Failure mechanism and quality evaluation in micro-resistance spot welds of braided Ag-plated Cu conductor to Cu interconnector

The service stability of flexible solar arrays relies on high-quality joints between braided Cu conductors and ultrathin interconnectors, which can be efficiently achieved through Micro-Resistance Spot Welding (MRSW) process. However, this unconventional MRSW joint, due to the discrete and multi-body nature of braided conductor, possesses a morphology that significantly differs from traditional Resistance Spot Welding (RSW) joints, resulting in a lack of effective standards for evaluating its mechanical performance, thereby hindering its widespread application in large-scale manufacturing. We introduce a double-sided MRSW process for joining Ag-plated Cu conductors and Cu interconnectors and, for the first time, proposes a quantitative evaluation method to elucidate the failure process and fracture mechanisms of these discrete, complex joints. The approach entails the analysis of the macro- and micro-structures of the joints under different heat inputs to clarify the bonding mechanism evolution: from single solid-state bonding interfaces to composite interfaces of brazing and solid-state bonding, and eventually to the composite of fusion zone and brazing interfaces. The corresponding failure modes transitioned from interfacial fracture to irregular plug fracture and ultimately to plug fracture. In light of these findings, a critical quality indicator dominated by fusion and brazing connections was identified, and a quantitative mechanical performance evaluation model based on the valid connection length of the joint was established. Experimental results demonstrate that the proposed model accurately predicted bonding strength based on the joint quality indicator, achieving a Root Mean Square Error (RMSE) of 2.736 N for peak shear strength prediction. This study paves the way for developing quality assessment criteria for MRSW joints between braided Ag-plated Cu conductors and Cu interconnectors.