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 flip-chip. The experiments with peripherally bumped chips show that reliable flip-chip interconnections can be achieved with solder-filled epoxy-based anisotropically conductive adhesive. However, if bump or substrate metallization is either copper or nickel, Sn-based solders react strongly with them forming various intermetallic layers. Due to the small size of the solder particles the intermetallic reactions consume practically all the tin from the particles leaving, in the case of the SnBi-filler, almost pure Bi in between the intermetallic layers. This can be avoided, however, by using a new bonding approach: a metal that cannot form intermetallic compounds with tin is used as a bump and substrate coating. The results with flexible circuits show that homogenous microstructures and better electrical properties are achieved with tin-coated conductors bonded together with the SnBi-filled epoxy.