LED lighting modules: taking the heat out of the situation
The appeal of using LEDs in lighting applications is growing rapidly. The numerous and significant benefits of using modules that incorporate a matrix of LEDs are being recognized by design engineers in several key industry sectors, including aerospace, architectural lighting, and the “golden egg” automotive market. Attributes such as design flexibility, low power consumption, even and reliable light, and long lifetime distinguish LED modules from designs based on traditional filament lamps and fluorescent tubes. LEDs can also have knock-on benefits, such as greatly reducing the size and complexity of the module and simplifying the lens design. A good example of some other benefits of LED lighting is demonstrated by an application in the cabin of a passenger aircraft. A retrofit LED unit that replaced a fluorescent-tube lighting module enabled finely controlled dimming and also provided mood lighting through the use of differently coloured LEDs.
Thermal management Perhaps the most challenging issue when realizing a module design that uses LEDs is to manage the temperature of individual device junctions during normal operation. If the considerable amount of heat produced by all the devices in a module is not managed correctly then the junction temperatures may reach a level where the LEDs’ expected life is shortened and reliability is compromised (see Links). LED modules typically comprise a matrix of many surface mount devices. These LEDs are soldered to an etched copper layer that provides the interconnects between the individual LEDs as well as other passive and active components that are required to complete the circuit. The small size of the LEDs and the close proximity with which they can be mounted means that designers have a huge amount of design freedom and can achieve complex lighting patterns with high levels of brightness.
The etched copper circuit is separated from a base plate – usually made of aluminum – by a thermally efficient, electrically isolating dielectric material. The characteristics and capabilities of the dielectric layer are key to the design flexibility and performance of the overall module. Dielectric materials are made by blending thermally efficient materials such as alumina and boron nitride with other ingredients, to provide a flexible yet resilient coating on the base plate. An important characteristic of the dielectric layer is the amount of electrical isolation it provides between the copper on the topside and the metallic base plate on the underside. This is known as its dielectric strength. A typical dielectric material may possess a dielectric strength of around 800 V/mil and be coated onto the base plate to a thickness of 8–12 mils (1 mil = 1 inch–3 = 25.4 µm). Dielectric materials used on insulated metal circuit boards usually have a thermal conductivity figure in the region of 3W/mK. This is approximately 10 times the performance achieved by FR4 (flame retardant woven glass reinforced epoxy resin) PCB material. A further key requirement of the dielectric layer is to be able to compensate for the different coefficients of thermal expansion of the copper track on the topside of the assembly and the aluminum base plate/heat spreader on the bottom side.
Going three-dimensional Flat sheets of insulated metal circuit board comprising copper foil, a dielectric layer and an aluminum base plate have been available for several years. In the eyes of the forward-thinking LED module designer, the main problem has been that flat sheets of insulated metal circuit board limit them to 2D shapes. To address these limitations, new dielectric materials are becoming available that have a low modulus, meaning that they are compliant with mechanical stress and strain. These materials not only accommodate the coefficient of expansion of the metal elements of the construction, but also enable parts to be formed into right angles, and even through 360˚. This enables designers to realize complex-shaped designs and ones that form a complete 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 several benefits to this, including enhanced reliability resulting from having fewer junctions and interconnects. Despite the slightly higher cost of the new materials, the overall 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 provide a good example of how LED modules can provide superior performance compared with traditional 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 means that they should not require any attention for the life of the vehicle. The long life of LEDs also simplifies the designers’ task because it is less important 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. Temperature modelling Thermal analysis software packages are available to help prove LED based module designs before they are committed to manufacture. These software packages gather data from an integrated database about LED performance and specifications along with those of other devices that are mounted on the insulated metal circuit board. This data is combined with other information about elements of the design, including the copper traces, power and ground planes, and vias. The collated information is then processed to produce an accurate representation of the thermal performance of the design. User-friendly graphical representations of the results enable the design engineer to quickly pinpoint areas that may require attention, right down to component and track level.
Thermal analysis software can bring significant commercial and design benefits by helping speed the time to market and reducing the number of iterations needed to reach a production-ready solution.