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High-output thermal substrates ‘must’ for LEDs industrial applications

High-output thermal substrates 'must' for LEDs industrial applications

By BizLED Bureau

Feb 25, 2016: Industrial market is growing in demand with ultraviolet LED (UV LED) and visible LEDs being slated to be used in curing, machine-vision, and other applications. These applications require high-output sources including modules that are thermally efficient and protected from any immediate operating environment.

UV LEDs have many varied applications and is used in curing equipment used for adhesives, drying paints and other curable materials. Visible and UV light applications are workable in a range of wavelengths less than 450 nm, typically applying UV-A band wavelengths between 365-405 nm.

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A reduced operating temperature is required to produce a stable and uniform output while maintaining high reliability. Reason being, lower operating temperatures are necessary for the extraction of maximum optical efficiency. Lower temperature is essential in achieving a standard color temperature and better color rendering. Operating systems are supposed to be maintained efficiently to benefit the module from the LED chips’ internal high efficiency and to have a long lifetime.

An LED module designer main objective is to have the least possible thermal resistance, which allows the excess heat to be dissipated away from the chip quickly to balance minimum temperature. As chip-level powers are increasing and system architectures are evolving to become more complex, giving the appropriate thermal design for modules and for system-level assembly is becoming a challenge. Hence, new technologies are being researched to further improve thermal management.

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Options in substrate

Substrate material compositions offer a variety of options for application-specific LED module designs which helps them to attain maximum performance at a lower cost. On the other hand, Chip-on-board (CoB) based industrial modules are typically used in industrial applications. However, they are different from CoB LED arrays with a single large, circular light-emitting surface. Rectangular in form and featuring discrete LEDs, Industrial CoB modules and substrate materials share the same concept of emitters attached to a thermal substrate.

The industrial modules range from a few watts up to 100W per COB, with COB areas up to around 10 cm2. These modules have ability of delivering optical power density in excess of 10 W/cm2. Luminaires and larger illumination sources are generally assembled from blocks of these smaller COBs, rather than manufacturing large-size, extremely high-wattage COBs.

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Polycrystalline ceramic substrate materials are primarily used for low- and mid-power modules. Alumina and aluminum nitride (AlN) materials, with thermal conductivities greater than 20 W/mK and 150 W/mK, are most common ceramics in use. Dielectric ceramic substrates like AlN, is a preferred choice for demanding applications as the conductor layer can be directly processed onto the material without the need for an isolation layer. It is only the high cost of the material that results from its complex production process and inherent fragility are preventing wider adoption.

Single crystalline silicon with high thermal conductivities exceeding 140 W/mK and low thermal expansion of 2.5 μm/mK makes an attractive substrate choice. For high-integration-level modules, silicon has the option for monolithic integration of passive components and is a naturally convenient platform for hybrid integration of drivers. Also emerging gallium-nitride-on-silicon (GaN-on-Si) technology for processing white-light, blue-pump emitters directly onto silicon is assisting that material as an attractive module-level substrate material.

Nanotherm is another emerging novel substrate material for LED applications. Currently, it offers maximum thermal performance for a metal-clad PCB substrate due to its unique construction. With best-in-class thermal conductivity, a nanoceramic process converts the top layer of aluminum to form an extremely thin ceramic dielectric layer. The nanoceramic dielectric thickness is applicable as a 3-μm layer -many times thinner than conventional dielectrics. The combination of the highest conductivity with thinnest dielectric layer in the industry yields the lowest thermal resistance of any material option.

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