Comparative study of ENEPIG and thin ENEPIG as surface finishes for SAC305 solder joints

Abstract

A new multilayer metallization, electroless-nickel electroless-palladium immersion gold (ENEPIG) with a thin 0.1-μm-thick Ni(P) layer (thin-ENEPIG), was plated onto a Cu printed circuit board substrate for fine-pitch package applications. We evaluated the interfacial reactions and mechanical strengths of Sn–3.0Ag–0.5Cu (SAC305) solder on thin ENEPIG-coated substrates over various reflow times and compared them to those of a conventional ENEPIG-coated substrate with a 6-μm-thick Ni(P) layer. Thin Au, Pd, and Ni layers on the thin ENEPIG substrates were exhausted during the initial reflow time, which brought the underlying Cu layer in direct contact with the molten SAC305 solder, resulting in the formation of scallop-shaped (Cu,Ni)6Sn5 intermetallic compounds (IMCs) at the interface. The interfacial IMC layer for the thin ENEPIG substrate was thicker than that for the normal ENEPIG substrate due to the direct contact between the SAC305 solder and the Cu layer. In the low-speed shear test, all the fractures occurred in the bulk solder regardless of the different substrates and reflow times. In the high-speed shear test, the fracture mode was changed from ductile to brittle on increasing the reflow time. The P-rich Ni layer and thick Cu–Sn IMC formation deteriorated the shear strengths of the normal and thin ENEPIG joints, respectively. The thin ENEPIG joints showed better mechanical strength in the solder joint than the normal ENEPIG joints, despite the thick interfacial IMCs.