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 minute 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