According to a report from the market research firm Strategies Unlimited, Mountain View, Calif., sales of high-brightness light-emitting diodes (LEDs) are expected to grow to $20.2 billion by 2015. That’s nearly double the number of sales since 2010. One reason for this growth is increased use of LEDs in exterior lighting on cars and trucks. Exterior lights designed with LEDs offer several functions such as high and low beams. However, these high-brightness LEDs require more power than interior LEDs. This power generates more heat and creates higher operating temperatures. Hot LEDs experience a shortened lifetime, degraded efficiency, and diminished color. These are unacceptable problems in headlights, turn signals, brake lights, and other traffic-indicating lights where safety concerns are paramount.
LEDs become inefficient at elevated temperatures. An LED with a junction temperature at 100°C can last about 80,000 hr. If the temperature rises to 135°C, the LED loses 60,000 hr. The problem with using LEDs in exterior automotive lights is an even higher junction temperature, sometimes reaching upwards of 180°C.
LED manufacturers search for ways to mitigate these heat-related problems but some obvious methods come up short. Fans and liquid-cooling systems are rarely used because automotive LED designs are tight on space. Smartcircuit drivers are another solution. They prevent failure by limiting the amount of current to the LED when junction- temperature limits are reached. However, they lower the light output which can require an added push of current. If an LED is normally powered at 2 W, its dissipation could reach 15 W just to output the required illumination for a headlight.
Manufacturers are working on new materials to work in small spaces, control heat, manage current, and keep light consistently bright. Some of their ideas help remove air resistance. Air is a poor conductor of heat and adds resistance to heat flow. Removing air resistance helps heat travel from the source to where it is dissipated.
An LED’s substrate and thermal-interface material (TIM) help satisfy these goals. A substrate is a material on which a process is conducted. It sits between the LED and the TIM. The substrate serves as the basis for the component design and is selected based on certain properties required by the application, including electrical insulation or conductivity, adhesion capability, inherent strength, or ability to perform at required temperatures. The right material selection can often mean either the success or failure of a project. Substrates can include thermal-clad laminates that incorporate metal or metal foil, dielectrically formulated polymer (which can include prepreg), thermally conductive inorganic fillers, or a metal base which is often aluminum or copper.
TIMs help reduce air resistance between LEDs and heat sinks. An LED and heat sink both have high contact points because of their microscopic surface roughness. The gaps between the contacted points form air-filled voids. TIMs fill these gaps which reduce air resistance and, therefore, improve heat flow. Two basic forms of thermally conductive TIMs are wet dispensed and fabricated.
It’s important to identify the right TIM for each lightsystem design. There are several types of materials that act as either wet-dispensed or fabricated TIMs. And each type offers individual benefits. The overall benefit of wet-dispensed materials is that they can be easily integrated into existing production lines using dispensing equipment.
Thermal-liquid adhesive is a type of wet-dispensed TIM. Thermal-liquid adhesive can be a one and two-part silicone with fast thermal curing or room-temperature vulcanizing (RTV) curing. The silicone resists humidity, provides good dielectric properties, and is low stress and noncorrosive.
Filled acrylic polymers and epoxies are examples of thermal- liquid adhesives. Filled acrylic polymers are highly conformable and slightly tacky. Epoxies are used in applications requiring high adhesive strength, good surface wet-out, and gap fillers.
Encapsulants and gels are two types of wet-dispensed TIMs. Encapsulants are two-part silicones with flowable liquid that cure to a flexible elastomer at a constant cure rate. Gels include two-part silicone-based formulations loaded with conductive fillers. They offer low-to-moderate viscosity, long working times, fast thermal cure, good dielectric properties, and low stress and corrosion. They are designed to overcome the pump-out and dry-out issues sometimes found with thermal greases.
One type of wet-dispensed TIM is sometimes referred to as a “form-in-place” compound. These include noncuring, thermally conductive RTV silicones that can be used to form thermal paths where the distance between an LED and a cold surface varies. Compounds offer high thermal conductivity and high temperature stability.
