A NEW APPROACH TO USING ANISOTROPICALLY CONDUCTIVE ADHESIVES FOR FLIP-CHIP ASSEMBLY

The use of anisotropically conductive adhesives (ACA's) for the direct interconnection of silicon chips to printed circuits offers numerous advantages including: reduced package thickness, improved environmenta l compatibility, lowered assembly temperatures, increased metallizatio n options, and decreased equipment needs. To increase throughput and t o lower costs, we have developed a new approach to pip-chip assembly w ith ACA's. Our process uses two unique features: an ACA thixotropic pa ste formulation and a batch curing fixture. The thixotropic paste, whi ch replaces the more conventional film form of the adhesive, can be ea sily dispensed onto the substrate with a stencil printer. Chips placed into the ACA paste are held securely due to the ''tacky'' nature of t he material much like surface mount components are held by solder past es. As a result, no heating of the chips is required during assembly, increasing throughput and relaxing co-planarity tolerances in the alig nment equipment. As with all ACA's, the paste must be cured by the sim ultaneous application of heat and pressure. In our process, curing is accomplished in a fixture capable of holding multiple chips and/or cir cuit boards simultaneously. Uniform pressure is applied to components during the 3-5 min thermal cure cycle via a conformable silicone rubbe r bladder. Initial yield and temperature cycling data are reported in this paper. Silicon chips with gold metallization show small (<15%) in creases in contact resistance after more than 1000 test cycles (betwee n 0-100 degrees C); bumping the chips was not required. Aluminum metal lized chips proved to be unreliable after temperature cycling tests.