LED heat dissipation is a much more serious issue than thermal conductivity to the extent that it remains a major bottleneck for systems so far. With in-house developed technology, TeamChem has invented a flexible ceramic thermal conductive adhesive to replace traditional thermal conductive adhesive and the anodizing process. Todd Yeh of TeamChem's R&D unit elaborated on the problems of thermal conductivity and heat dissipation as well as applications of the company's products.
Thermal conductivity and heat dissipation
Thermal conductivity and heat dissipation are just like traffic problems. Germany has a relatively big population. The country has a sizeable population in its suburban areas partly because there is no speed limit on its highway. However, highways without speed limit can still suffer traffic jams due to poor interchange design. Both factors have to be taken into consideration to achieve smooth traffic and better commuting experience.
Thermal conductivity and heat dissipation are two different concepts. For LED, substrates act as a thermal conductor instead of a heat dissipater, since heat dissipation means spreading heat into the air. As heat dissipation is a bigger issue than thermal conductivity, the 5W and 10W thermal conductive substrates commonly seen now actually are approaching the issue in the wrong direction and often turn out to become bottlenecks.
Conventional thermal management methods include fans and heat sinks. But these are not applicable to LEDs. Fans, with their problems of energy conservation and dust accretion, reduce the durability of LEDs, while heat sinks are less effective. We still need to find better solutions to LEDs' heat dissipation problem.
What are the possible solutions? Should we resort to air flow, increase the surface area or reduce the mass (thickness) in order to decrease heat storage?
It is generally believed that a higher k value means better thermal conductivity, but thickness is often not taken into consideration. Thermal impedance will decrease with lower thickness, and heat dissipation can only be completely achieved with low thermal impedance.
For traditional MCPCB, the thickness measures more than 1,600µm, with 35µm of copper foil, 100µm of adhesive and 1,500µm of aluminum plate (their k values are 400, 1 and 200W/m-K, respectively). The thermal impedance results are 0.1, 100 and 7.5µm2-K/W, respectively, adding to a total of 1.08cm2-K/W thermal impedance. This is actually the whole picture of MCPCB's heat dissipation performance. Thermal impedance measurements of several commercialized MBPCBs range from 2-4c m2-K/W.
However, as the aforementioned figures are the sum of individually-calculated thermal conductivity measurements from three different layers, problems may occur in the future if interfacial thermal impedance rises. Moreover, the substrate surface needs to be filled by liquid because it is uneven from a microscopic view. Whether the surface of the thermal conductive board (TCB) can become totally even is one problem, and aluminum plates often suffer inconsistent heat dissipation.
As for heat dissipation, its measurements involve still air and moving air, as air is the source of thermal impedance. Thermal impedance in still air ranges from 400-3,000cm2-K/W, and 200-1,000 cm2-K/W in moving air. You can multiply these thermal impedance figures.
LED-related applications have their own heat-dissipation characteristics. For example, as there is no moving air to accelerate heat dissipation without fans, thermal impedance will be pretty high around 1,000cm2-K/W in still air. The fact that the surface temperature of most substrates is kept below 60 degrees Celsius is also a major limitation in need of solutions. Besides, while heat dissipation does represent a bottleneck, thermal conductivity is not much a problem in comparison.
A number of factors must be addressed in order to promote growth of the entire LED industry, including lower costs, simplified structures and assembly processes, and better thermal management and reliability. These can all be achieved with the introduction of new materials such as low-cost thermal conductive FPCB (while aluminum costs are four to five times higher), low-temperature-curable silver conductive adhesive, and optically clear thermosetting adhesive film.
Trends in product applications
TeamChem's flexible ceramic thermal conductive adhesive - a waterproof, antistatic and self-cleaning material with high tolerance pH range of 3-11, insulating properties, weatherability and great heat dissipation - can be used for outdoor LED lamps. According to tests by clients who are LED lamp makers, system temperature is lower than traditional treatment by 10 degrees Celsius, and 7 degrees Celsius lower than anodizing treatment. The cost is also low at below NT$90 for every square meter of TC-19E thermal conductive spray coating.
Conventional LED aluminum substrate fabrication is a time-consuming and costly method using copper foil, multi-layer thermal conductive adhesives and aluminum plates before anodizing the lower part of the substrates. The new fabrication method also requires cooper foil and aluminum plates but there is only one layer of thermal conductive adhesive, and the thermosetting, high-temperature-resistant, shockproof TC-19EW coating can be easily spayed and effectively bring down the temperature. The material is also acid/alkali resistant, anti-static and can prevent dust from sticking to the surface.
Other applications include large thermal conductive aluminum plates for LED TVs, capacitors, or various devices, such as motors, heat exchangers, transformer boxes and internal combustion engines, that generate heat and may be exposed to natural elements.
Todd Yeh of TeamChem's R&D unit
Photo: Digitimes, October 2010