Reactive wetting behavior of Ni-alloy platings was investigated by combining wetting measurements with interfacial analyses. Ni(P) platings were prepared by electroless deposition in which the P content varied up to 16.6at.%. Wetting balance measurements showed that P content in the as-deposited plating had little effect on wetting by the Sn-3.8Ag-0.7Cu lead-free solder. However, when the P-rich plating was annealed, wetting force was reduced and wetting kinetics slowed. Such degradation in wetting properties was shown to result from precipitation of Ni3P particles, which inhibited dissolution of the Ni-plating by the liquid solder.
Different from the non-reactive element of P, the addition of Fe in the Ni-based substrate not only changed the type of IMCs between substrate and solder but also enhanced the wetting property of the substrate. When Fe content was greater than 29% but less than 65%, NiFe platings had better wetting property than pure nickel plating. By SEM and EDX analysis, Ni3Sn4 formed at the interface between liquid solder and pure Ni plating, while FeSn2 formed at the interface between liquid solder and NiFe substrate. The improved wetting on Ni-Fe platings is believed to be related to the greater driving force for interfacial reaction associated with FeSn2 formation.
The FeSn2 compound formed during solder reflow was much finer than Ni3Sn4 and grew into a densely packed thin continuous layer at the interface. The coarse Ni3Sn4 grew non-uniformly at the interface at a faster rate than the growth rate of FeSn2.
The wetting property of the NiFe platings with Fe content more than 65% was much worse than that of platings with less Fe content with the wetting property of pure Fe platings being the worst. XPS analysis indicated that the oxide and hydroxide layers in the samples with higher Fe contents were much thicker than those with lower Fe contents. The oxide coverage on the pure Fe plating was thickest. Thus, the decline of solderability of platings with higher Fe contents resulted from the thicker oxidation and hydroxid layer on the surface.
For the solid reaction of eutectic SnBi solder with various Ni-alloy platings during aging at different temperatures (60~120℃), the IMC formed on pure Ni and pure Fe platings was Ni3Sn4 and FeSn2 , respectively, while on NiFe plating the interfacial IMC was a layer of FeSn2 adjacent to the substrate and some separated scallop-shaped Ni3Sn4 above the FeSn2 layer. In all Fe-containing platings, the reaction rates of the substrates with the solder were much lower than that of pure Ni plating. The reduced reaction rate was directly related to the different IMCs formed on different substrates.
修改评论