UV-curable Encapsulants for LED

Abstract

UV cured epoxy systems represent complementary technology to the established heat cured epoxy systems for coating, adhesion and encapsulation of electronic devices. The UV cured epoxy systems offer significant benefits in terms of fast cure and thus very short production cycle time; low stress and better colour due to room temperature cure; process wise no mix ratio problem or anhydride handling issues and yet produce comparable or even better physical and chemical property than the more common heat cured systems. This paper deals with the evaluation and comparison among four different type of curing systems for LED encapsulants: 1. Free radical UV system with faster curing rate but only surface layer cure. 2. Cationic UV system with slower curing rate but deeper layer cure. 3. Hybrid of both free radical and cationic UV system. 4. Existing heat cure system. Characterisation topics included the state-of-the-art LED package strength indicated by DMA modulus, TMA coefficient of thermal expansion, photostress, liquid nitrogen and ring washer crack test; outdoor performance indicated by heat age at 120 o C, 180 oC, atmospheric moisture absorption and boiling water absorption; solder heat resistance indicated by Tg, % cure and solder (TTW) delamination and last but not least the optical clarity indicated by % Transmission under UV-visible range and the refractive index.