Adhesive flip chip bonding on flexible substrates

Flip chip attach provides the highest interconnection density possible, making this technology attractive for use with high density flex substrates. This paper presents three approaches to a flip chip adhesive process based on flexible polyimide and polyester substrates using Au, Ni-Au and Au stud bumps with anisotropic adhesives, isotropic conductive adhesives and nonconductive adhesives. Isotropic conductive adhesives conduct equally in all directions. For flip chip application of such adhesives, the material must be applied precisely on the points to be electrically connected and not allowed to flow and short circuit between circuit lines. Anisotropically conductive adhesives are prepared by dispersing conductive particles in an adhesive matrix at high enough concentration 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 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.