Reduced Gold-Plating on Copper Leads for Thermocompression Bonding-Part I: Initial Characterization

In this two part series, Au-plated Cu leads with reduced Au thicknesses are evaluated for thermocompression (TC) bonding. Part I is concerned with initial characterization, which makes use of recent measurements of grain boundary diffusion coefficients (D') of Cu through Au. Part II considers the problem of predicting the long term reliability (40 years at 50°C) of these TC bonds, using recent measurements of the volume interdiffusion coefficients (D). Oxygen-free Cu leads plated with as little as 0.7 µm Au were compared to the more typical plating thickness of about 2.5 µm. The properties evaluated in Part I are: 1) initial TC bondability; and 2) prebond shelf life, i.e., TC bondability after storage of the Au-plated leads. Results of these evaluations are the following. 1) Leads plated with as little as 0.7 µm Au can have acceptable initial TC bond strengths especially when bonded at 30 to 50 percent deformation. 2) High-temperature storage of the leads followed by TC bonding indicates a shelf life of greater than two years at 50°C. This is in agreement with predictions based on grain boundary diffusion of Cu through Au: D' = 8 x 10-5exp (-0.91 eV/kT) [cm2/s]. Thus the initial characterization was successful for Au thicknesses down to 0.7 µm.