Packaging challenges of high-power LEDs for solid state lighting

With the invention of high brightness LEDs (to penetrate illumination markets), the requirement for drive current has increased significantly, thereby increasing power dissipation. A I-mm LED dissipating 1W corresponds to 100W/cm(2) of heat flux, which is twice the amount of the heat flux generated in a conventional microprocessor chip. Therefore, effective removal of heat (to maintain a safe junction temperature) is the key to meet the future flux per LED requirements. Effective thermal management of high-power LEDs is, complicated since a standard LED structure has to rely primarily on natural convection cooling; whereas incandescent lighting technology relies on radiation as well as natural convection for thermal management. In addition to low thermal resistance, an LED package must meet a few additional constraints such as low cost, no active cooling and reliable in various environmental conditions. Using state of the art flipchip technologies and careful materials selection such as using a silicon submount to dissipate heat and match CTE, Lumileds has developed a high-power, low resistance LED package-Luxeon(TM). A Luxeon package offers record performance and reliability of high-power white and colored LEDs with thermal resistance ranging from 4-10K/W that can handle 0.6 to 5W of heat dissipation. Luxeon offers well-controlled high-intensity beam in a compact package. In this paper, we address some of the design issues we tackled and the technical approaches we took in the development of the Luxeon package. Research results involving materials selection for die interconnects and die attach, and process improvements (to achieve defect-free interfaces) are presented. Cost vs. performance and reliability of different materials options are also addressed.