Influence of carbon, metal-coated polymer, and nano powders on sintering and electrical performance of nano-micro-filled conducting adhesives for z-axis interconnections

This paper discusses micro-filled epoxy-based conducting adhesives modified with nanoparticles, carbon, and metal-coated polymer fillers for z-axis interconnections. A variety of conducting adhesives with particle sizes ranging from 5 nm to 15 mum were incorporated as interconnects in printed wiring board (PWB) or laminate chip carrier (LCC) substrates. Scanning electron microscopy (SEM) and optical microscopy were used to investigate the micro-structure, and conducting and sintering mechanisms. Sheet resistance of Ag, carbon, and metal-coated polymer filler was low. Among all, metal-coated polymer showed the highest resistance. Sheet resistance decreased with increasing curing temperature. Drop in resistance for carbon-doped samples was 90% from 200degC to 275degC. It was found that with increasing curing temperature, the resistance of the conducting paste decreased due to sintering of metal particles. Sintering temperature and corresponding grain growth of nano-micro adhesive was further evaluated using different size nano particles, and shows optimum sintering at 240degC. Adhesives formulated with highly filled silver nano-micro particles exhibited a Z-axis coefficient of thermal expansion (CTE) of 17 ppm/degC, and as high as 41 ppm/degC for carbon doped, highly filled silver nano-micro systems. Similarly, Z-CTE of highly filled metal-coated polymer fillers was 28 ppm/degC. As a case study, a variety of z-axis interconnect constructions for a flip-chip plastic ball grid array package, as well as for PWB were fabricated and evaluated at both the subcomposite and composite levels to understand and reduce paste-to-package CTE mismatch. Several conductive adhesives were used in the z-axis interconnect constructions for LCC and PWB. The present process allows fabrication of z-interconnect conductive joints having diameters in the range of 55 to 300 mum. The processes and materials used to achieve smaller feature dimensions, satisfy stringent registration requirements,- and achieve robust electrical interconnections are discussed.