Adhesive flip chip bonding on flexible substrates

Flip chip attachments provide the highest interconnect density possible, making this technology attractive for use with flexible high density substrates. This paper presents three flip chip adhesive process methods based on flexible polyimide and polyester substrates using gold, nickel/gold and gold stud bumps. Isotropic conductive adhesives are promising for adhesive joining, as they conduct electricity equally in all directions. To use such adhesives in flip chip applications, the material must be applied precisely to the points to be connected, and must be not allowed to flow and short circuit between circuit lines. Anisotropically conductive adhesive materials are prepared by dispersing conductive particles in an adhesive matrix at a concentration high enough to assure reliable conductivity between substrate and IC electrodes. Another possibility is the use of nonconductive adhesives and Au-bumped chips, which are bonded via thermocompression to the substrate. During bonding, the bumps pierce a nonconducting adhesive foil and make the electrical contact while the adhesive supplies mechanical stability. Moreover, the adhesive fills the gap between chip and substrate, relieving the bumps of mechanical stress due to the different CTEs. Reliability evaluation was performed with specific regard to the interface reactions between polymers and metal surfaces in adhesive contacts. The electrical and mechanical performance of the adhesive bonds were studied by evaluating initial contact resistance as a function of temperature and humidity