Effective (Pd,Ni)Sn4 diffusion barrier to suppress brittle fracture at Sn-58Bi-xAg solder joint with Ni(P)/Pd(P)/Au metallization pad

초록

In this study, we investigated the interfacial reactions and mechanical properties of Sn-58Bi solders alloyed with Ag nanoparticles (NPs) after bonding with electroless nickel-immersion gold (ENIG) or electmless nickelelectroless palladium-immersion gold (ENEPIG) surface finish. On the ENIG surface finish, Ni3Sn4, Ni-Sn-P, and Ni3P layers were sequentially formed on the Ni(P) layer. On the ENEPIG surface finish, (Pd,Ni)Sn-4 intermetallic compound was formed on thin Ni3Sn4/Ni3P on the Ni(P) layer, and it survived nine reflow cycles, unlike Sn-Ag-Cu solder joints in previous studies. This effectively stopped the formation of the Ni-Sn-P layer, which acted as an effective diffusion barrier to suppress the consumption of Ni in the Ni(P) layer. A mixed-mode fracture-partially through the bulk solder (ductile mode) and partially through Kirkendall voids in the Ni-Sn-P layer (brittle mode)-was observed with the Sn-58Bi-xAg/ENIG joint, whereas only ductile fracture was observed with the Sn-58Bi-xAg/ENEPIG joint. Alloying with 0.5 wt% Ag NPs strengthened the composite solder by refining the eutectic microstructure of the Sn-58Bi solder and by inducing precipitation hardening with fine Ag3Sn particles. However, the addition of more Ag NPs led to mechanical degradation by re-coarsening of the eutectic microstructure and by providing an easy fracture path along the coarsened Ag3Sn/solder interface. The highest shear strength was obtained with Sn-58Bi-0.5Ag/ENEPIG, which was suggested to be the most reliable system for low-temperature soldering at 200 degrees C.

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