Thermal measurements and analysis of flip-chip LED packages with and without underfills

Flip chip technology is one of the advanced chip packaging techniques in IC packaging industry. This technology may provide a solution to enhancing heat-dissipation and optical performance of the LED when applying to the LED packaging. The goal of this study is to experimentally and numerically study the thermal performance of flip-chip (FC) LED (with the size of 1 × 1 mm) with and without underfills, by comparing with wire bond LED package. In test specimen fabrication, the LED was mounted on an aluminum nitride sub-mount with Au bumps (with 110 μm in diameter and 80 μm high), and such device was housed in the chip-on-plate (COP) package. Three types of the LEDs packages (including conventional wire bond LED, FCLED with and without underfills) were evaluated in term of junction temperature (T_j) and thermal resistance (R_th) experimentally by junction temperature tester and Infrared thermal imager and numerically by ANSYS simulation. After the simulation model validation, the effect of bump number and underfills thermal conductivity on T_j and R_th was further investigated. The results show that the T_j and R_th for these three packages from experimental measurements are reasonably consistent with those from ANSYS simulation. It is also shown that the T_j and R_th of FCLED with four Au bumps without underfills are higher than those of the wire bond LED. And a large thermal gradient was found on the surface of this FCLED and can be alleviated by increasing the number of Au bumps or applying underfills between the chip and sub-mount. Moreover, for improving the thermal performance of this FCLED package, the number of Au bumps and the underfills with different thermal conductivity were studied by the validated ANSYS model. The results indicated that the R_j-a of this FCLED can be lowered by 11% relative to wire bond LED by using either up to 20 Au bumps or- - a underfills with thermal conductivity of 3.8°C/W.