Formation and resettlement of (AuxNi1-x)Sn4 in solder joints of ball-grid-array packages with the Au/Ni surface finish

The interfacial reactions between eutectic PbSn solder and the solder ball pads with the Au/Ni surface finish were studied. Solder joints subjected to up to three repeated reflow-and-aging treatments were examined. For the reflow, the peak reflow temperature was 225 degrees C, and the reflow time was 115 s. Each aging process was performed at 160 degrees C for 500 h. After the first reflow, all the Au would disappear from the interface, and formed many (AuxNi1-x)Sn-4 particles inside the solder joints. The value of x was between 0.99 and 0.75. In addition, there was a thin layer of Ni3Sn4 (1.4 mu m) at the interface. After one reflow and one subsequent aging, most of the (AuxNi1-x)Sn-4 would relocate from inside the solder joint to the interface, and the value of x for (AuxNi1-x)Sn-4 at the interface decreased to 0.45. This (AuxNi1-x)Sn-4 resettlement process repeated itself for additional reflow-aging cycles. More reflow-aging treatments, however, made the microstructure of (Au0.45Ni0.55)Sn-4 at the interface become more non-planar. It was shown that gravitational effect was not the driving force for the resettlement of(AuxNi1-x)Sn-4. It is proposed that the driving force is for (AuxNi1-x)Sn-4 to seek Ni at the interface so that it can become more Ni-rich. In other words, the driving force is lowering the Gibbs energy of (AuxNi1-x)Sn-4 by dissolving more Ni. A decomposition-diffusion mechanism is proposed to explain what happened. Kinetic rationales for this rapid resettlement of (AuxNi1-x)Sn-4 at such a low temperature were also discussed.