Optical Interaction Between LED Backside Reflectors and Die Attach Adhesives

Abstract

Adding optical reflectors to the backside of GaN-based light-emitting diode (LED) chip has been adopted for enhancing the light output. The optical effectiveness of backside reflectors (BRs) in enhancing light output is investigated by studying the light output of packaged mid-power blue and white LED emitters based on BR-based and BR-free chips. It is found that the effectiveness is strongly dependent on the optical properties of die attach adhesive (DAA) materials. When epoxy-silver DAA is applied, the difference in light output between the gold BR-based and BR-free blue LED chips is about 9% points at 40% coverage. However, with optically clear or reflective DAAs, BR-free chips show higher light output than the gold BR-based chips. For the optically clear DAA with 40% coverage, a 20% point enhancement of the output power of blue LED chips is obtained by the high transmittance of the DAA and high reflectance of the silver plating on the leadframe. With the optically reflective DAA, the enhancement for blue LED chips is $\sim 22$ % points due to the high reflectance of the DAA. This letter suggests a cost-effective method for enhancing the light output of mid-power LED emitters based on relatively low-cost BR-free LED chips by using optically clear or reflective DAAs depending on the applications and with a low fillet coverage for both materials.