Adhesive flip chip bonding on flexible substrates

Flip chip attachments provide the highest interconnection density possible, which makes this technology very attractive for use with flexible high density substrates. This technology stimulated the development of new interconnection techniques, such as anisotropic adhesives, isotropic conductive adhesives and non-conductive adhesives. This paper presents three approaches to a flip chip adhesive process based on flexible polyimide and polyester substrates using gold, nickel/gold and gold stud bumps. Promising candidate for adhesive joining technique are the isotropic conductive adhesives. These adhesives are isotropically conductive, which means that they conduct electricity equally in all directions. To use such adhesives in flip chip applications, the material must be applied precisely onto the points to be electrically connected, and not allowed to flow and short circuit between circuit lines. The anisotropically conductive adhesive materials are prepared by dispersing electrically conductive particles in an adhesive matrix at a concentration that is high enough to assure reliable conductivity between the substrate and the IC electrodes. Another possibility is the use of nonconductive adhesives and goldbumped chips, which are bonded via thermocompression to the substrate. During the bonding the bumps pierce through a non-conducting adhesive foil and make the electrical contact while the adhesive supplies the mechanical stability. Moreover, the adhesive fills the gap between chip and substrate, relieving the bumps of the mechanical stress resulting from the different CTE's. The 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.