The thermal implications of small, portable UV LED disinfection
What are the benefits of UVC disinfection?
UVC disinfection is an extremely safe and effective way of killing bacteria and stopping the spread of pathogens. UVC light disrupts the genomes of microscopic organisms to kill bacteria. Thus, UVC light will disinfect any surface it is directed at. Unlike chemical approaches to disinfection, UVC provides rapid inactivation of microorganisms through a physical process. When bacteria, viruses and protozoa are exposed to the germicidal wavelengths of UV light, they are rendered incapable of reproducing and spreading infection.
UV light even works on chlorine-resistant pathogens like cryptosporidium and giardia, and can be used (alone or in conjunction with hydrogen peroxide) to break down toxic chemical contaminants while simultaneously disinfecting. In addition, UVC requires no transportation, storage or handling of toxic or corrosive chemicals, and UV treatment creates no carcinogenic by products that could adversely affect whatever is being disinfected.
There are also significant cost benefits of using UVC for disinfecting. Operating costs are largely limited to maintenance and electrical consumption. There’s less need to pay for cleaning chemicals (and the costs associated with the safe transportation and delivery of these chemicals). With UVC light you can also bypass or reduce the costs associated with chemical exposure, risk management and emergency planning and operator training.
Overall the benefits of UVC are huge. But there’s a catch that limits the number of applications it can be integrated into.
Current technology limits UVC applications
Traditionally, UVC disinfection devices use mercury lamps. These have large, fragile bulbs and the mercury contained in the them is a hazardous material, making UVC mercury lamps unsuitable for use anywhere where they could easily be broken.
This has tended to limit the use of UVC disinfection to fixed installations in applications such as municipal water purification, HVAC sterilisation, room and medical equipment sterilisation in hospitals, and sterilisation in factories that produce food and drink or pharmaceutical products.
UVC is now at a tipping point
The advent of LED technology that can operate effectively in the germicidal UVC range is set to transform this market, opening up the benefits of UVC to a whole host of new applications. The potential of the technology is so big that market analysts Yole Développement predict that the $7 million UVC market will explode to $610 million by 2021.
Opening up a new world of UVC disinfection
UVC LED technology is significantly smaller and more flexible, controllable and cost-effective than its mercury lamp alternative. In particular, the small size of UV LEDs, and the fact that these LEDs can be arranged to fit a much wider range of form factors, allows manufacturers to produce smaller, portable (even hand held) disinfection devices. The size and robustness of UVC LED technology means that such LEDs could even be embedded within devices themselves, turning them into self-disinfecting devices. These factors open up a host of new applications.
In homes, portable and convenient UVC LED ‘wands’ could be used to disinfect surfaces around the home or your computer keyboard, while baby bottles could be sterilised in UVC enabled containers. Sterilisation could take place in the pipes of a home, or even the tap itself. Even toothbrush holders could be embedded with the technology to disinfect toothbrushes at the push of a button.
In hospitals, UVC disinfection robots (albeit not currently using UVC LEDs) are already growing in use, but this is just the start. UVC LED technology could be embedded within instruments such as stethoscopes and scalpels to sterilise them within just a few seconds when required. This could help to stem the alarming growth in hospital-based infections such as MRSA due to inadequate sterilisation of hospital objects and patients: ‘One in every 25 hospital patients contracts at least one healthcare-associated infection during their visit. Globally, each year over 700,000 patients suffer from an infection while hospitalized, leading to 75,000 deaths.
Small, affordable, accessible UVC LED form factors also hold the potential to enable sterilisation of water at the point of consumption. This would be of use anywhere that clean water is unavailable, and in particular in countries that lack centralised water sterilisation infrastructure or are affected by natural disasters. As such, UVC LED technology holds the potential to prevent millions of deaths every year.
But for the market to meet the weighty expectations placed on it by the analysts, designers will have to address one critical factor — how they will deal with the thermal demands of UV LEDs while ensuring that components remain cost-effective, durable, machinable and resistant to wear from the UV light source itself.
Why UVC presents such a significant thermal challenge
UVC LEDs convert only 5% of the power put in to light, with the remaining 95% converted into heat. And UVC LEDs, like any electronic component, are extremely vulnerable to heat. This heat needs to be removed otherwise the LED die risks overheating and failing, which will impair its lifespan and light quality.
The size of UV LEDs is too small to allow for any significant loss of heat through ambient radiation or convection. The only way heat can be removed is through conduction through the back of the LED to the PCB and then to the ambient atmosphere via a heat sink. It stands to reason that the PCB on which the LED is mounted must have a high thermal conductivity.
raditional visible-light, high-power LEDs use metal-clad PCBs (MCPCBs) to provide the thermal performance required. However, most MCPCBs aren’t suited to UVC applications because of the way they’re manufactured. Most are manufactured from a sheet of metal (usually aluminium or copper) with the copper circuit layer attached with a layer of thermally conductive, but electrically insulating, dielectric. This dielectric poses the problem: Conventional MCPCBs use an organic epoxy to attach the copper circuit layer which aren’t suitable for UVC LEDs as UV degrades organic matter. This means the options are limited to inorganic ceramic PCBs.
One choice is aluminium nitride (AIN) which has excellent thermal conductivity (140–170 W/mK) but is also expensive. Alumina (Al2O3) is a cheaper alternative but has limited thermal conductivity (20–30 W/mK). And thermal conductivity and price aside, both materials are very brittle, which means they can’t easily be mounted with screws and risk breaking in applications that operate in harsher environments.
Sputtered nanoceramic (direct metallisation) as an alternative
Cambridge Nanotherm has come up with a solution that manages to overcome the limitations demonstrated by both MCPCBs and ceramics, resulting in an inorganic PCB with a thermal conductivity of 150W/mK, enough for all but the most aggressive UVC LED applications. This approach cleverly weds the best of both worlds; the robustness of aluminium with the thermal efficiency of ceramics.
Using a patented electro-chemical oxidation (ECO) process, Nanotherm converts the surface of an aluminium board into a layer of alumina (Al2O3) ceramic that is just microns thick. This acts as an electrically-isolating dielectric layer, preventing the circuit from shorting out against the aluminium heat spreader. Alumina isn’t the most thermally-effective substance, but because the layer is extraordinarily thin it conducts heat extremely effectively.
The circuit layer is attached to the nanoceramic using a direct plated copper process that bonds the copper to the ceramic dielectric at a molecular level without using any organic epoxy. This minimises impedance along the thermal path and ensures that heat is transferred between the layers as efficiently as possible. And as there’s no epoxy it makes the whole stack 100% inorganic so there’s nothing for UVC light to degrade.
The result is a board that demonstrates thermal qualities close to those of AlN while retaining the machineability and robustness of aluminium. Furthermore, this approach allows for much larger tile sizes to be manufactured which in turn means that designers can start to bring some of the economies of scale found in the MCPCB industry to bear on the UVC market.
Nanoceramic is enabling UV LED designers to overcome one of the key challenges they face in designing small, portable and low-cost UVC disinfection devices and helping this nascent industry to reach its full potential.
About the author:
John Cafferkey is marketing manager at Cambridge Nanotherm – www.camnano.com