Comparative study between Sn37Pb and Sn3Ag0.5Cu soldering with Au/Pd/Ni(P) tri-layer structure

The soldering reactions between an Au/Pd/Ni(P) (0.1 μm/0.2 μm/7 μm) tri-layer structure and two kinds of solders (Sn37Pb and Sn3Ag0.5Cu) were investigated in this study. In the Sn37Pb soldering system, it was found that the Au layer could disappear within 5 s, which exposed the underlying Pd to form a (Pd,Ni)Sn4 layer. The (Pd,Ni)Sn4 then broke off from the roots of the grains and spalled into the solder after soldering for 15 additional seconds. In turn, the Ni(P) came in contact with the molten Pb–Sn, forming the intermetallics Ni3Sn4 and Ni3P. The Ni3P formed a uniform layer that was adjacent to the Ni(P), while the Ni3Sn4 exhibited needle- or chunk-like grains that were scattered inside the solder, instead of the layered-type morphology that appears in most eutectic Pb–Sn/Au/Ni(P) systems. Interestingly, the series of soldering reactions changed dramatically when the Sn37Pb was replaced with Sn3Ag0.5Cu. First, both the Au and Pd were quickly exhausted within 5 s. Subsequently, a dense (Cu,Ni)6Sn5 rather than the scattered Ni3Sn4 became the dominant reaction product that formed adjacent to the solder. Additionally, a thinner Ni3P formed at the interface, specifically for the regions where were covered with the (Cu,Ni)6Sn5. The sluggish Ni3P can be attributed to the relatively low Ni consumption caused by forming a dense (Cu,Ni)6Sn5 layer over the Ni(P). A more detailed analysis on the growth of Ni3P in these two soldering systems is provided in this study.