ABSTRACT

In the push for Pb-free interconnect technologies, adhesively-bonded interconnect assemblies have emerged as a viable alternative for low-temperature applications.

Adhesives not only offer lower process temperatures compared to Pb-free solders (curing versus reflow), but also require fewer process steps, and have the ability to accommodate much smaller pitches, as in Flip-Chip-on-Flex (FCOF) assemblies. In this technology, the interconnect is usually between gold bumps on the silicon die and gold-plated copper bumps on a flex substrate (Figs 1 and 2). Conductive adhesives (CAs) (Fig. 1) or non-conductive adhesives (NCAs) (Fig. 2), are used as an underfill to bond the die to the flex substrate. The CA shown in Fig. 1 is an anisotropic conductive adhesive (ACA) with higher conductivity across facing bumps than between adjacent bumps. Some other technologies use isotropic conductive adhesives (ICAs).