Abstract
Finding new under bump materials (UBM) is of significant importance with the shrinkage of pitch size in advanced high density packaging. FeNi alloy has been found to be a good UBM candidate for its slower interfacial reaction during reflow and aging. However, solderability of FeNi has to be improved. 58Fe42Ni and 92Fe8Ni alloy platings with electrodeposited tin finish were investigated by wetting balance method in a SnAgCu liquid bath. Results showed that tin finish can reduce the initial wetting time t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> of 58Fe42Ni and change the non solderable 92Fe8Ni to be the same wetting performance as 58Fe42Ni. The maximum wetting force of tin finished both FeNi alloys were larger than the bare alloy. However, after the maximum was reached, a slight decline of the wetting force was observed. The decline depended on the thickness of tin finish, with the thicker tin finish resulting in a higher steady plateau on the wetting force curve.
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