Joints Soldered on Electroless Ni–Au Surfaces Using Cu-Containing Flux: Strength, Microstructure and Mechanism of Improvement

Abstract

The critical issue for soldering on electroless Ni-Au surface finishes is fragile solder joints. Previous studies have indicated that these weak joints are a result of the formation of a P-rich layer at the joint interface. In this study, the new flux, which contains a Cu compound, demonstrates improved solder joint strength for solder ball attachment on electroless Ni-Au surface finishes. Cross-sectional analysis revealed that this new flux gives a thinner P-rich layer at the joint interface than that seen with a conventional flux. Moreover, the formation of a uniformly thin Cu-Sn intermetallic layer on the Ni-P plated surface is observed after reflow soldering. We conclude that this Cu-Sn layer formation during reflow impedes extra diffusion of Ni into solder from the plating surface so that the growth of a P-rich layer can be effectively inhibited and, thus, the joint strength is improved.

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

“…As a way to decrease the growth of a P-rich layer, the authors of this current study previously reported that the use of a flux containing 10 wt.% of Cu(II) stearate could suppress Ni diffusion in a Sn-based solder on an ENIG surface finish [9][10][11]. Due to their tendency to ionize, Cu ions derived from Cu(II) stearate would be reduced through a substitution reaction with Ni, as described by Eq.…”

Joint Strength and Microstructure for Sn-Ag-(Cu) Soldering on an Electroless Ni-Au Surface Finish by Using a Flux Containing a Cu Compound

“…18 In this case, the substitution reaction between Ni and Cu, as described in Eq. 1, is utilized in forming a Cu layer that serves as a temporal barrier on the Ni-P plating.…”

“…18 Because of the presence of a Cu resource near the joint interface, a flux that contains Cu(II) stearate, known as barrier flux, is believed to form an effective Cu barrier layer before the solder melts on the conventional electroless Ni-P surface finish. This Cu layer contributes to the suppression of excess diffusion of Ni into the solder, and hence the formation of a P-rich layer at the joint interface is reduced.…”

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

“…Therefore, to resolve this, the present authors applied the notable barrier function of Cu to the property of flux used for the BGA solder ball attachment on electroless Ni-plated surface finish in our previous studies. [17][18][19] The addition of the Cu compound, copper(II) stearate, to the flux controlled the growth of both intermetallic compounds and the P-rich layer at the joint interface in several soldering systems, such as those using Sn-Pb, Sn-Ag, and Sn-Ag-Cu. Observation of the microstructure revealed that refined intermetallics, as well as a thin P-rich layer, were formed at the joint interface when using the flux containing the Cu compound.…”