Why thermal management is key to ensuring the success of UVC LED disinfection
31 August 2017
LEDs are changing our understanding of how we can use light. In horticulture, LED lighting is transforming tower blocks and basements into urban farms, enabling farmers to produce food in the heart of our cities and towns.
This article originally appeared in the September 2017 issue of Electronic Product Design & Test; to view the digital edition, click here – and to register to receive your own printed copy, click here.In local government, LEDs are driving down the cost and carbon footprint of street lighting, helping taxpayers’ money go further while protecting the environment. And in the home, LED lighting is reducing our energy bills while opening up a range of smart lighting possibilities.
Growth of LEDs in UVC disinfection
One particularly profound innovation enabled by LEDs is in the UV lighting industry. Sitting at the edge of the visible electromagnetic spectrum, with a wavelength between 200 to 400 nanometres, ultraviolet radiation (UV) is split into three bands: UVA (longwave), UVB and UVC (shortwave). UVA has applications in curing (for example, inks and glues) and UVB in medical phototherapy, while UVC has the unique ability to disrupt the DNA of bacteria – so is widely used for disinfection and sterilisation applications.
Prior to the development of UV LEDs, traditional lamps were used to generate UV light, particularly mercury vapour lamps. However, these are large, fragile, expensive – and use hazardous chemicals. Because of these factors, use of UVC disinfection has generally been restricted to large-scale commercial applications, such as municipal water purification, room and medical equipment sterilisation in hospitals, and sterilisation in food and drink or pharmaceutical factories.
However, the development of viable UV LED technology is about to transform the UVC market, enabling a much wider variety of applications. With LED technology, manufacturers can now produce lower cost, smaller and more user-friendly disinfection devices for everyday use. And UVC LED technology can even be embedded directly into objects themselves, making them self-sterilising. The types of objects that could feature this kind of technology is virtually unlimited.
For the home, taps could automatically sterilise water when turned on. Door handles, which hoard bacteria, could disinfect themselves immediately after someone has touched them. Baby feeding bottles, keyboards, smartphones, tablets, toothbrush-es – almost anything could become self-disinfecting, or could easily be disinfected with a small, portable device at the point of use.
The home isn’t the only area set to benefit from advanced UVC LED technology. Hospitals can benefit from improved disinfection techniques, helping to reduce the number of infection-related deaths each year. Hospital equipment such as stethoscopes and scalpels could have UVC LED technology built into cases and holders – making sterilisation as simple as putting the instrument into its case.
The potential of this technology is so significant that market analyst firm Yole Développement predicts that the $7 million UVC market will explode to $610 million by 2021. However, to fully realise the benefits of small form-factor UVC LED devices – and the UVC LEDs embedded within devices – there are still some major technical barriers to overcome. One of the most significant is thermal management.
The hot topic of UVC thermal management
LEDs, like any electronic component, are vulnerable to heat. If the LED junction is not kept below its maximum rated temperature, the LED is likely to fail – potentially catastrophically. UVC LEDs convert only around 5% of the power put in to light; the remaining 95% is converted into heat.
This heat needs to be removed from the LED die as quickly as possible. The maximum operating temperature of LEDs, at around 125°C, is too low (and their size too small) to permit any significant direct loss of heat to ambient by radiation or convection. Therefore, the only way the heat can escape is by conduction from the backside of the LED: through the PCB, into the heatsink and finally out to the ambient atmosphere. To cool effectively by conduction, the PCB on which the LED is mounted must have high thermal conductivity.
Visible light high-power LEDs use cost-effective metal-clad PCBs (MCPCBs) to manage the heat. Unfortunately, MCPCBs aren’t suitable for UVC applications. MCPCBs are manufactured from a sheet of metal (usually aluminium or copper) with the copper circuit layer attached to one side, bonded to a layer of thermally conductive – but electrically insulating – dielectric. The dielectric layer is a combination of organic epoxy filled with varying amounts of ceramic to improve its thermal performance. It’s the organic nature of the dielectric that presents the problem, as UVC degrades organic material. Exposure to UVC light will decompose the organic dielectric layer and trigger a number of component failure mechanisms.
This limits the choice of PCB to an inorganic ceramic. The standard options are aluminium nitride (AIN), which has excellent thermal conductivity (140 to 170 W/mK), but is expensive; or alumina (Al2 O3), a cheaper alternative, but with limited thermal performance (20 to 30 W/mK). Thermal conductivity and price aside, both materials are very brittle, which makes them unsuitable for more rugged applications and extremely difficult to mount on a heat sink (using screws or clamps) without fracturing the PCB.
Sputtered nanoceramic (direct metallisation) as a solution
A new solution overcomes these limitations, while offering thermal conductivity at 150 W/mK – which more than meets the needs of most UVC LED applications.
This new approach from Cambridge Nanotherm uses a patented electro-chemical oxidation (ECO) process to transform the surface of the aluminium board into a very thin layer of alumina ceramic. This alumina layer acts as a dielectric between the circuit above and the aluminium heat spreader below. While alumina doesn’t offer particularly high thermal conductivity, the extraordinary thinness of the layer makes it exceptionally thermally effective.
After the ECO process, the board undergoes thin-film processing (sputtering), which attaches the copper circuit layer to the nanoceramic dielectric; this creates a direct bond and ensures the highest possible thermal performance. And since no organic epoxy is used, there is nothing that UV can degrade.
This approach results in a nanoceramic MCPCB with a close thermal match to AlN, but which brings the robustness and the tile sizes of the MCPCB industry. This helps to bring some of the PCB industry’s economies of scale to the UV ceramics market.
For the UVC LED market to reach its true potential (and anywhere near that of the predictions by market analysts), manufacturers need to address the thermal management challenge head on. Cambridge Nanotherm is currently working with multiple manufacturers to tackle the issue, ultimately unlocking a whole host of LED UVC design options – and enabling the next generation of LED UV applications.
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