Effects of Zn-Containing Flux on Sn-3.5Ag Soldering with an Electroless Ni-P/Au Surface Finish: Microstructure and Wettability

Abstract

The microstructure resulting from Sn-3.5Ag soldering on an electroless Ni-P/Au pad using flux containing Zn(II) stearate was investigated. The content of zinc compound in the flux was 0 wt.% (Z-0), 20 wt.% (Z-20) or 50 wt.% (Z-50). A study of the interfacial microstructure revealed that both Z-20 and Z-50 fluxes yielded a thinner P-rich layer at the interface than did the Z-0 flux. In addition, compared with the bulky Ni–Sn intermetallics of the Z-0 joint interface, refined interfacial intermetallic compounds (IMCs) were observed when using Zn-containing fluxes, Z-20 and Z-50. Based on qualitative analyses of both Z-20 and Z-50 joint interfaces, it was presumed that their intermetallic layers would consist of Ni, Zn, and Sn. Additionally, the Ni content in the IMC layer of the Z-50 joint was lower than that of the Z-20 joint. Electron probe microanalysis (EPMA) of the initial Z-50 joint interface revealed Zn in the interfacial reaction layer, suggesting that Zn participated in the reaction between solder and the surface finish at an early stage of soldering. Consequently, the supply of Zn from the flux diminished Ni diffusion into the molten solder during heating. This effect may have caused a thin P-rich layer to form at the joint interface.