Comparative study of parallel gap resistance welding joints between different interconnected foils and GaAs solar cells: Microstructure and thermal reliability

Deployed in space orbits, solar cell arrays are subjected to daunting challenges posed by exceedingly harsh thermal environments. During their operational lifespan in space, potential failures at the joints between solar cells and interconnecting strips can adversely affect the longevity of solar cell arrays. To enhance the thermal reliability of solar cell joints in intricate space conditions, this study delved into the influence of thermal cycle on mechanical properties and microstructures of parallel gap resistance welding (PGRW) joints utilizing both silver (Ag) and Ag-plated Kovar foils. Operating at respective current densities of 410 A/mm2 and 420 A/mm2, these two types of foils exhibit solid-phase interfaces characterized by elemental diffusion. Interface of the joint of Ag foil experiences microscopic deformation and accumulated strain during the thermal cycle, with cracks initiating and propagating at the edges of joint interface. With an escalation in the number of thermal cycles, tensile-shear performance deteriorates, and the fracture morphology shifts from a ductile fracture with evident dimples to a deformation-intensive fracture attributed to thermal fatigue, draped in Ag grains. Conversely, the joints of Agplated Kovar foil demonstrate minimal alterations in mechanical properties, microstructures, and fracture morphology due to thermal cycling. This phenomenon reflects noteworthy thermal reliability spanning from -160 degrees C to 120 degrees C. The investigation into PGRW joints of Ag-plated Kovar foil holds significant implications for satisfying the high-reliability of solar cell arrays in hostile space environments.