LED lighting modules: taking the heat out with the situation
Managing temperatures employing suitable packaging supplies is vital to ensure the reliability of high-power LED applications, explains James Stratford of Universal Science.
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The appeal of working with LEDs in lighting applications is expanding rapidly. The numerous and significant benefits of utilizing modules that incorporate a matrix of LEDs are getting recognized by design engineers in quite a few crucial business sectors, including aerospace, architectural lighting, and also the golden egg automotive market place.
Attributes for instance style flexibility, low power consumption, even and dependable light, and lengthy lifetime distinguish LED modules from designs according to regular filament lamps and fluorescent tubes. LEDs may also have knock-on benefits, for example tremendously decreasing the size and complexity from the module and simplifying the lens style.
A superb instance of some other benefits of LED lighting is demonstrated by an application inside the cabin of a passenger aircraft. A retrofit LED unit that replaced a fluorescent-tube lighting module enabled finely controlled dimming as well as supplied mood lighting through the use of differently coloured LEDs.
Perhaps one of the most difficult concern when realizing a module style that makes use of LEDs would be to handle the temperature of person device junctions throughout typical operation. If the considerable quantity of heat created by all of the devices in a module will not be managed properly then the junction temperatures could reach a level exactly where the LEDs expected life is shortened and reliability is compromised (see Hyperlinks).
LED modules normally comprise a matrix of lots of surface mount devices. These LEDs are soldered to an etched copper layer that supplies the interconnects among the individual LEDs as well as other passive and active components which might be required to total the circuit. The modest size in the LEDs plus the close proximity with which they’re able to be mounted implies that designers possess a substantial amount of style freedom and can attain complicated lighting patterns with high levels of brightness.
The etched copper circuit is separated from a base plate generally made of aluminum by a thermally efficient, electrically isolating dielectric material. The characteristics and capabilities with the dielectric layer are crucial for the design flexibility and overall performance in the overall module.
Dielectric materials are produced by blending thermally effective materials such as alumina and boron nitride with other ingredients, to provide a flexible but resilient coating on the base plate. A crucial characteristic with the dielectric layer would be the quantity of electrical isolation it offers in between the copper on the topside and the metallic base plate on the underside. This can be identified as its dielectric strength. A typical dielectric material might possess a dielectric strength of around 800 V/mil and be coated onto the base plate to a thickness of 812 mils (1 mil = 1 inch3 = 25.4 m).
Dielectric supplies utilised on insulated metal circuit boards usually have a thermal conductivity figure inside the region of 3W/mK. This can be roughly 10 occasions the performance accomplished by FR4 (flame retardant woven glass reinforced epoxy resin) PCB material.
A further key requirement of the dielectric layer is always to have the ability to compensate for the distinct coefficients of thermal expansion from the copper track on the topside from the assembly plus the aluminum base plate/heat spreader on the bottom side.
Flat sheets of insulated metal circuit board comprising copper foil, a dielectric layer and an aluminum base plate happen to be obtainable for a number of years. Inside the eyes on the forward-thinking LED module designer, the main challenge has been that flat sheets of insulated metal circuit board limit them to 2D shapes.
To address these limitations, new dielectric supplies are becoming out there which have a low modulus, which means that they’re compliant with mechanical stress and strain. These supplies not simply accommodate the coefficient of expansion with the metal components from the construction, but in addition allow parts to become formed into suitable angles, and also through 360˚. This enables designers to realize complex-shaped designs and ones that form a total circle with either internal or external copper traces.
When designing with new, formable insulated metal circuit board materials it is possible to route the tracks around corners, which alleviates the need to use connectors and hard wiring. There are a number of rewards to this, such as enhanced reliability resulting from having fewer junctions and interconnects. Despite the slightly higher cost with the new supplies, the general cost is reduced because fewer components are needed, and assembly time is reduced.
Strength and durability
LEDs themselves are inherently durable. Mounting them on metal based circuit boards only serves to enhance their robustness and that of the finished module, providing excellent resistance to vibration and mechanical shock.
Automotive lighting clusters supply an excellent instance of how LED modules can supply superior overall performance compared with regular filament lamps. On-vehicle applications experience high levels of vibration and wide operating temperature ranges that can cause premature failure of filament lamps. In some operating conditions LEDs can last up to 100,000 hours, which indicates that they should not require any attention for the life of your vehicle.
The lengthy life of LEDs also simplifies the designers task because it is less essential to make the lighting module accessible for servicing in the finished product. This can result in a neater, more integrated installation and also in potential cost savings.
Thermal analysis software packages are readily available to help prove LED based module styles before they are committed to manufacture.
These software packages gather data from an integrated database about LED efficiency and specifications along with those of other devices which might be mounted on the insulated metal circuit board. This data is combined with other information about elements of your style, including the copper traces, power and ground planes, and vias. The collated information is then processed to produce an accurate representation on the thermal performance on the style.
User-friendly graphical representations from the results enable the design engineer to quickly pinpoint areas that might require attention, proper down to component and track level.
Thermal analysis software can bring considerable commercial and style rewards by helping speed the time to market and lowering the number of iterations needed to reach a production-ready solution.