Book Excerpt: Thermal Management With Insulated Metal Substrates, Part 2

Estimated reading time: 2 minutes

The following is an excerpt from the second half of Chapter 1 of The Printed Circuit Designer's Guide to... Thermal Management With Insulated Metal Substrates written by Ventec International Group’s Didier Mauve and Ian Mayoh. In this free eBook, the authors provide PCB designers with the essential information required to understand the thermal, electrical, and mechanical characteristics of insulated metal substrate laminates.

The thermal conductivity of a substance k is an intrinsic property that indicates its ability to conduct heat. Some relevant examples are illustrated in Table 1.1.

Heat can be dissipated from QFN components by mounting them on thermal pads connected to internal copper planes by thermal via holes. Problems of solder wicking into the holes and causing voided joints can be avoided by filling the holes with thermally conductive resin and plating them over with copper, or by completely filling the holes with electroplated copper. Solid copper “coins” can be bonded into recesses milled into the PCB to conduct heat away from individual power devices. All of these techniques can be effective, but they usually involve additional materials and processing, and a significant cost premium.

Many power electronics and under the hood automotive devices are built on direct-bonded copper (DBC) substrates because of their strong thermal conductivity. They are based on a ceramic tile (commonly alumina) with a sheet of copper bonded to one or both sides by a high-temperature oxidation process. The top copper layer can be formed prior to firing or chemically etched using PCB technology to form an electrical circuit, while the bottom copper layer is usually kept plain so that it can be soldered to a heat spreader by soldering the bottom copper layer to it. Beryllia, silicon nitride, and aluminum nitride are more effective heat conductors than alumina, but cost considerably more. Further, thick-film technology can be used in some high-reliability applications. Thick-film technology offers a higher degree of design freedom than direct-bonded copper, but it may also be less cost-efficient.

LED manufacturers have adopted packaging technologies from the power electronics field with the result that they can now offer efficient thermal coupling from the semiconductor to the primary interconnecting substrate of the package. However, it remains that the only path for heat out of the LED is via the bottom of the LED package to the PCB, which must not present a thermal barrier. For high-power LEDs, the thermal conductivity of an FR-4 PCB is insufficient to enable effective heat transfer, and insulated metal substrate materials offer a better solution. These materials generally consist of a thermally-conductive dielectric layer heavily loaded with ceramic-type fillers that are sandwiched between copper foil and an aluminum or copper plate. The dielectric may be unreinforced or woven-glass reinforced.

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