Advances in solid state light emitting diodes (LEDs) over the last several years have opened new applications for these devices. Traditionally used only in low power, low light output applications, modern high power LEDs are finding their way into a wide variety of applications.
LEDs for lighting applications offer several advantages over traditional incandescent lighting methods.  They significantly lengthen the life of the lighting system (an important consideration in that lighting defects account for a considerable portion of warranty and maintenance budgets), offer cooler operating temperatures, and they are considerably more efficient compared to incandescent bulbs, using significantly less energy for a given light output. LEDs are also rugged and highly resistant to shock and vibration, which is especially important in automotive, heavy vehicle and industrial lighting applications.
LEDs, however, suffer from one drawback in that their light output diminishes as their temperature increases, typically by as much as 1.25% per degree C. As a result, thermal management is critical. Fortunately, there are several advancements in the electronics industry today that offer design engineers better options for dissipating heat in direct-mounted LED applications.
High power LED applications invariably require the use of thermally conductive substrates to manage the heat produced, and one of the most widely selected materials for this application is aluminum alloy substrate. Many manufacturers cover a bare piece of aluminum with a film of epoxy to insulate the mounted components as well as transfer the heat. Even when filled with thermally conductive media, filled epoxies are generally not efficient thermal conductors. Also, reliability problems can be encountered if the aluminum is not perfectly clean; if the epoxy is not cured properly; or if the epoxy layer delaminates during heat fluctuations.
However, recent developments in anodized aluminum alloy substrates give lighting system designers a lighter, cooler and less expensive method of mounting visible LEDs. IRC has developed an aluminum alloy substrate for surface mounted LEDs using an aluminum core with a chemically grown dielectric layer and printed conductor tracks. This technique produces an insulated, yet thermally conductive PC board with extremely low thermal resistance from the die or chip to the substrate in power LED applications.
These substrates reduce operating temperature, allow higher operating power/density, improve reliability and reduce failures due to thermally induced problems. This material is sold and marketed under the Anotherm name.
Because the insulation layer between the aluminum and printed traces is very thin, and its composition is aluminum oxide (a fairly good thermal conductor), the thermal performance is very good. Tests results comparing aluminum alloy substrates with common epoxy insulated metal substrate boards are summarized below.
Although possessing many positive features, there is a limitation to this technology that deserves mentioning.  First, the maximum voltage withstanding capabilities of the anodization insulation material is directly dependent upon its thickness. It becomes difficult to anodize very thick layers of anodic coatings on aluminum due to the increased time and the dissolution of the anodized coating back into the bath. As a result, the maximum thickness that can routinely be achieved (cost-effectively) is approximately 55 microns, resulting in a typical breakdown voltage of about 500 VAC (rms).  This factor currently prevents this material from use in high voltage applications.
The aluminum substrate’s thermal conductivity (the ability to dissipate heat) is characterized at 173W/m-K — far superior to other types of traditional substrates (0.8W/m-K for FR4 PC board; 17.3W/m-K for 304 stainless steel; or 21W/m-K for 96% alumina ceramic).
An aluminum alloy substrate consisting of either 3003 or 6061 aluminum delivers high thermal conductivity and low cost. The thermal expansion coefficient of this material corresponds favorably with traditional PC board materials as shown in the table below.  Long term thermal shock testing confirms the ruggedness of the dielectric medium.   
Aluminum alloy substrates have been proven through many hours of laboratory testing. One such test, consisting of 2000 thermal shocks (between -55ºC to +125ºC) resulted in continued superior performance with no degradation in thermal characteristics or delamination. Also, thick film power resistors using similar aluminum alloy process have demonstrated reliability in automotive applications for more than eight years. Presently more than 25 million units are in operation with excellent results.
The insulation system used with aluminum alloy substrate systems is an anodically grown coating (similar to hard coat anodizing), that deposits a dense, thin film of aluminum oxide approximately 0.0014” thick (0.035mm) onto the aluminum surface. This inorganic dielectric layer gives a high quality insulation that is not affected by temperature or chemicals.
This process, as one of chemical conversion, results in an insulating layer that is robust and unlike other methods of insulating aluminum, is highly resistant to delamination and separation. An additional benefit that the anodization process gives is the uniform dielectric thickness even around corners and holes. This characteristic is important as other methods of insulation often give thin areas around corners or on the inside of holes which may be subject to voltage breakdown. Additionally, a wide range of substrate configurations can be processed, such as finned, extruded, die cast, and special shapes can be accommodated.
Because of the its extremely low thermal impedance and versatility, aluminum alloy substrate material is quickly gaining in popularity for a host of power LED applications, including automotive, interior/exterior light fixtures and display signage, laptop computer backlighting and flashlights.
The thermally-conductive aluminum alloy material enables design engineers to mount high power LEDs and other components directly to it, thus eliminating the need for attached heatsinks, mounting hardware and the associated assembly costs. Economical solderable thick film conductors can be screen-printed directly to the anodized substrate to connect surface mount packaged components as well as wire-bonded die.

Conclusion
Aluminum alloy thermally conductive substrates offer several advantages in applications that require effective thermal management such as in high brightness automotive LED applications. Among the advantages offered are exceptional thermal conductivity, versatility in size and configuration, and cost effectiveness.
Aluminum substrate technology has been in use for nearly eight years in automotive resistor applications, and has recently been adopted for use with high power (1 watt and above) visible LEDs.
Benefits of Aluminum alloy substrates include:
• Very low thermal impedance to the substrate (<0.01ºC/watt/in2)
• Conductors that are directly solderable using  standard solders (Sn62/Pb36/Ag2 or Sn96/Ag4)
• Breakdown voltage >150VDC
• Crossovers
• Inorganic composition allows extended operation at high temperatures without degradation
• Increase power density
• Lower operating temperatures
• Eliminate assembly of heatsinks and mounting hardware
• Large boards-up to 8” X 12”
• Finned heat sinks