A new approach to using anisotropically conductive adhesives for flip-chip assembly

The use of anisotropically conductive adhesives (ACAs) for the direct interconnection of silicon chips to printed circuits offers numerous advantages including: reduced package thickness, improved environmental compatibility, lowered assembly temperatures, increased metallization options, and decreased equipment needs. To increase throughput and to lower costs, we have developed a new approach to flip-chip assembly with ACAs. Our process uses two unique features: an ACA thixotropic paste formulation and a batch curing fixture. The thixotropic paste, which replaces the more conventional film form of the adhesive, can be easily dispensed onto the substrate with a stencil printer. Chips placed into the ACA paste are held securely due to the “tacky” nature of the material much like surface mount components are held by solder pastes. As a result, no heating of the chips is required during assembly, increasing throughput and relaxing co-planarity tolerances in the alignment equipment. As with all ACAs, the paste must be cured by the simultaneous application of heat and pressure. In our process, curing is accomplished in a fixture capable of holding multiple chips and/or circuit boards simultaneously. Uniform pressure is applied to components during the 3-5 min thermal cure cycle via a conformable silicone rubber bladder. Initial yield and temperature cycling data are reported in this paper. Silicon chips with gold metallization show small (<15%) increases in contact resistance after more than 1000 test cycles (between 0-100°C); bumping the chips was not required. Aluminum metallized chips proved to be unreliable after temperature cycling tests