Contact resistance degradation in z-axis connectors operated at burn-in temperatures

Elevated temperature tests were performed on molybdenum wire button z-axis socket assemblies in an effort to develop and qualify a high footprint density burn-in socket for pinless modules. Stress tests were performed at 200°C, with periodic mating and unmating of the substrate from the socket. These tests revealed that after 172 hours of stress, the mean contact resistance increased by over an order of magnitude. Individual measurements of the stressed circuit board, socket and substrate revealed that the substrate contributed the majority of the observed resistance increase and that the degradation was due to diffusion and oxidation of the Cr/Cu/Ni/Au pad metallurgy on the pinless ceramic substrates. The resistance contributions of the socket and circuit board pads were also analyzed and various wire button contact metallurgies were evaluated in order to minimize their role in the degradation. Overall, this work demonstrates the importance of maintaining the integrity of all metallurgical interfaces when using z-axis interconnects at elevated temperatures