As an important constitutional part of Flip-Chip technology, under bump metallization have played a crucial role in reliability of FC packaging. Although electroless Ni(P) UBM have good solderability and lower IMC growth rate with SbPb solder, it have met difficulties to meet with lead-free solders. In lead-free technology, because the reflow temperature is high, the growth rate of IMC between solder and Ni(P) is rapid, and will eventually cause interfacial brittleness. Besides the consideration of soldering process, mechanical properties for more reliable performance have to be taken into account. Thermal expansion coefficient of FeNi alloy is quite close to the die made of silicon. This jointed couple can induce less thermal stress. Thus, FeNi alloys would be a good choice to meet the requirement of thermal match in connecting with silicon die.
In this study, wetting balance method was employed to evaluate the solderability of electro-plated NiFe alloys with and without immersion gold. The effects of iron concentrations in the substrates and the thickness of gold films on wetting performances were discussed.
The wetting balance test was clearly enough to show that the immersion gold could enhance the solderability of NiFe alloys dramatically. The solderability of high iron concentration plating, Ni-71Fe, which could not be wetted without immersion gold plated was turn out to have good solderability with immersion gold plated.
The same as platings without immersion gold, the solderability of NiFe(Au) platings were affected by iron concentrations in the substrates. From the test result of XPS on platings with immersion gold, it was probably because of the different surface oxidation degree of the platings which cause the differences of the solderability. Because Ni-71(Au) and Ni(Au) have much surface oxide than Ni-30Fe(Au) and Ni-47Fe(Au), so the solderability of former platings was poorer than the later.
By controlling the immersion gold time, gold films with different thicknesses were obtained. The longer the platings times the thicker the gold films were produced. With the presence of thicker immersion, the surface oxidation was greatly reduced. When the gold plating time is longer than ten minutes, the wetting performances become stable and do not change anymore.
The wetting frontier of the substrates with and without gold was observed by SEM. The AuSn IMC was found in the wetting frontier of NiFe with solder, which could be an evidence of the iron involved in the wetting reaction. The varied thicknesses of immersion gold have caused differences in the morphology of wetting frontiers. The thicker the immersion, the more AuSn IMC was found on the wetting frontiers, and less of FeSn IMC.
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