Volume effect on interfacial microstructure and mechanical properties of Ni(UBM)/Sn3.0Ag0.5Cu/Ni(UBM) joints

The effect of solder volume on the interfacial microstructure and mechanical properties of micro-scale line-type Ni/Sn3.0Ag0.5Cu/Ni joints was investigated. Ni wires of 300 μm in diameter were used as under-bump-metallization (UBM) to be connected by Sn3.0Ag0.5Cu solder balls. The solder joints were assembled by modeling the thermal cycle of the reflow process with the accurately controlled assembly joint gap (i.e., joint thickness or height) of 100, 75, 50 and 25 μm, respectively. The mechanical behavior of the joints was evaluated by a dynamic mechanical analyzer (DMA) under uniaxial tensile loading. The interfacial microstructure and fractographic morphologies of the joints were analyzed by SEM equipped with EDS. Results show that both (Ni,Cu)₃Sn₄ and (Cu,Ni)₆Sn₅ IMC phases formed at the interface of all the as-assembled solder joints regardless of solder volume or joint thickness, i.e., the solder volume has little influence on the interfacial reaction products. Furthermore, with decreasing the joint thickness, tensile strength of the joints increases obviously. Fractographic morphology analysis results indicate that there are three different failure modes, that is, ductile, quasi-brittle and brittle mode. The joints with thicknesses of 100 and 75 μm all failed in a ductile failure mode, the joints with a thickness of 50 μm failed in both ductile and quasi-brittle mode, while for the joints with a very small thickness of 25 μm, brittle fracture happened and quasi-brittle failure mode still possessed the most part.