Other materials that act as wet-dispensed TIMs include phase-change materials and solder. Phase-change materials are formulated with silicone or other polymer resins that are loaded with conductive fillers. These materials perform like thermal grease after they reach melt temperature, creating high viscosity and good gap filling. Compressive force is used to help a phase-change material form a thin bond line. Solder and solder hybrids that contain polymers act as TIMs. Both flow at room temperature and provide a thin bond line.
Unlike wet-dispensed materials, fabricated TIMs are solids that have been laminated and die cut to specific shapes for easy application. These materials can be applied cold, precured, cut to fit, and with a release liner for easy handling. Because they can be highly compressible on both sides, they excel at filling gaps on even the most irregular surfaces and won’t pump-out or dry out.
Fabricated TIMs include adhesive tapes, phase-change materials, thermal-insulating pads, and gap filler pads. An example of an adhesive tape is silicone-treated polyester transfer tapes. These tapes offer high mechanical strength, good surface wet-out, and good shock and vibration performance. Acrylic soft tapes are another example of an adhesive tape. They are flame retardant and offer good gap filling and thermal transfer. Acrylic or silicone-based pressure-sensitive adhesive tapes also act as fabricated TIMs. They bond heat sinks securely to power-dissipating components.
Phase-change materials in the fabricated category come in tape or pad form to exactly match a component shape. They have high thermal grease performance in a “peel-and-stick” format. Their compression brings the surfaces together and causes material flow.
Thermal-insulating pads and gap filler pads are additional types of fabricated TIMs. Thermal-insulating pads work almost like thermal grease, but apply more easily. Pads can be silicone or acrylic based, providing superior thermal performance with or without bonding. Gap filler pads can be cut into complex shapes and can incorporate EMI absorbing qualities. The pads also come in wet-dispensable form.
Converting and testing
The quest for the right thermal-management approach is often a process of elimination. Knowledge of how much heat the component generates, its place within the overall product, and other thermal-management details, all help shorten the process of selecting and matching appropriate substrates and TIMs.
Important steps to consider when starting a thermalmanagement project include determining the converting process and available testing capabilities. Fabrication processes include precision die cutting, multilayer laminating, slitting to tight tolerances, servo-driven rotary die cutting, CNC die cutting, laser die cutting, and waterjet cutting.
Choose a servo-driven rotary die cutter for the complex, multilayer die cutting and lamination that thermal-interface pads or tapes may require. The process maintains tight tolerances ranging from 0.015 to ±0.005 in. at speeds up to 500 fpm.
For complex foam tape die cutting, waterjet technology provides clean edges with no distortion. Laser die cutting, kiss cutting, slitting, and laminating can also be used in converting applications.
For a wet-dispensable TIM, plan for easy integration into existing processes with the help of dispensing equipment.
Quality is king so choosing the right testing and verification system is important. For instance, measurement of material strength ensures the material meets application requirements. While static shear testing measures the cohesive strength of the adhesive and whether it can withstand a fixed load over time.
Material weight measurement determines accurate adhesive coating weight, while microscopic imaging determines the differences between adhesive and material over time. Use an adhesive/release liner to help determine converting properties for high-speed application characteristics.
Dielectric testing is used to determine a material’s electrical insulation properties. Resistance and voltage testing provides a complete profile of the electrical properties of a material or adhesive. And, of course, thermal testing ensures materials and adhesives are meeting specifications.
LED design terms
Prepreg is a term for “preimpregnated” composite fibers where a material, such as epoxy, is already present. These usually take the form of a weave or are unidirectional. They already contain an amount of the matrix material used to bond them together and to other components during manufacture. The resin is only partially cured to allow easy handling.
Room-temperature vulcanizing (RTV) is a type of silicone rubber made from a two-component system (base plus curative; A+B) available in a hardness range of very soft to medium — usually from 15to 40 Shore A. RTV silicones can be cured with either a platinum or tin catalyst. Applications include low-temperature overmolding, making molds for reproducing, and some optically clear grades have lens applications.
EMI absorbers are designed to absorb electromagnetic/ radio-frequency interference in the broadband range.