Flip chip assembly on organic substrates using ACAs (anisotropic conductive adhesives) have received much attention due to many advantages, such as easier processing, good electrical performance, lower cost, green processes, and low temperature processing compatible with organic substrates. ACAs are generally composed of epoxy polymer resin and small amounts of conductive fillers (less than 10 wt.%). As a result, ACAs have almost the same CTE values as an epoxy material itself (60-80 ppm), which are much higher than those of conventional underfill materials (25-30 ppm) that contain lots of silica fillers. Therefore, it is necessary to lower the CTE value of ACAs to obtain more reliable flip chip assembly on organic substrates. New ACA composite materials with added conductive fillers and non-conductive fillers were invented. In this paper, we investigated the effect of fillers on the thermo-mechanical properties of newly modified ACA composite materials and the resulting reliability of flip chip assembly on organic substrates using modified ACA materials. Electroless nickel and gold stud bumps are used in this experiment. As the non-conducting filler content increased, CTE values decreased and storage modulus at room temperature increased. Contact resistance changes were measured during reliability tests such as thermal cycling, high humidity and temperature, and high temperature at dry condition. It was observed that lowering ACA materials CTEs significantly enhanced thermal cycling reliability. And results also showed that contact resistance of flip chip assembly using modified ACA composites was almost the same as conventional ACAs without non-conducting fillers. Electrical high frequency properties of newly invented ACAs were also improved by the incorporation of low k non-conductive fillers. As a result, significant reliability enhancement of flip chip on organic substrates was obtained using newly invented ACAs. These materials can open new low cost flip chip on organic substrates applications such as smart cards, rf, memory devices, and so on. High speed memory CSPs using newly invented ACA materials are successfully demonstrated.
Key words: ACA, low CTE, reliability, flip chip on organic substrate.