Reaction kinetics of solder-balls with pads in BGA packages during reflow soldering

The Au/Ni/Cu three-layer structure is one of the most common solder-bah pad finishes for the ball-grid-array (BGA) packages. The first layer, which is to be in direct contact with the solder, is a 1-mu m Au layer. Beneath the Au layer is the Ni layer, whose thickness is about 7 mu m. The Cu layer is part of the internal wiring of a EGA package. In this study, eutectic PbSn solder-balls were reflowed on the Au/Ni/Cu pads at 225 degrees C for reflow times from 7.5 s to 1003 s. It was found that the Au layer reacted very quickly with the solder to form AuSn4 and AuSn2. The growth rate of AuSn4 + AuSn2 was very high, approaching 1 mu m/s. When the reflow time reached 10 s, all the Au had been consumed, and AuSn2 had been converted to AuSn4. Moreover, 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 s, all AuSn4 grains had left the interface and a thin layer of Ni3Sn4 formed at the solder-Ni interface. The growth rate of this Ni3Sn4 layer was very low, reaching only 6 mu m for 1003 s of reflow. This study showed that during reflow the Au layer reacted with Sn to form AuSn4 first, and then broke off and fell into the molten solder. In other words, the Au layer did not dissolve into the molten solder directly during reflow.