Selective Coating of White Silicone to Improve the Optical and Thermal Characteristics of White LED Packages

We investigated the effect of selective coating of white silicone on the output and thermal performance of white LED packages for middle power lighting application. The LED package, where a Zener diode part was coated by TiO ₂ particles-mixed white silicone, exhibited lower surface temperature and heat flux than those with the uncoated Zener diode. The white silicone-coated packages resulted in lower thermal resistance (R _th ) than the uncoated ones. For example, the R _th at 85 °C of the uncoated and coated white LED packages in the junction-PCB was 26.30 K W ⁻¹ and 21.98 K W ⁻¹ , respectively. The coated blue and white LED packages yielded 6.5% and 4.5% higher luminous fluxes than the uncoated packages, respectively. Far-field emission patterns showed that the coated white LED packages had view angle of 118.06° and 118.10° along the two orthogonal directions, respectively, whilst the uncoated package gave 117.37° and 117.55°. After operation under 60 °C/80% relative humidity condition for 1000 h, the uncoated white LED package yielded maintenance rate of 96.85% whereas the coated packages gave 98.17%. At 85 °C under 200 mA, the thermal resistance of the white silicone-coated white LED packages was lowered by 17.7% than the uncoated packages.