Demonstration of Ni-free Surface Finishing with IGEPIG for Solid-State Diffusion Bonding on Ultra-fine Pitch Chip-on-Film (COF)

Abstract

This paper demonstrates the latest ultra-fine pitch scalability of off-chip IC assembly on an 18 $\mu {\mathrm{ m}}$ pitch chip-on-film (COF) package fabricated by novel fully additive process (FAP) with Ni-free surface finish plating, immersion Au/electroless Pd/immersion Au (IGEPIG). The COF assembled with Au-Au diffusion bonding method is evaluated in terms of electrical and mechanical performance. Comparison is made with various finishes including Electrolytic Ni/Au and electroless Ni/electroless Pd/immersion Au (ENEPIG). Excellent alignment with sufficient contact area is achieved even on such narrow traces. The substrate fabricated by FAP possess desirable trace profile suitable for fine pitch bonding. With different finishing, it is seen that surface roughness is the main contributing factor that interrupt diffusion and creep process in forming void on the joint interface while the elastic plastic properties of the material construction and tri-layer thickness affect the deformation of the interconnect structure. Correlations between the surface finish and the Au-Au solid state diffusion bonding on the COF is established. In terms of long term reliability, all finishes show stable contact resistance below 30 $\text{m}\omega $ with no open joint on the daisy chain connectivity after high temperature humidity storage at $85^{\circ }\text{C}$/85% RH (1000 hours) and air-to-air thermal shock test at $125^{\circ }\text{C}$/-$55^{\circ }\text{C}$ (1000 cycles).