The microstructure of ultrafine eutectic Au-Sn solder joints on Cu

The present work studies the microstructure and microstructural evolution of small volumes of nominally eutectic Au-Sn solder on Cu. The study includes solder bumps 140-145 mu m in diameter and 55-65 mu m tall deposited on Cu-plated Sit and solder joints 60 mu m in diameter and 25 mu m in height that join Cu-plated ceramic and polyimide substrates. The results show that the microstructure is strongly affected by the addition of Ou from the substrate during reflow, which produces a thick intermetallic layer along the interface. In the case of the joints, normal processing produces a coarse microstructure that includes only a few grains between thick intermetallic coatings. Aging at high temperature causes a further monotonic increase in Cu content, which alters the intermetallic structure at the inter faces and call lead to intermetallic bridging across the joint. Thermal fatigue tests suggest that cyclic deformation breaks up the intermetallic structure, increasing the rate of Cu addition to the joint, but refining the apparent grain size.