The microstructure of eutectic Au-Sn solder bumps on Cu/electroless Ni/Au

In this work we studied the initial microstructure and microstructural evolution of eutectic Au-Sn solder bumps on Cu/electroless Ni/Au. The solder bumps were 150-160 mum in diameter and 45-50 mum tall, reflowed on Cu/electroless Ni/Au, and then aged at 200 degreesC for up to 365 days. In addition, Au-Ni-Sn-alloys were made and analyzed to help identify the phases that appear at the interface during aging. The detailed interfacial microstructure was observed using a transmission electron microscope (TEM). The results show that the introduction of Au from the substrate produces large islands of zeta -phase in the bulk microstructure during reflow. Two Au-Ni-Sn compounds are formed at the solder/substrate interface and grow slowly during aging. The maximum solubility of Ni in the phase was measured to be about 1 at.% at 200 degreesC, while Ni in the delta -phase is more than 20 at.%. The electroless Ni layer is made of several sublayers with slightly different compositions and microstructures. There is, in addition, an amorphous interaction layer at the solder/electroless Ni interface.