SUMMARY : LEDs generate a significant amount of heat as a by-product of the visible light they produce. If not managed properly, this heat can lead to an early demise for some consumer electronics. Insulated metal substrates can help manage this problem and xtend the useful life of products.
Arguably, the invention of incandescent light bulbs in 1879 started a global revolution. Light quickly became a daily necessity and was soon expected when entering a building, when guiding the way on a long journey or when us ing modern conveniences like the flashlight app on a smartphone. As the world has produced more and more light bulbs, their power demands have drastically increased. Energy is becoming more expensive and scarce and the world is looking to energy efficient products to help ease the stress of keeping the lights on. The light bulb has evolved from its incandescent roots through fluorescent adolescence to its current iteration as a mature, environmentally friendly LED. Its power demands have evolved over time, as well.
A typical residential LED bulb can use as little as 10% of the power of a traditional incandescent bulb and can last up to 40x longer. These capabilities make it easy to see why there has been an explosion in LED use over the last few years and why global brands like IKEA have undertaken plans to phase out all non-LED lighting by 2016. With LEDs finding their way into everything from consumer electronics to the neighborhood streetlamp, manufacturers must consider the bad and the good when projecting or guaranteeing the longevity of their products. On one side there is a product that can shine brighter and longer than any traditional bulb, but at the same time these activities cause products to run at a very high temperature, which can create significant challenges for manufacturers.
A LED uses only a fraction of the energy it absorbs to create light while converting the rest into heat. This unfortunate by-product must be managed effectively or it will take a destructive toll on the completed product. Heat has become a leading threat to the useful life of LED-based products. When a consumer purchases a 90” LED television to watch the Superbowl, there is an expectation that it will continue to be the life of the party for years to come. These expectations will continue to rise as consumers invest more and more in premium products such as the forthcoming ultra HD 4K and 8K resolution television technologies (UHDTV). The bar for longevity is also set high for LED-based technologies used in safety-critical equipment.
Consumers want peace of mind that automotive headlights will shine from coast to coast when taking the family for a long summer road trip. Government legislation will be sending halogen household bulbs the way of the dodo bird in the very near future. In the United States, light bulbs will be required to be up to 75% more efficient by the end of the decade. This seismic switch will see consumers investing billions of dollars in energy efficient lighting, one bulb at a time. Many of these light bulbs are sold with the expectation to last from 30,000 to as much as 100,000 hours.
There is a responsibility on behalf of the manufacturer to keep the lights on for years to come while guiding the world to a more environmentally conscious existence. Super long-lasting LED bulbs are extremely sensitive to heat and without the technology of a sophisticated heatsink, reaching these significant milestones would be impossible. A common design for effectively dealing with the excess heat generated by a LED is to mount it on a metal core printed circuit board (MCPCB or MPCB), which becomes a heatsink. This design guides the unwanted heat through the metal core of the MCPCB to the metal side where it can be dissipated and left to do no harm. The metal side can also be attached to an additional heatsink if further cooling is required. The PCB design is printed on an insulated metal substrate (IMS) laminate, which consists of a very thin dielectric layer sandwiched between a metal baseplate (generally made of aluminum) and a thin layer of copper. LEDs and other components are only mounted on the thin copper side. IMS materials are also available with a copper baseplate ideal for high power thermal performance. The PCB industry is used to working with FR-4 laminates and may not be aware of some of the behaviours of aluminium- based substrates such as IMS. The operational temperature of an aluminium substrate tends to be very high as electronic designers aim to transfer the heat from the components such as LEDs into the aluminium substrate and use it as a heatsink.
Maintaining a proper maximum operating temperature (MOT) of the substrate is very important, especially in the interconnect layer under the heating element. When the substrate is heated past its MOT, degradation of the resin inside the dielectric layer can occur with high risk of dielectric reduction or delamination. Today the use of LEDs of four or more watts is not uncommon when only a few years ago they would rarely exceed one watt. The increased demands of these high-power LEDs makes keeping the recommended temperature stable (~consider when selecting a laminate is that only manufacturers with more than 10 years of experience in IMS laminates (such as Aismalibar, Bergquist and Denke) can guarantee life of over 60,000 hours, as newer manufacturers have not existed long enough to fully test their material under these conditions. In Figure 2, projections are at 100,000 hours and actual results at 60,000 hours of durability for the dielectric strength. Data was extracted from the tests done by UL for the calculation of the electric RTI of Aismalibar’s Cobrithermand graphics were obtained from UL tests. RTI (relative temperature index) testing has two parts: mechanical and electrical. The RTI electrical test is performed by baking panels to specific temperatures and then running voltage through the copper side of the IMS material. Coupons of the panels are then measured at identified time intervals to record their retention of dielectric properties.
Once the test reaches retention below approximately 50%, the test is stopped and the total hours are recorded. The RTI mechanical test is performed in a very similar way; however, it measures the peel strength at identified time intervals. The UL numbers in Figure 2 show that Aismalibar’s Cobritherm (which the LED lighting across the entire surface of the Torre Agbar Barcelona is mounted on) can stand 100,000 hours at 118°C while keeping 50% of the initial dialectical strength value. The initial dialectic strength of Cobritherm was 10kV. After 100,000 hours it can be seen that Cobritherm will stand. By guaranteeing these values it can be assured that LEDs will dissipate their heat to the aluminium or copper substrate correctly and the IMS material will not be the cause of failure for the LED.
Manufacturers should choose their IMS laminates wisely and look closely at the history of the supplier to ensure the reliability of the material. Only after the technology of the typical LED is broken down can one fully understand the need to move heat effectively and have reliable dielectric strength. When guaranteeing the useful life of your LED product be sure to consider the quality of the IMS material on which it is mounted to ensure the claims on the box match the results the consumer achieves.