Elpor material from Heatron finds applications in high-power LED light engines

Nov. 25, 2004
The use of a proprietary fired-on ceramic dielectric material makes even steel-core MCPCBs suitable for use with high-power LEDs.
High-power LED light engines can generally be constructed in two ways, using either large-area, high-power LED chips, or by building arrays of closely spaced die. Heat removal from the die is essential in order to prevent shifts in wavelength and forward voltage and reductions in lumen output and lifetime.
LiveLED 100

A popular solution is to mount the LEDs on a metal-core printed circuit board (MCPCB). In some cases, the board has an organic dielectric material to electrically insulate the metal core.

Heatron, a company based in Leavenworth, Kansas, produces steel-core and aluminum-core boards in its Erie, Pennsylvania Thick Film Products plant. These boards utilize a proprietary inorganic porcelain dielectric material called Elpor®, which offers a number of advantages over organic dielectrics. However, the use of Elpor in thermal design has been questioned due to the low thermal conductivity value of the porcelain enamel (which is 2 W/mK). This value, in comparison, is equal or better than competitive organic dielectrics used in traditional laminated MCPCBs.

Vehicle lighting Elpor is a thin (3-4 mil or ~ 75-100 microns) fired-on glass dielectric material on steel or aluminum that offers excellent heat dissipation. The absence of organic materials means there is no delamination or degradation with time or temperature. In addition, resistors can be printed and fired onto Elpor steel boards.

The combination of the thick-film dielectric on the metal core provides effective heat transfer, says HB Turner, executive VP of Heatron. "The dielectric helps heat get through quickly, while the metal core acts as a heat spreader," he says. Surprisingly, test data confirms that there is only a small difference between the steel-core and the aluminum-core Elpor boards. "The steel processing is a lot less expensive, so for high-volume applications steel is probably a much better option."

Thermal results

The bulk thermal conductivity of aluminum is about three times higher than that of steel (160 W/mK vs. 55 W/mK respectively). However, comparisons of high-power (350 mA) LEDs mounted on either Elpor steel or Elpor aluminum boards showed that the thermal resistance (LED case to rear of Elpor board) was only about 1 °C/W higher for steel than for aluminum.

Heat dissipation For Golden Dragon LEDs from Osram, the case-board thermal resistance was 2.2 °C/W for aluminum and 3.5 °C/W for steel. For Lumileds Luxeon devices, the corresponding values were 1.3 °C/W and 2.7 °C/W. These numbers include the thermal resistance of the material used to attach the slug to the board: solder for the Dragons and thermally conductive epoxy for the Luxeons.

The non-intuitive behavior of Elpor on different metal core materials is attributed to the non-linearity of bulk material properties in relatively thin metal core and dielectric coatings. Also, the test samples were coated on both sides with Elpor material, which radiates heat more evenly and more effectively than either bare steel or aluminum.

Turner says that the thermal resistance of both Elpor material systems is equal to or better than the reported thermal resistance of an MCPCB material consisting of an aluminum board with an organic dielectric.

As well as the low cost of the Elpor steel product, and the incremental improvement offered by going to aluminum, Heatron has also developed a patent-pending white surface coating for its steel boards that is non-yellowing and provides high reflectivity for increased light output.

Light engine development

Aircraft lights As well as supplying Elpor material and circuit boards, Heatron also builds LED modules and light engines for customers' proprietary designs. Turner says that his company built the modules used in Honeywell's aircraft wingtip marker lights (see photo and Honeywell applies LEDs to aircraft position lights).

"We also built the light engine incorporated into Advanced Lighting's LiveLED 100 wall-wash luminaire," says Turner. The LiveLED 100, launched at LDI, incorporates 44 Luxeon devices operating at one or three watts.

"We are going full speed in the Erie plant with new light assemblies now going into production just from the last 12 months of custom projects," says Turner. "These include channel letter assemblies, airport lighting, 3-4 projects with Honeywell Grimes, fork-lift lights, snowmobile lights, strobes, emergency vehicle light bars, the Live LED unit, and many others."