Thermal issue has always been a key part in the development of the LED industry. Polytronics Technology offers aluminum substrates as the best solution for edge-lit LED TVs. Yi-An Sha from Polytronics is optimistic about the growth of LEDs' application in backlighting and general lighting. He discussed the technological development trend of thermal substrates and analyzed the pros and cons of different technologies.
The trend of backlighting and lighting market
The market trend of LED replacing CCFL is obvious. Taiwan's top CCFL manufacturers are gradually turning their focuses to LED-related products, whose share of their revenues has reached 52%. Meanwhile, the proportion of revenues from CCFL products has come down to 42%. LED backlighting is applied to various LCD panels, using such backplanes as FPCB (screen size below 15 inches), MCPCB (screen size above 15 inches, mostly edge-type) and FR4 (screen size above 6 inches, mostly direct-type).
According to DisplaySearch, the penetration rate of LED backlighting in the notebook market is estimated at 90% in 2010 and 100% in 2011. In the LCD monitor market it is estimated at 25% in 2010, 50% in 2011 and 85% in 2016. In the large-size TFT-LCD market it will reach 50% in 2011 and 80% in 2012. Now the sales of LED TVs are facing short-term fluctuations mainly because of price differences, inventory digestion of CCFL sets, and adjustments in LED-backlit product lineups.
As for the cost of LED TVs, the edge type is cheaper than the direct type. In the case of edge type, to further reduce the cost of backlighting, the amount of LEDs and light strips has to be reduced. Meanwhile, the brightness of LEDs, the light extraction efficiency and heat dissipation of modules need to be improved as well. Heat dissipation can be improved by using high thermal conductive board (TCB).
IEK estimates the LED lighting market will reach US$5.2 billion in 2012, and its compound annual growth rate (CAGR) from 2007 to 2012 will reach 28.5%. At present LED lighting is mainly found in architectural, industrial and commercial applications. The LED streetlamp sector now embraces the concept of Energy Performance Contracting (EPC), which came into existence amid the energy crisis in the 1970s. The proprietor does not have to pay, while the supplier earns from the price difference resulting from energy savings. For the supplier, the better their technical capabilities, the more their profits. But they need to keep considerable capital in the short term.
Industrial application and technological development
The trends of industrial application and technological development include brightness enhancement, optical improvement, energy saving, product reliability and reduction of cost. Among them product reliability is highly related to heat dissipation. The key issue of LED heat dissipation is to provide thermal conductive media to transfer the heat from the LED to the thermal substrate and then to the thermal module. If the base area of the LED is widened, then the area of heat dissipation will increase. Power supply is needed as well.
The materials for composing thermal substrates are related to three areas: copper foil electric circuits, ceramic powder plus macromolecules, and aluminum substrates. The thermal conductivity of ceramic powder is 20W/m-K, macromolecules is 0.1W/m-K, and FR4 is 0.36W/m-K.
Heat dissipation materials include fused silica (SiO2), aluminum oxide, crystalline silica, silica, aluminum nitride, and boron nitride. Fused silica and aluminum oxide are low-cost, and aluminum oxide has high thermal conductivity of 30W/m-K; so they are the main stream in the market. Aluminum nitride has the best thermal conductivity of 320W/m-K, but costs are high and reliability is low.
Glass-fiber thermal substrates are made with macromolecules, high thermal conductive ceramic powder and glass fiber cloth. The advantages of glass-fiber thermal substrates are their easy manufacturing process, better thickness uniformity and low production costs. Their disadvantages include: good thermal conductivity on the Z axis but bad on the X-Y plane; and serious warpage.
As a kind of traditional PCB, FR4 is often used in low-power electronic components or LEDs. Its thermal conductivity is merely 0.36W/m-K, therefore lower reliability. Its disruptive voltage is around 40KV/mm and price is low. Ceramic thermal substrates are suitable for high-power LEDs or electronic components. They have thermal conductivity of 2-12W/m-K and thus are more reliable. Its disruptive voltage is around 60KV/mm, but the price is three times that of FR4.
Technological development of metal thermal substrate
There are two ways to produce metal thermal substrates: the dry process and wet process. The former is processed with macromolecules through continuous injection, and no solvents are needed, which reduces pollution. The latter is simple but has lower reliability. It uses solvents for blending and its chief process is printing (screen or blade printing), which needs to be dried.
Polytronics' high thermal conductive silicon film adopts the "roll-to-roll' continuous dry process. It is the only 12W/m-K thermal film roll in the world. Its dry process is environmentally friendly.
Yi-An Sha from Polytronics
Photo: Digitimes, October 2010