High performance isotropic polymer/metal composite for interconnect applications

A paste composed of thermoplastic binder, silver particles, and solvent is compounded for directly interconnecting a chip to a substrate. The thermoplastic nature of the binder has three distinct advantages: (i) the interconnection is fully reworkable, (ii) the bonding time interval between the dispensing and assembling operation is indefinite, enabling a true `bumping´ technology, and (iii) the paste is stored at room temperature with well over 6 month shelf-life. The bond is formed by drying the solvent and applying pressure at a bond temperature above the glass transition temperature of the polymer. The resultant polymer/metal composite (PMC) bond is an isotropic conductor with electrical conductivity provided by continuous channels of Ag particles. With PMC electrical conductivity higher than solder and fine-pitch screenability, we are developing this material as an interconnect for flip-chip attach (FCA) technology. In this paper, we report our results on the dispensability and rheological behavior of the paste and some results on our bonding process optimization study using response surface analysis