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
wt.% (C-), 2
wt.% (C-2) and 4
wt.% (C-4). The amount of the deposited Cu on an ENIG surface finish at 21
°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-4 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- joint consisted of Ni
3Sn
4. On the other hand, Cu-bearing fluxes C-2 and C-4 produced ternary Ni–Cu–Sn IMCs at the joint interface. In the case of C-2, the joint interface was composed of a thin (Ni,
Cu)
3Sn
4 layer with peninsular particles of (Cu,
Ni)
6Sn
5. When using C-4, the faceted-like (Cu,
Ni)
6Sn
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)
6Sn
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–3.5Ag/ENIG joints formed by the use of C-, C-2 and C-4 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-4 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.
