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Effects of Zn-Bearing Flux on Joint Reliability and Microstructure of Sn-3.5Ag Soldering on Electroless Ni-Au Surface Finish

Joint reliability and microstructure for Sn-3.5Ag soldering on an electroless Ni-P/Au surface finish were investigated, using the flux bearing Zn(II) stearate. The content of the zinc compound in the flux varied from 0 mass% to 50 mass%. The results of both shock and pull strength tests for as-reflowed solder joints showed that Zn-bearing fluxes gave higher joint strength than did flux without Zn compound. Additionally, the use of flux containing 50 mass% of Zn(II) stearate provided the smallest reduction rate in pull strength in regard to aging treatment at 150°C for 1000 h. According to the microstructure of the aged joints, both an interfacial intermetallic layer and a P-rich layer of the joint obtained by the use of the flux without Zn(II) stearate became thick with prolonged aging. On the other hand, a slow growth of the interfacial reaction layer was observed for the joint with Zn(II) stearate in flux during annealing. It is presumed that the formation of the interfacial intermetallic compound with Zn at reflow works effectively in suppressing the diffusion of Ni into the solder matrix during subsequent aging, and this can maintain a relatively high joint strength for those joints with Zn(II) stearate in flux. After damp heat treatment with 85°C/85% RH up to 500 h, no sign of corrosion was observed in either joint. Moreover, there was no significant change of the microstructure, nor of the pull strength, with or without Zn(II) stearate in flux.