Effect of ENEPIG metallization for solid-state gold-gold diffusion bonds

Gold-gold (AuAu) diffusion bonding behavior of different tri-layer thicknesses of Electroless Ni/Electroless Pd/Immersion Au (ENEPIG) plating on a high-density system on a flex (SOF) package was examined. Plating thickness has a significant effect on surface roughness and void formation at the AuAu bonding interface, which exhibits degraded bond strength with an affected failure mode. It is seen that relatively smooth surface roughness (Ra < 100 nm) of thicker Ni(P) plating samples facilitates the shrinkage of voids and significantly increases bonding strength. Higher surface roughness in the low Ni(P) sample has a poor surface profile, which results in large lenticular shape voids and requires more energy to shrink by diffusion and a creep process. Enhancing bonding parameters constitutes an essential feature to compensate the physical and mechanical properties of ENEPIG plating. Based on this study, the authors recommend a suitable ENEPIG plating thickness for a high quality metallurgical bond, which passes different reliability tests.