Detection of the non-uniformity of junction temperature in large light-emitting diode using an improved forward voltage method

Abstract

Large size light-emitting diodes (LED) are frequently used in the application that requires high luminous intensity. Among diverse types of LED chips, the wire-bonding LED chip has become the most common type for its simple and mature manufacturing processes. For the commercial product manufacturing, chip bonding is one of the most critical procedures. Conventional chip bonding uses adhesive filled with thermal conductive particles. As the size of the LED chips is being enlarged, it becomes much more challenging to achieve perfect bonding. Defects may often occur in the bonding layer in practical manufacturing. Previous studies have revealed that defects inside the bonding layer will block the heat flow from the LED junction to the carrier and create non-uniform junction temperature distribution. The ordinary method to measure the junction temperature is the forward voltage method which uses the negative forward voltage-junction temperature characteristic of LEDs. Nevertheless, this standard method could not offer any information about the non-uniformity of junction temperature. As it is well understood that the junction temperature is critical to the performance of an LED device, it is necessary to find out a method to evaluate this non-uniform junction temperature phenomenon. In this study, we prepared several custom-made LED samples with artificial bonding defects to generate non-uniform junction temperature. An improved forward voltage method was developed to detect the junction temperature non-uniformity. The introduced method was experimentally validated.