As the active area of the LED chip is small and its working current is big, the operating temperature of the LED ship can increase in a short time. The rise of pn junction temperature of LED can make the optical output power decrease, accelerate the aging of a chip, and shorten the service life of the device. With the rise in junction temperature, the LED wavelength will have “red shift”. (The drift visual effect of the orange and amber LED is more obvious). So in view of adverse effects of color drift in the practical application, the thermal design will limit the highest junction temperature. In the high power LED cooling channel, the heat radiating substrate is the key link between internal and external cooling channel. It has the following functions at least.
(1)A cooling channel of the LED chip.
(2)An electrical connection sub state of the LED chip.
(3)A physical support of the LED chip.
Substrate materials of the high power LED must meet requirements of the high insulating property, high stability, high thermal conductivity, and also have similar coefficients of thermal expansion, smoothness and high strength with chips.
The traditional materials of
heat sinks are aluminum alloy or copper. Aluminum thermal conductivity can reach 209w/m.k, and aluminum has good processing characteristics and low cost. So it is widely used. Copper thermal conductivity can reach 390w/m.k, and it increases by 70% compared with aluminum. But the disadvantage is that its weight is about three times as much as aluminum and it has bad processing characteristics.
However, when applications are limited to the conduction characteristics as focus, the copper is used usually. In addition to the two kinds of materials, the increasing radiating materials are high-profile, such as compound materials based on carbon, metal powder sintered material, combined diamond, graphite, etc. aspic is the most new materials at present. It consists of many kinds of aluminum alloy and has special physical properties like control of thermal expansion properties, high conductivity and significant strength. Because of the cost, these new materials are used in the bottom of the power module and the substrate which is directly connected with chips.
