Formation and resettlement of (Au_xNi_1−x)Sn₄ in solder joints of ball-grid-array packages with the Au/Ni surface finish

Abstract

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°C, and the reflow time was 115 s. Each aging process was performed at 160°C for 500 h. After the first reflow, all the Au would disappear from the interface, and formed many (Au_xNi_1−x)Sn₄ 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 µm) at the interface. After one reflow and one subsequent aging, most of the (Au_xNi_1−x)Sn₄ would relocate from inside the solder joint to the interface, and the value of x for (Au_xNi_1−x)Sn₄ at the interface decreased to 0.45. This (Au_xNi_1−x)Sn₄ resettlement process repeated itself for additional reflow-aging cycles. More reflow-aging treatments, however, made the microstructure of (Au_0.45Ni_0.55)Sn₄ at the interface become more non-planar. It was shown that gravitational effect was not the driving force for the resettlement of (Au_xNi_1−x)Sn₄. It is proposed that the driving force is for (Au_xNi_1−x)Sn₄ 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 (Au_xNi_1−x)Sn₄ by dissolving more Ni. A decomposition-diffusion mechanism is proposed to explain what happened. Kinetic rationales for this rapid resettlement of (Au_xNi_1−x)Sn₄ at such a low temperature were also discussed.