Mechanisms of Contact Formation and Electromigration Reliability in Wirebond Packages

This paper concerns the microstructural mechanism responsible for the variation seen in the contact resistance of ball bonds made on Al pads with wire materials and dimensions and discusses its potential impact on the contact properties related to the physical reliability of wirebond packaging. Specifically, this paper reports the characterization on the contact resistance, a reflection of contact area, with wire dimension and wire materials (Cu and Pd coated Cu), and their susceptibility to the electromigration (EM) induced failures. This study is motivated by a belief that the property of contact interface is the one of leading factors affecting the overall reliability of the bond especially against delamination and EM, yet related studies are critically lacking. To solve this problem, we conducted a comprehensive characterization study on the contact resistance and the ball microstructure and related those to samples which were evaluated under EM loads. This study reveals that one of the critical contributing factors to the contact formation is the work-hardening property of the ball because it limits the level of plastic deformation that occurs during the ball bonding process. This is supported by the fact the contact formation in PCC (Pd coated Cu) has a high contact resistance since Pd alloying in the ball leads to the formation of a smaller contact area because the solid solution hardening causes the ball to attain a high work-hardening rate, and thus resist against deformation. We believe that this reduction in the contact area may cause the bond interface formed by PCC to have a lower adhesion strength and also possibly be more susceptible to EM failure, unless the contact is made with a greater force. We believe that the correlation between the strain hardening and the contact area found in our study can provide guidance for wirebonding with enhanced reliability.