A critical aspect of effective
LED –based product designs is ensuring that individual lamps are thermally managed. Although device manufacturers are continuing to improve the durability of LEDs at raised operating temperatures, excess heat in LEDs still has both an immediate impact on performance as well as longer term consequences as can be seen in Fig.1. Short-term, mostly reversible effects include colour shift and reduced light output, while the long-term impact of poor thermal management is accelerated
lumen (light output) depreciation meaning a curtailed useful life.
Fig. 1: Impact of temperature on LED life expectancy (Source: Cree)
As design approaches change and LED technology advances, there is a definite market shift towards the use of mid-power (sub one watt) as opposed to high-power devices (one to three watt) for many LED lighting applications.
By using a higher number of mid-power devices more closely positioned to one another in an array as opposed to a lesser number of high power devices, a more even light can be achieved. This is especially important in larger designs such as where LEDs arrays are used instead of CFL tubes. This kind of arrangement also simplifies the design, and therefore cost, of the lens/cover fitted over the array as it is not required to mix and spread the light output so much as with ‘point’ light source of a small number of high power devices.
The approach to thermal design for high-power LEDs is inappropriate and ‘overkill’ for mid-power devices and with careful and early consideration to the design there are opportunities to reduce overall design cost, weight and complexity without impacting performance, reliability and expected life of the lighting assembly in any way.
Thermally managing high-power LED designs
Despite the growing popularity of mid-power LEDs, higher power devices are still likely to remain the best choice for a significant number of applications until further advances in technology allow even greater lumen per watt output. The power dissipated by high power LEDs is still of course much, much lower than that of other older lighting technologies, however, heat generated in normal operation is concentrated in a much smaller area, base of the individual LED device. By effectively managing the heat in the base, the junction temperature of the diode can be maintained at a level where short-term & long-term performance is not impacted.
The use of insulated metal circuit boards offers one of the most efficient way of dissipating heat at the required rate and magnitude from designs that utilise high power LEDs. The construction of insulated metal circuit boards comprises a sandwich construction of an aluminium sheet, coated with a very thin electrically insulating dielectric material that possesses a high thermal conductivity. This is then topped with a bonded copper layer that is etched using conventional PCB processes. The LEDs & any other devices required to complete the circuit are then soldered onto the etched copper traces.
The thin, high thermal efficiency dielectric layer ensures that heat is dissipated to the aluminium baseplate efficiently and effectively. However, in most high power LED designs the thermal performance of the baseplate alone is not enough to provide effective thermal management during the prolonged operating cycles typical in lighting applications. Therefore, the insulated metal circuit assembly must be attached to a larger heatsink or the metal chassis of the equipment or overall product. Using mechanical fixings and a material such as thermal grease tend to provide inconsistent results due to the difficulty in maintaining repeatability of thermal grease application. Also, performance over time may degrade due to the grease flowing from the interface or a relaxing of the closure force of the assembly. A more effective approach that also yields cost savings due to the need for mechanical fixings being alleviated is to use a thermally efficient structural adhesive tape. Materials such as Bondline 200 from Universal Science feature a strong, pressure sensitive adhesive on both sides of a thin foil of aluminium having a thermal conductivity of around 140 W/mK. These types of material exhibit cold flow characteristics when installed between two metals surfaces which means that micro air voids are effectively removed and heat transfer between insulated metal circuit & chassis or heatsink is maximised; this performance is enhanced due to a thin bondline of around 0.16mm which shortens the thermal path between the metallic surfaces. Typically available in rolls in a range of standard widths, or for high volume production, as custom die-cut shapes thermally adhesive tapes allow fast repeatable assembly & deliver long-term, high levels of stable thermal performance.
Taking a different approach for mid-power device-based applications
Mid-Power LEDs – those rated at around 0.5W to 1.0W dissipate less power and so do not require the high thermal performance of an insulated metal circuit to dissipate heat from their junctions. This presents the designer with an opportunity to reduce the cost and weight of their design by using a standard thin (0.6 – 0.8mm) FR4 printed circuit board material. Mid-power LED lighting product designs can use a format of FR4 that is familiar to designers and PCB fabricators/ processors throughout the electronics industry: that is a thin sheet of glass-reinforced epoxy laminate clad top and bottom with a thin copper foil. During the chemical etching process the copper is selectively removed from the board to leave a finished circuit of copper lands where the individual devices – in this case LEDs and supporting passive components – are mounted, and joined by interconnecting tracks to complete the electrical circuit. On the underside of the board it may be necessary to have some tracks connected to the top side using plated via holes in order to achieve the desired circuit layout. However, to maximise the heat spreading effect, as much copper as possible should be left in place on the underside of the board. In order to promote the optimum heat flow from the individual LEDs mounted on the PCB, the copper pads on which the individual devices are mounted are drilled with multiple vias which are then plated in order to create a small matrix of ‘heat pipes’ to draw the heat from the die of the LED through the PCB to the larger copper area on the underside of the board. If required, the thermal efficiency of this top-to-bottom connection can be enhanced further by ‘flooding’ the via holes with solder.
Photo Credit: Bondline 200 thermal adhesive tape star
Photo Credit: Bondline 700 thermal adhesive tape
While mid-power LEDs generate a lot less heat than high-power devices, it is still necessary to attach the FR4 circuit to a more substantial metal surface in order to continually remove the heat and maintain acceptable device operating temperatures. Using a thermal adhesive tape can provide the best approach for achieving this and also removes the need for screws and other mechanical fixings. For the reasons previously described, thermal grease does not provide a consistent, reliable long-term solution and also necessitates the need for screws and other fixings. Modern thermal adhesive tapes such as Universal Science’s Bondline 700 can provide the required combination of adhesion and thermal performance and are specifically designed for bonding FR4 circuits to chassis or other heatspreaders/heatsinks.
In addition to providing a permanent bond that increases to a maximum level over the first 24 hours after installation, Bondline 700 also flows to fill the micro air voids that can form between the adhesive layer and the underside of the FR4 circuit due to the small ‘steps’ that exist between the copper areas and the FR4 where the copper has been selectively etched away. Other undulations and surface irregularities that need to be filled may also exist on the chassis / heat spreader side of the sandwich assembly. With FR4 the right selection of the interface material and mechanical attachment is much more critical than with MCPCB as can be seen in Fig. 2. Where even higher levels of thermal performance are required Bondline 1800 provides conductivity of 1.8W/mK.
Fig. 2: Image of FR4 using thermal camera
Summary
With both high power and mid-power LEDs providing options for designers a wide range of commercial and industrial lighting solutions, it is important that the correct decisions are made with regard to thermal management. As well as giving the appropriate degree of heat spreading and dissipation, the right choices can also save money, reduce weight and minimize design complexity.
James Stratford, CEO of Universal Science Group, got trained as an electro-mechanical design engineer with Her Majesty’s Government Communications.
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