In this paper, we evaluated the solderability of thin electroless nickel-electroless palladium-immersion gold (ENEPIG) plating layer for fine-pitch package applications. Firstly, the wetting behavior, interfacial reactions, and mechanical reliability of a Sn-3.0Ag-0.5Cu (SAC305) solder alloy on a thin ENEPIG coated substrate were evaluated. In the wetting test, maximum wetting force increased with increasing immersion time, and the wetting force remained a constant value after 5 s immersion time. In the initial soldering reaction, (Cu,Ni) 6 Sn 5 intermetallic compound (IMC) and P-rich Ni layer formed at the SAC305/ENEPIG interface. After a prolonged reaction, the P-rich Ni layer was destroyed, and (Cu,Ni) 3 Sn IMC formed underneath the destroyed P-rich Ni layer. In the high-speed shear test, the percentage of brittle fracture increased with increasing shear speed.
Thermo-compression bonding (TCB) properties of Cu/SnAg pillar bumps on electroless palladium immersion gold (EPIG) were evaluated in this study. A test chip with Cu/SnAg pillar bumps was bonded on the surface-finished Cu pads with the TCB method. The surface roughness of the EPIG was 82 nm, which was 1.6 times higher than that of the ENEPIG surface finish because the EPIG was so thin that it could not flatten rough bare Cu pads. From the cross-sectional SEM micrographs, the filler trapping of the TC-bonded EPIG was much higher than that of the ENEPIG sample. The high filler trapping of the EPIG sample was due to the high surface roughness of the EPIG surface finish. The contact resistance increased as the thermal cycle time increased. The increase of the contact resistance with 1500 cycles of the thermal cycle test was 26% higher for the EPIG sample than for the ENEPIG sample.
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