On the Physics of Purple-Plague Formation, and the Observation of Purple Plague in Ultrasonically-Joined Gold-Aluminum Bonds

The gold wire-aluminum thin-film joint is treated as a diffusion couple at low concentration, but above dilute levels. Electrical resistance of the joint is directly related to the solute concentration and, therefore, is a measure of the thermal annealing process. Annealing temperatures are related through the diffusion length Dt. Effects of temperature and time can be superimposed thereby. Experimental results on ultrasonically-joined gold wirealuminum thin films showed resistance increase with decreasing bond strength. From an Arrhenius-type plot at 105°C, 156°C, and 203°C, the apparent activation energy for thermal annealing of ultrasonically-joined gold wire-aluminum thin-film bonds is found to be 12.8 kcal/mole. The diffusion model is independent of the method by which the two metals are joined, however.