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Failure mechanisms related to gold bonding were determined using the scanning electron microscope, Auger electron spectroscopy, X-ray energy dispersive spectroscopy, electron microprobe analysis, and deuteron probe analysis. Power transistors from different lots were analyzed; there were four types of bond defects. Type I defect is darkened carbon inclusions in the bond area. Type II defect is a carbon buildup region on the posts probably in the form of graphite particles which adhered to the gold plating during processing. Type III defects are adsorbed carbon surface films. Type IV defects are gross discontinuities in the gold plating. Auger electron spectroscopy analysis and electron probe analysis proved that defects I, II, and III are subsurface and that further exposure to time-temperature and stress will result in carbide precipitates and hence cracks at the interface. Deuteron probe (DP) analysis of the same bond area has shown the presence of subsurface carbon (the predominant contaminant) thus verifying the Auger results. Contaminants such as manganese, iron boron, cobalt, nickel, chlorine, and sulfur were detected by energy dispersive X-ray analysis in concentrations of 100 ppM or more. These contaminants result in microcracks and voids which are formed by a time-temperature process. All bond-pull data had a bimodal distribution similar to that reported by Horsting. The bimodal distribution of pull-strength, carbon subsurface inclusions, and excessively high levels of contamination (from the plating bath), all indicate that a contamination-related failure mode exists.
Date of Publication: Aug. 1980