Effect of die attach adhesive defects on the junction temperature uniformity of LED chips

In the literature, the junction temperature of a light-emitting diode (LED) has been considered having a uniform distribution over the whole chip. Such a perception is based on the assumption that the die attach adhesive (DAA) covers the whole bottom of the LED chip with a uniform thickness. However, in reality, there may be insufficient DAA during the chip mounting process. As a result, non-uniform junction temperature may be induced and hot spots could occur. Therefore, it is essential to investigate the effect of DAA on the creation of hot spots at the LED junction. In the present study, LED chips were mounted with various amounts of DAA on a leadframe designed for high power LED. A high resolution infra-red (IR) camera was used to measure the temperature on the surface of the LED chip. The obtained IR images were calibrated carefully in order to eliminate the influence from IR emissivity. Hot spots were observed at the junction areas where DAA was missing. Detailed analysis for illustrating the effect of DAA distribution on hot spots is presented and reasons for such a phenomenon are discussed.