Reduced Gold-Plating on Copper Leads for Thermocompression Bonding--Part II: Long Term Reliability

This two-part series is an evaluation of the acceptability for thermocompression (TC) bonding of Cu leads having reduced thicknesses of Au-plating at the bond sites. Part I was concerned with initial characterization which made use of recent grain boundary diffusion work in the Cu-Au system. In this second part, long term reliability is considered where use is made of recent work in volume interdiffusion in the Cu-Au system. Three properties are evaluated: 1) Long term reliability based on accelerated aging of bonded leads; 2) temperature cycling; and 3) cyclic 45° bend fatigue. Results of these evaluations are the following. 1) A lifetime of greater than 40 years at 50° C is predicted for Cu leads with as little as 0.7 µm Au-plating. This conclusion was reached using volume interdiffusion as the rate controlling mechanism for the observed decrease in strengths. The same conclusion was reached from a statistical analysis of the lifetime data without making recourse to a physical model for the degradation. 2) No degradation in bond strength was observed for up through 300 temperature cycles between -40 and +150°C. 3) Cyclic 45° bend fatigue of the bonded leads was not affected by the Au-plating thicknesses studied. In summary, when the results of both parts are combined, the overall conclusion is that oxygen-free Cu leads plated with as little as 0.7 µm Au are judged acceptable for TC bonding.