Consistent colour, long lifetime and high reliability for LEDs

Ms Rachel Gordon
Consistent colour, long lifetime and high reliability for LEDs
Managing temperatures using appropriate electronics packaging materials is essential to ensure the reliability of high-power LED applications.
The appeal of using LEDs in lighting applications is being recognized by design engineers in many industries, including aerospace, automotive and architectural lighting. LED units are being retrofitted into cabins of passenger aircraft to replace fluorescent tubes and so provide dimming capabilities and mood lighting.
Perhaps the most challenging issue when designing an LED module is to manage the temperature of individual device junctions during normal operation. If the heat produced by all the devices in a module is not managed correctly then the junction temperatures may reach a level where the LEDs' colour spectrum is changed, lifetime is shortened and reliability is compromised. More information about thermal management can be found in the IDTechEx report, Thermal Interface Materials 2016-2026.
LED modules typically comprise a matrix of many surface mount devices. These LEDs are soldered to an etched copper layer that provides 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. However, it also means a large amount of heat is generated in a small area.
LEDs themselves are inherently durable. Mounting them on metal based circuit boards enhances their robustness and that of the finished module, providing excellent resistance to vibration and mechanical shock, making them useful in automotive and aerospace applications.
The etched copper circuit is separated from the aluminium base plate by a thermally conductive but electrically isolating dielectric material. Dielectric materials are made by mixing thermally conductive materials such as alumina and boron nitride particle with polymers, to provide a flexible yet resilient coating on the base plate.
Dielectric materials used on insulated metal circuit boards usually have a thermal conductivity figure in the region of 3 W/mK. The dielectric layer must be able to compensate for the different coefficients of thermal expansion of the copper track on the topside of the assembly (~12 x 10-6 K-1) and the aluminium base plate on the bottom side (~23.1 x 10-6 K-1). Flat sheets of insulated metal circuit board comprising copper foil, a dielectric layer and an aluminium base plate have been available for many years, but are limited to 2D shapes.
New dielectric materials are becoming available that have a low modulus, meaning that they are compliant with mechanical stress and strain. These materials accommodate the coefficient of expansion of the metal elements of the construction, and also enable parts to be formed to realize complex designs. Despite the slightly higher cost of the new materials, the overall cost is reduced because fewer components are needed, and assembly time is reduced.
Desirable attributes of LEDs include design flexibility, low power consumption, even and reliable light, and long lifetime. These distinguish LED modules from designs based on traditional filament lamps and fluorescent tubes. Using LEDs can have knock-on benefits, such as reducing the size and complexity of the module and simplifying the lens design.
Automotive lighting clusters provide a good example of how LED modules can provide superior performance compared with traditional filament lamps. Automotive applications experience high levels of vibration and wide operating temperature ranges. LEDs can last up to 100,000 hours, which means that they should last longer than the vehicle. The long life of LEDs means the lighting module accessible doesn't need to be accessible for servicing. This can result in a neater, more integrated installation and also in potential cost savings. More information about electronics in the automotive industry can be found in the IDTechEx report Printed and Flexible Electronics in Automotive Applications 2016-2026.