Flip chip attach provides the highest interconnection densitypossible, making this technology attractive for use with high densityflex substrates. This paper presents three approaches to a flip chipadhesive process based on flexible polyimide and polyester substratesusing Au, Ni-Au and Au stud bumps with anisotropic adhesives, isotropicconductive adhesives and nonconductive adhesives. Isotropic conductiveadhesives conduct equally in all directions. For flip chip applicationof such adhesives, the material must be applied precisely on the pointsto be electrically connected and not allowed to flow and short circuitbetween circuit lines. Anisotropically conductive adhesives are preparedby dispersing conductive particles in an adhesive matrix at high enoughconcentration to assure reliable conductivity between substrate and ICelectrodes. Another possibility is the use of nonconductive adhesivesand Au-bumped chips, which are bonded via thermocompression to thesubstrate. During bonding, the bumps pierce a nonconducting adhesivefoil and make electrical contact while the adhesive supplies mechanicalstability. Moreover, the adhesive fills the gap between chip andsubstrate, relieving the bumps of mechanical stress due to the differentCTEs. Reliability evaluation was performed with specific regard to theinterface reactions between polymers and metal surfaces in adhesivecontacts. The electrical and mechanical performance of the adhesivebonds were studied by evaluating initial contact resistance as afunction of temperature and humidity
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