of the energy based fatigue curve for SAC305 solder joints