ANISOTROPIC ADHESIVES FOR FLIP-CHIP BONDING

Feasibility of anisotropically conductive adhesives (ACAs) as bonding material for flip-chip will be discussed and compared with solder hip- chip. The experiments with peripherally bumped chips show that reliabl e flip-chip interconnections can be achieved with solder-filled epoxy- based anisotropically conductive adhesive. However, if bump or substra te metallization is either copper or nickel, Sn-based solders react st rongly with them forming various intermetallic layers. Due to the smal l size of the solder particles the intermetallic reactions consume pra ctically all the tin from the particles leaving, in the case of the Sn Bi-filler, almost pure Bi in between the intermetallic layers. This ca n be avoided, however, by using a new bonding approach: a metal that c annot form intermetallic compounds with tin is used as a bump and subs trate coating. The results with flexible circuits show that homogenous microstructures and better electrical properties are achieved with ti n-coated conductors bonded together with the SnBi-filled epoxy.