Cambridge Nanotherm has announced how its Nanotherm LC thermal management solution addresses the unique needs of chip-scale packaging (CSP) LEDs. CSP LEDs have several benefits over traditional high-power LEDs (HP LEDs), giving module designers the ability to produce smaller, brighter and more cost-effective luminaires. The market for CSP LEDs is growing rapidly. Yole Développment predict CSPs will make up 34 percent of the HP LED market by 2020.
However, CSPs also present a significant thermal challenge. Traditional HP LEDs have a ceramic submount onto which the LED die is mounted. This spreads the heat from the die before it reaches the printed circuit board, helping to keep the junction temperature of the die within its approved operating temperature. CSPs do not use a submount; instead the P and N contacts are metalised enabling the die to be soldered directly onto the PCB (usually a metal-clad PCB — MCPCB). This approach reduces the cost, and size, of the finished LED package.
The downside is removing the ceramic submount (and its heat spreading abilities) makes CSPs an intense ‘point source’ of heat that most MCPCBs simply cannot handle. The challenge is conducting the concentrated thermal flux through the dielectric layer of the PCB and into the metal board where it can be spread and removed by the heatsink. If the heat is not removed quickly enough there’s a significant risk of the LED overheating and failing catastrophically. This situation is exacerbated by the ability to mount CSP LEDs extremely closely together — a benefit for shrinking module designs, but a major headache when it comes to thermal management as the intensity of the heat is increased significantly.
The key to understanding this challenge is the dielectric layer. MCPCBs are usually made of an epoxy resin mixed with ceramic to create a thermally conductive, but electrically isolating, barrier. However, there’s a limit to how much ceramic can be added before the composite become friable, restricting the thermal conductivity of the layer.
Cambridge Nanotherm’s approach to thermal management provides a unique solution to this challenge. A heavily patented electro-chemical oxidation (ECO) process converts the surface of an aluminium board into a super-thin alumina dielectric layer. This nanoceramic alumina has a thermal conductivity of 7.2 W/mK, which, coupled with being just tens of microns thick, makes for a composite thermal performance of 115 W/mK — much higher than any competitive MCPCB. This means the heat from the CSP LEDs is conducted efficiently through the dielectric and into the aluminium board, ensuring the LED junction temperature is kept at a stable temperature.
Cambridge Nanotherm sales and marketing director Mike Edwards said: “CSPs, particularly Nichia’s D.M.C. LEDs, bring significant cost and manufacturability benefits to LED designers. However, by removing the heat spreading submount they push the thermal challenge from the LED manufacturers to the module and luminaire designers who now need new and innovative ways to handle the heat.
“Epoxy-filled MCPCBs struggle to cope with the thermal profile of CSP designs, particularly when they are mounted close together on a module. Nanotherm’s unique nanoceramic MCPCBs overcome these limitations, enabling designers to build increasingly power dense modules. This, coupled with our comprehensive manufacturing capabilities, offers designers the optimum route to realising their CSP designs.”