Filler Particle-Induced Light Absorption in Underfill-Encapsulated Flip-Chip Light-Emitting Diodes

Light-emitting diode (LED) devices depend strongly on proper thermal management to mitigate negative effects caused by high operating temperature, especially in general lighting applications. Flip-chip packaging has been used as a solution to reduce thermal resistance, with some iterations involving an epoxy underfill material to mechanically support solder joints and assist in thermal management. The benefits of underfill encapsulation on the thermal resistance of flip-chip LED (FCLED) packages have been studied, but the effects on optical performance have not been addressed. This paper identifies that the radiant power of the FCLED die suffers as a result of the underfill fillet. The selected underfill materials were applied to FCLEDs, and radiant power was measured. Radiant power was found to decrease as filler loading increased.