There are three types of bases of SMD LED Street Light chips: glass fiber, plastic, ceramic material. When in packaging, each LED chip will be fixed directly in a copper sheet, for that copper has a very high thermal conductivity. However, the volume of copper sheets is so small that heat will be radiated into external PCBs though a very short path. Experiments prove that SMD LED Street Lights chips with good cooling designs have a much longer life than plug-in LED chips. Similar low power LED chips but different packaging (low power supports/high power bases) will have different working lives. For example, 9 10 x 12 mil2 LED chips fixed in parallel on base surface, under a total working current of 350 mA. After 7600 hours’ constant work, the light attenuation is about 23%. In accordance with this trend, the estimated working life will be 18000 hours. High power packaging chips have surface current density (0.324 mA/mil) twice as much as low power packaging chips (0.167 mA/mil), but has a much longer life than them (almost 9 times), as a result of using materials with higher thermal conductivities and bases with larger area. By doing so, heat can be transmitted not only in radial but also in lateral.
Other materials used in package:
Heat is the main cause of the light attenuation of white LED lamps. Thus every material related to heat radiation should be taken into account. In previous section, we discussed the LED base problem. Now let us talk about other materials used in packaging process: solid crystal glues, mix powder glues, sealants. Tests results indicate that the life of white LED lamps using silver glues is longer than using epoxy resins, but initial luminous flux is lower by 1/3. Using epoxy resins as mixed glue has a shorter working life than using silica glues, but initial luminous flux is increase by 25%. The most commonly used sealants are epoxy resins and silica glues. As a matter of fact, these two types of materials have very weak heat radiating capabilities. The thermal conductivity of iron is 230 times as much as that of epoxy resins and 150 times as much as that of silica glues. In addition to their thick, they are somehow with no capability of heat radiation.