Effects of Cu contents in flux on microstructure and joint strength of Sn-3.5Ag soldering with electroless Ni-P/Au surface finish

The effects of additive amounts of Cu(II) stearate in flux on the interfacial microstructure and joint strength between Sn-3.5Ag solder and an electroless nickel immersion gold (ENIG) pad were investigated. The content of the copper compound used in the flux was 0 wt.% (C-0), 20 wt.% (C-20) and 40 wt.% (C-40). The amount of the deposited Cu on an ENIG surface finish at 210 degrees C depended on the content of Cu(II) stearate in the flux. Microstructural observations of the joint interfaces showed that the thickness of the P-rich layer became thinner with increasing amounts of the Cu compound in the flux used. The remarkable reduction in the growth of P-rich layer observed at the C-40 joint interface was attributed to the high concentration of Cu precipitates at the joint interface. In addition, a difference in the morphologies of the interfacial intermetallic compounds (IMCs) was observed among the fluxes used. Qualitative analyses of the joint interface revealed that a thick IMC layer of C-0 joint consisted of Ni3Sn4. On the other hand, Cu-bearing fluxes C-20 and C-40 produced ternary Ni-Cu-Sn IMCs at the joint interface. In the case of C-20, the joint interface was composed of a thin (Ni,Cu)(3)Sn-4 layer with peninsular particles of (Cu, Ni)(6)Sn-5. When using C-40, the faceted-like (Cu, Ni)(6)Sn-5 IMCs were predominantly observed at the joint interface. The formation of the Ni-Cu-Sn IMCs indicated the intervention of Cu from a flux in the interfacial reaction during soldering. Besides, the high concentration of Cu deposited on an ENIG tended to preferentially form the Cu-rich interfacial IMCs, (Cu,Ni)(6)Sn-5. According to the results of shock strength tests, the joint strength improved with increasing amounts of the Cu compound in the flux. The minimum shock strengths for the Sn-3.5AVENIG joints formed by the use of C-0, C-20 and C-40 were 11.4 N, 17.2 N and 19.8 N, respectively. According to the observation of the fracture surfaces, the high reliability obtained by the use of C-40 flux was attributed to the formation of a quite thin P-rich layer at the joint interface. Consequently, it can be concluded that additive amounts of Cu compound in the flux have a significant impact not only on the growth of the interfacial reaction layer but also on the joint reliability between Sn-3.5Ag and an ENIG surface finish. (C) 2012 Elsevier Ltd. All rights reserved.