Comparative study of Au/Pd/Ni(P) surface finish in eutectic PbSn and Sn3Ag0.5Cu soldering systems

Abstract

Recently, the Au/Pd/Ni(P) tri-layer has become one of the promising replacements for the Au/Ni(P) surface finish in array-array packaging applications. The key motivation for this developed trend is the additional Pd layer can prevent the Ni(P) from the galvanic hyper-corrosion that has long been recognized to be the root cause of “black pads”. In the present study, the solderability of the Au/Pd/Ni(P) [0.1(±0.03) µm /0.2(±0.02) µm /7 ±m in thickness] are evaluated using Sn37Pb and Sn3Ag0.5Cu for various soldering times of 5 – 300 seconds. Comparison of the results shows that the interfacial reactions in both soldering systems are strong time dependence. During Sn37Pb soldering, it is found that the Au layer can disappear in 5 seconds, exposing the underlying Pd to solder as a (Pd,Ni)Sn4 layer. The (Pd,Ni)Sn4 then breaks off at the roots of grains and spall into the solder after soldering for 15 additional seconds. In turn, the Ni(P) would contact with solder and forms a discontinuous, chunk-like Ni3Sn4. Beneath the Ni3Sn4, there is a layer of Ni3P. Interestingly, the reactions change dramatically when the Sn3Ag0.5Cu replaces the Sn37Pb for soldering. Firstly, both Au and Pd can be depleted by Sn3Ag0.5Cu in the beginning 5 seconds of soldering. Additionally, a dense (Cu,Ni)6Sn5 rather than Ni3Sn4 becomes the dominant reaction product over the Ni(P). Another interesting difference is that a much thinner Ni3P forms at the interface excepting the region that the Ni(P) is direct in tough with the solder. The variation in Ni3P can be attributed to a lower Ni consumption by forming a dense (Cu,Ni)6Sn5 than a scattered Ni3Sn4.