Microstructural variation and high-speed impact responses of Sn–3.0Ag–0.5Cu/ENEPIG solder joints with ultra-thin Ni–P deposit

Electroless Ni–P/electroless Pd/immersion Au (ENEPIG) with ultra-thin Ni–P deposit serve as a potential replacement of traditional ENEPIG surface finish because of its superior electrical performance in flip chip solder joints interconnection. However, the interfacial reaction and mechanical reliability of solder joints in ENEPIG with ultra-thin Ni–P layer is not yet well evaluated. In this study, we investigated the characteristic microstructure of interfacial intermetallic compounds and high-speed impact responses of Sn–3.0Ag–0.5Cu/ENEPIG attachments with 4.8, 0.3, and 0.05 μm Ni–P deposit. ENEPIG with Ni–P layer of 0.3 μm exhibited the eutectic structure dispreading in the solder alloys and layer-type P-rich IMCs at solder/metallization interface, while there was (Cu,Ni)6Sn5 precipitation in the solder but no P-rich IMCs layer formed in ENEPIG with 0.05 μm Ni–P layer. Slower interfacial reaction rate in ENEPIG with 0.3 μm Ni–P layer was attributed to the effect of electroless Ni–P diffusion barrier layer, which would further provide better impact resistivity than that of ENEPIG with 0.05 μm Ni–P deposit. Moreover, breach in P-rich IMCs and underneath (Cu,Ni)6Sn5 patch were observed in ENEPIG with 0.3 μm Ni–P layer. The growth mechanism was closely related to the Ni diffusion from surface finish and element redistribution.