Reflow soldering and isothermal solid-state aging of Sn-Ag eutectic solder on Au/Ni surface finish

The reaction between the eutectic Sn-3.5Ag solder and the Au/Ni surface finish during reflow as well as during isothermal aging was studied. The Au layer was electroplated and had a thickness of one pm. The peak reflow temperature was fixed at 250 degreesC while the reflow time was varied between 10 sec and one h. Samples that went through 90 sec reflow time were then subjected to 160 degreesC isothermal aging for up to 875 h. It was found that during reflow the Au layer reacted very quickly with the solder to form AuSn4, One mum of Au layer was consumed in less than 10 sec. As the aging time increased, AuSn4 grains began to separate themselves from the Ni layer at the roots of the grains and started to fall into the solder. When the reflow time reached 30 sec, all the Au intermetallic had left the interface, and Ni3Sn4 started to form at the interface. The Ni3Sn4 growth rate followed linear kinetics initially (< 240 sec), but the growth rate slowed down afterward. During the isothermal aging, only a small amount of (AuxNi1-x)Sn-4 resettled back to the interface, and a continuous (Au0.45Ni0.55)Sn-4 layer did not form at the interface, unlike the case for the Sn-37Pb solder. This is an important advantage for Sn-3.5 Ag over Sn-37Pb because a continuous (Au0.45Ni0.55)Sn-4 layer inevitably will weaken a solder joint. Our observation indicated that many (AuxNi1-x)Sn-4 particles were trapped by the Ag3Sn particles, and were hindered frofrom resettling back to the interface.