Encapsulant thickness options as a factor to impact thermal performance of Chip-on-Board (COB) light emitting diodes

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Abstract
There is an important trend in light emitting diodes (LED) industry to produce continuously thinner package. Chip-on-Board (COB) packaged LED, as a promising thermal efficient package structure, has rarely been investigated for its thermal performance under smaller package thickness. In this work, encapsulant or overall package thickness as a factor to influence the junction temperature as well as the maximum phosphor temperature for COB LEDs is presented. Finite element method (FEM) is employed and validated by forward voltage method (FVM). As shown in the results, for single-chip COB LEDs with standard MESA structure and volume distributed phosphor, there is a lowest point in the maximum temperature while thinning encapsulant layer. But for multi-chip COB LEDs, the junction and phosphor temperature will be reduced with thinner encapsulant. In both cases, the largest deviation of the maximum temperature with different encapsulant thickness can be more than 10°C. Parametric studies of other critical factors such as different Die Attach Adhesive (DAA) materials and radius of encapsulant while thinning encapsulant thickness are also provided. There are two ways to enlarge the input power of COB LED: one is to raise the input power into each chip; the other is to increase the number of chips. It's interesting to find the different thermal behaviors for the two cases. Larger input power will cause a more stable maximum temperature for COB LEDs. While more chips leads to higher variation of the maximum temperature with respect to encapsulant thickness.