Effect of Thinning Encapsulant Layer on Junction and Phosphor Temperature of White Light-Emitting Diodes

Abstract

Thinning of an encapsulant layer for light-emitting diodes (LEDs) is a current major technological trend. The influence of encapsulant thickness on the junction temperature as well as the maximum phosphor temperature for blue and white LEDs is studied systematically for the first time. Using the finite-element method and forward voltage method, it is demonstrated that, in contrast to common belief, a thinned encapsulant layer for relatively low-power LED emitters with an input electrical power of 0.5 W will lead to an increase in the junction temperature for the white LEDs fabricated using different methods, including the phosphor volume distributed in-cup method and chip-coating method, as well as for monochromatic blue LEDs. It is also demonstrated that, different from the effect of encapsulant thickness on the junction temperature, a reduction in the encapsulant thickness results in a decrease in the maximum phosphor temperature for the in-cup white LEDs and an increase in the phosphor temperature for the chip phosphor-coating white LEDs. For example, the largest phosphor temperature drop of the in-cup white LED can be as high as 5 °C. The effect of thermal conductivity of leadframe and phosphor concentration under different encapsulant thicknesses is also indicated.