Evaluation of bismuth as a filler material for anisotropically conductive adhesive

Summary form only given. Tin and bismuth are metallurgically compatible because they are unable to form intermetallics with each other. When in contact with each other, i.e. in a local chemical equilibrium, the melting point of the Sn/Bi interface is 139/spl deg/C. During the bonding procedure above 139/spl deg/C tin and bismuth atoms dissolve very fast into the liquid until local equilibrium has been reached. Between Sn-coated pads or bumps, Bi particles form small lentils, the volumes of which depend on the particle size and the bonding temperature. When the temperature is decreased the liquid lentils solidify and bond locally the mating pads together inside the adhesive. The procedure for the SnPb/Bi system is quite similar. Basically, the only difference is that the local melting of the SnPb/Bi interface is initiated at about 93 /spl deg/C. The bonding process of Sn-bumped chips on Sn-coated substrates with different adhesives being filled with Bi particles is also presented. The solder joints formed at different bonding temperatures and with different pressure settings are examined with optical and scanning electron microscopy as well as with energy dispersive spectroscopy.