In-situ characterization of moisture absorption and hygroscopic swelling of silicone/phosphor composite film and epoxy mold compound in LED packaging
Epoxy mold compound (EMC) and optical grade silicone based materials have been widely used in manufacturing Light Emitting Diode (LED). Silicones are being used as encapsulant and lenses for next-generation LED packaging designs, such as chip on board (COB) LEDs. Silicones serve several roles including protective lenses, stress relieving encapsulant, mechanical protection and light path materials. Even though silicone-based materials are typically hydrophobic, they are not resistant to moisture absorption, which may greatly affect the key attributes of silicones such as high transparency, high refractive index, stable thermo-mechanical properties, and tunable hardness. On the other hand, EMCs are used in LEDs as packaging material because of their low cost and high productivity, whereas their moisture absorption is long known. In this study, a new generation of silicone/phosphor composite films and a commonly-used EMC material for COB LEDs are studied regarding their moisture absorption and hygroscopic swelling behaviors by in-situ measurements. Moisture sorption tests were performed using TGA Q5000SA at different humidity and temperature conditions, such as 30°C & 30% RH, 30°C & 60% RH, 30°C & 80% RH, 60°C & 30% RH, 60°C & 60% RH, 60° C & 80% RH, 80°C & 30% RH, 80°C & 60% RH and 80°C & 80% RH. Hygroscopic swelling was determined using DMA-RH Q800 at 60°C. Material properties including diffusivity, activation energy, saturated moisture concentration and the coefficient of hygroscopic swelling are extracted from the experimental data. Based on the results, comprehensive discussions are made on the characteristics of moisture absorption and hygroscopic swelling for the two different materials.