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Home > News > Company News > More heat than light for UVC L.....

More heat than light for UVC LEDs

  • Author:Ella Cai
  • Release on:2017-09-08
he old saying about an argument generating ‘more heat than light’ also sums up the thermal challenge in the growing market for UVC LEDs being used to disinfect medical instruments, water and other everyday consumer products, writes John Cafferkey of Cambridge Nanotherm.

Ultraviolet C (UVC) technology has its roots back at the beginning of the 20th century when mercury vapour lamps were first mass-produced.

UV-emitting lamps were used for drinking water disinfection in 1910. However, the prototype plant proved to be less than reliable and was shut down.

In the 1950s new UVC water treatment systems were trialled and by the mid-1980s there were about 1,500 plants across Europe.

Beyond water treatment UVC is used in applications ranging from cleaning medical equipment and hospital rooms to disinfecting heating, ventilation, and air conditioning (HVAC) systems to prevent the spread of pathogens.

While mercury vapour lamps are extremely effective at these large‑scale applications the fragile nature of the bulbs and the use of hazardous mercury means they are unsuitable for more portable and consumer-friendly applications.

In recent years LED manufacturers have developed increasingly effective UVC LEDs. While not as efficient as UVA LEDs (which are used for curing inks and paints) they are becoming viable for low-power applications.

It’s this move into new markets that has led industry analysts Yole Développement to predict an explosion in UVC LED growth from $7m in 2015 to a staggering $610m by 2021.

This boom is predicated on UVC LEDs creating a market in portable, consumer-friendly UVC applications. For example, consumers will be able to buy portable disinfection ‘wands’, which they can use to sterilise everyday items, like smartphones, tablets or keyboards.

Consumer goods manufacturers will be able to embed LED UVC technology into products to make self‑disinfecting items. For example, a toothbrush could disinfect itself after you put it back into its holder, a baby’s milk bottle could self-sterilise at the push of a button, and a tap could sterilise water as and when you use it – the possibilities are endless.

Life-saving potential

However, it is some of the more profound applications of UVC that are really grabbing attention. Portable water sterilisation bottles could improve the way developing countries offer clean water to their citizens by sterilising water at the point of consumption. This will be especially valuable in areas where centralised water sterilisation infrastructure does not exist, or at disaster sites where safe water could be quickly provided.

New applications aside, even hospitals (where UVC has been used for years) can benefit. Globally, each year over 700,000 patients – equating to one in every 25 patients – suffer from an infection while hospitalised, leading to 75,000 deaths. UVC technology could be embedded in medical implements like stethoscopes and scalpels, which could be sterilised in seconds.

UVC LEDs hold the potential to bring the sterilising power of UVC to a mass market and could have big implications for public health.

The thermal challenge

The technology is still in its infancy, and among other challenges one barrier is the thermal management of the UVC LEDs. Like any electronic component, LEDs are sensitive to heat.

UVC LEDs have a particularly low external quantum efficiency (EQE) – they only convert about 5% of the power input into light. The remaining 95% of the power is converted to heat which must be quickly removed to keep the LED junction below its maximum operating temperature. Failure to keep the LED die cool will at best shorten its life, and at worse cause it to fail catastrophically.

As UVC LEDs become more powerful (the latest is 75mW) manufacturers need to look at new ways to address this challenge. The question remains how to deal with the high thermal demands of UV LEDs while ensuring that components remain cost effective, durable, machinable and resistant to wear from the UV light source itself.

As UV LEDs are too small to radiate significant amounts of heat from their surface the only way it can escape effectively is through the back of the LED. Heat must be conducted from the LED chip, through the module PCB, before reaching the heatsink which releases it out to the atmosphere.

Heat is a PCB issue too

The PCB on which the LED is mounted must have a high thermal conductivity, which for a visible light LED would usually be a metal-clad PCB (MCPCB). However, these are not suited to UVC applications. MCPCBs are manufactured from a sheet of metal (usually aluminium or copper) with the copper circuit layer attached with a dielectric layer of thermally conductive, but electrically insulating, epoxy.

MCPCBs based on epoxy dielectrics are useful in visible light applications but UV (and in particular UVC) degrades organic substances such as epoxy, significantly reducing the lifespan of MCPCBs used in UV applications. The only viable alternative is to use electronics-grade ceramics.

Of these, aluminium nitride (AIN) features excellent thermal conductivity (140W/mK-170W/mK), but it is also expensive. Aluminium oxide (Al2O3) is a more cost-effective alternative but doesn’t offer the thermal conductivity needed for UV LEDs (20W/mK-30W/mK). Both are brittle and can damage easily, which means they cannot easily be screw-mounted and are not suitable for more rugged applications.

The alternative is nano-ceramic with a sputtered circuit layer. This provides a more‑than‑adequate thermal conductivity of 150W/mK, well within the thermal range required.

Using a patented electro-chemical oxidation (ECO) process, Cambridge Nanotherm used this approach to transform the surface of an aluminium board into an incredibly thin layer of alumina (Al2O3) ceramic measuring just tens of microns thick.

This layer acts as the dielectric between the circuit above and the aluminium below. As the layer produced by the ECO process is extraordinarily thin, heat can easily pass through, giving the substrate its exceptional thermal conductivity.

Thin-film processing, or sputtering, follows the ECO process, which directly attaches the copper circuit layer to the nano-ceramic dielectric to further improve the thermal efficiency of the stack. At no stage is organic epoxy used, so there is nothing for UV to degrade.

This whole approach creates an MCPCB with a thermal performance that rivals aluminium nitride (AlN) but without any of the problems associated with poor manufacturability or brittleness.

UVC sterilisation and disinfection enabled by LEDs is a technology that can have a genuinely transformative effect. But for analysts’ projections to become reality manufacturers and designers will need to ensure they can overcome the thermal challenges UV LEDs present.