Flo therm (V8.1, Mentor Graphics Ltd.) was used to evaluate the junction temperature of LED package . The actual shape of the LED package was conserved in the modeling for comparisons with experimental data. A 1 mm  1 mm size of GaN based L ED chip was used in this study and the size of silver (Ag) paste was assumed as same as the chip size. The package is composed of GaN chip, Sapphire substrate, Ag paste, copper (Cu) slug, lens-epoxy,thermal compound, MCPCB, and mold. The thermal conductivity,density and specific heat of each component are listed at Table . Under the DC power, constant heat generation is applied to the LED package. The ambient temperature of system was fixed
at 20 °C and the LED was modeled under the conditions of natural
convection, which agrees with the EIA/JESD .