Why the UVA LED Curing Industry Needs Nanoceramic Thermal Management
By John Cafferkey, Marketing Manager, Cambridge Nanotherm
Over the past 30 years or so, UVA curing has revolutionised the way manufacturers have made products. UV’s ability to change a liquid to a solid has paved the way to widespread ink curing, adhesive bonding and coatings used in a large number of applications, like general consumer electronics, automotive, telecoms, graphic arts and more.
For decades UVA curing used high-intensity discharge (HID) UVA lamps. Only recently has the industry seen LED alternatives come to the fore. Compared to HID lamps, LED form factors are smaller and less fragile, while operationally they use less power, run cooler, and power cycle instantly when needed. These characteristics are improving the efficiency of the UV curing industry while opening up new curing techniques through hand-held devices — something that would not have been possible with HID lamps.
While LEDs run cooler than HIDs, they are still relatively inefficient. LEDs only convert around 40% of the power that goes into them as light, converting the remaining 60% as heat. This heat is a huge problem because if it can’t escape quickly enough, it can cause the quality of the UVA light to deteriorate or, worse, cause the LED to fail and/or shorten its lifecycle. And as designers today seek to pack more, and more powerful, UVA LEDs closer together on modules, this heat problem is only set to worsen.
Why heat is a problem for LEDs
LEDs cannot convect or radiate enough of the heat away from the source through the ambient air surrounding them because the surface area is too small and the temperature too low for this process to take place. The only way LEDs can shed excess heat is by conduction out of the back of the die, through the materials in the PCB, to a heatsink and the surrounding environment.
Therefore, the substrate material that the PCB uses needs to a high level of thermal efficiency. With UVA applications, the module substrate tends to be either a thermally effective metal-clad PCB (MCPCB) or electronics-grade ceramics like aluminium nitride (AlN). In higher-power density applications, epoxy-based MCPCBs do not have the requisite thermal performance (<100W/mK vs 170W/mK for AlN), making AlN the substrate of choice where heat is a significant issue.
However, AlN isn’t perfect. First, the material itself is expensive. Secondly its characteristics can pose problems for manufacturers — AlN is inherently brittle, making it difficult to manufacturer into circuits. That brittleness restricts the tile size to just 4x4-inches (occasionally 7x5-inch). Any larger and the brittleness begins to wreck the yield rate. Even with 4x4-inch tiles, a yield loss of 20% is not uncommon.
The other issue with brittleness is when mounting the finished module onto its heatsink. Ideally, the circuit should be attached as firmly as possible to reduce any air gaps between the circuit and the heatsink. But screwing a brittle AlN module to a heatsink is likely to fracture it if too much pressure is applied.
Now, however, there exists an alternative material that offers a comparative thermal performance to AlN without the issues around manufacturing. That alternative is nanoceramics.
Nanoceramics — a better choice than AlN
LED thermal management innovator Cambridge Nanotherm has devised a way to produce nanoceramics specifically for the UV curing industry. The secret is in the process. Using a patented electro-chemical oxidation (ECO) process, Cambridge Nanotherm converts the surface of an aluminium board into thin layer of alumina (Al2O3) just tens of microns thick. This alumina layer performs the dielectric function, insulating the circuits from the aluminium below, while conducting excess heat through the back of the LED quickly. And while alumina is far less thermally conductive than AlN the sheer thinness of the layer more than cancels that issue out, removing heat efficiently.
After the ECO process, Cambridge Nanotherm sputters the copper circuit layer directly to the nanoceramic dielectric via thin-film processing, further improving the thermal efficiency of the stack. Nanotherm DMS (a direct replacement for AlN for UV modules) has a composite thermal performance of 152W/mK, slightly lower then high-grade AlN but more than enough to cope with all but the most demanding UVA applications.
In terms of manufacturability, nanoceramics can be treated in the same way as a standard MCPCB. Because it’s less brittle, there’s less loss of yield, and the product can be screw-mounted to a heatsink without breaking. In short, nanoceramics offer the best of both worlds — the thermal performance of AlN and the robust characteristics of an aluminium MCPCB.
LED technology is having a profound impact on the UVA industry, and has seen costs drop and efficiencies increase. However, heat remains a central issue with LEDs, and a challenge that needs addressing if the curing industry is to see further cost and efficiency benefits.