Bismuth-filled anisotropically conductive adhesive for flip chip bonding

In this communication a new bismuth (Bi) -filled anisotropically conductive adhesive (ACA), which can be used to bond pure tin (Sn) or any Sn-based contact areas, is presented. In this study the adhesive was used to join tin-lead (SnPb) -bumped test chips to SnPb-coated contact pads on FR4-substrate. Due to the metallurgical characteristics of the SnPbBi system, the process temperature and bonding pressure remain low. With the help of pure Bi particles, the SnPb bumps are fused with the SnPb-coated contact pads above 92 degreesC forming small local solder joints, while the adhesive acts as an underfill. The microscopic solder interconnections retain their electrically continuity even though the adhesive matrix relaxes in service. Good quality metallurgical interconnections were produced by the bonding procedure with relatively low bonding temperature (160 degreesC) and pressure (24.5 g/bump). No intermetallics is formed in this bonding system. The Bi-filled ACA exhibits good reliability under high temperature and high humidity conditions (85 degreesC/85%RH). Thermal shock test (-40 degreesC /+ 125 degreesC, total cycle time 1 hour) showed that good reliability can be achieved even though the bonding process is not yet optimized. The failure of the joints is due to the formation of small liquid phases in the Bi-rich (over 10 at-%) areas in the solid lentils at the upper thermal shock temperature. This can be avoided by using higher bonding temperature. The discrepancy of reliability results is explained by the fact that the temperature of bonding has not been exactly the same for all the samples bonded.