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

The present work studies the microstructure and microstructural evolution o f small volumes of nominally eutectic Au-Sn solder on Cu. The study include s 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 tha t join Cu-plated ceramic and polyimide substrates. The results show that th e microstructure is strongly affected by the addition of Ou from the substr ate during reflow, which produces a thick intermetallic layer along the int erface. In the case of the joints, normal processing produces a coarse micr ostructure that includes only a few grains between thick intermetallic coat ings. Aging at high temperature causes a further monotonic increase in Cu c ontent, which alters the intermetallic structure at the inter faces and cal l lead to intermetallic bridging across the joint. Thermal fatigue tests su ggest that cyclic deformation breaks up the intermetallic structure, increa sing the rate of Cu addition to the joint, but refining the apparent grain size.