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A new multilayer metallization, ENEPIG (electroless nickel electroless palladium immersion gold) with 0.1μm thin Ni(P) layer (thin-ENEPIG), was plated on a Cu PCB substrate for fine-pitch package applications. We evaluated interfacial reactions and mechanical reliability of a Sn-3.0Ag-0.5Cu (SAC305) solder alloy on a thin ENEPIG coated substrate during various reflow times. In the initial soldering reaction, (Au,Cu)Sn4 intermetallic compound (IMC) formed at the SAC305/ENEPIG interface. After prolonged reflow reactions, the Pd and Ni layers were consumed, and (Cu,Ni)6Sn5 IMC formed on the Cu layer. As the reflow time increased, the Cu and Ni contents in (Cu,Ni)6Sn5 IMC increased and decreased, respectively, due to the limited Ni layer in the ENEPIG plating layer. In the low-speed shear test, all fractures occurred in the bulk solder regardless of reflow times. In the high-speed shear test, the fracture mode was changed from ductile to brittle with increasing reflow time, due to the formation of the thick interfacial IMC.

Interfacial reactions and mechanical strength of Sn-3.0Ag-0.5Cu/0.1㎛-Ni thin ENEPIG solder joint