In order to reduce the thermal impedance, many foreign LED manufacturers set LED chips on heat sinks made of copper and ceramic materials. Then welding connect them with printed circuit boards. According to the test result of German OSRAM Opto Semiconductors Gmb, the gap thermal impedance between LED chips and welding points mentioned above could reduced to 9K/W, about 1/6 of the conventional LEDs. For example, if the input power is 2W, the bonding temperature of LED chips will be 18K higher than welding points. In normal conditions, the bonding temperature of LED chips will be impacted by PCBs. Thus reducing LED chip working temperature is necessary, namely reducing the thermal impedance between LED chips and welding points. This can effectively reduce the cooling operation burden of LED chips. Conversely, even if white-ray LEDs have special structure to limit the thermal impedance, if heat cannot be successfully conducted from packaging structure to PCBs, the luminous efficiency will reduce dramatically as a result of LED temperature rising. Thus Matsushita Electric developed a new PCB-package integration technology, which can package the 1mm square blue-ray LEDs to ceramic substrates in the form of flipchip. Then attach ceramic substrates to the surface of copper PCBs.
The tightness between heat sinks and PCBs directly affects the heat conducting effect. Therefore the design of PCBs became rather complex. As a result of these, many LED packaging manufacturers (like United States LumiLEDs, Japan CITIZEN) have developed high-power led tunnel light with simple cooling technologies. CITIZEN began to provide white-ray LEDs (new package samples) to their customers. The new heat dissipation technology could directly remove the heat on heat sinks (2-3mm) without any help from special bonding technique. According to CITIZEN, although the thermal impedance between LED chips and welding points is approximately 30K/W, much higher than 9K/W of OSRAM, this white-ray LED model still can maintain a normal working condition even no cooling ventilators are used.
LumiLEDs started to promote high-power white-ray LEDs from 2005, the maximum LED chip bonding temperature can reach up to +1850C, 600C higher than similar products from other companies. If using traditional RF4 PCB package, within the ambient temperature range of 0-400C, the equivalent power can be 1.5W (approximately 400mA).