Reaction-induced phase separation (decomposition) in a thermoset/thermoplastic system was successfully applied for the semiconductor packages of smartphones and tablet PCs. More than 50% of smartphones in the world now contains the thermoset/thermoplastic alloy as the adhesive layers between IC chips (die bonding materials). Epoxy/acryl rubber system was found to show UCST phase behavior. Spinodal decomposition proceeded by curing epoxy. Very late stage of the decomposition led to high adhesive strength. A morphology gradient formed in 10 ìm space between chips rendered a high level of thermal stress release. Such high performances made thinner both IC chips and adhesive layers to fabricate more than 20 stacks. Keywords: Polymer Alloy, Reaction-Induced Phase Separation, Semiconductor, Smartphone.