Book Excerpt: Thermal Management With Insulated Metal Substrates, Part 3

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The following is an excerpt from Chapter 3 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.

Chapter 3: Developments in Insulated Metal Substrate Laminates
The insulated metal substrate concept is not new. Materials were available as long ago as the mid-1960s for specific niche-market applications. However, the exponential growth in LED lighting has been the main driver for the development of improved versions in volume manufacture. Insulated metal substrate laminates are now firmly established as the preferred base material for the fabrication of printed circuits for high-brightness LED lighting and DC power conversion applications because they offer cost-effective performance with straightforward fabrication, good mechanical stability, and a range of thermal conductivities to suit particular configurations.

Although thermal PCB design technology has been predominantly single-sided, multilayered constructions are now possible through resin-coated foil and resin-coated film options. The use of thermally conductive prepregs and copper clad thin laminates manufactured with them, which can be bonded to the insulated metal substrate or co-laminated with high Tg or low Dk and Df cores and prepregs, have also made multilayered constructions possible.

The recent progress made by these thermal prepregs and thin cores allow engineers to design multilayered PCBs with integrated thin thermal layers. This opens up many possibilities, particularly when convection is not an option due to space, or real estate, and the cost of additional radiators is a concern.

Dielectric Layer
The key element of an insulated metal substrate material is the thermally conductive dielectric layer between the copper foil and the aluminum plate. This may be a woven-glass reinforced-resin composite (prepreg), as in a conventional laminate construction, or a layer of unreinforced resin. The resin itself is typically a halogen-free epoxy-laminating resin. Whereas a conventional FR-4 laminate would have very poor thermal conductivity, the thermal conductivity of the resin component is significantly improved by loading it with up to 70% of a thermally-conductive ceramic filler. The resin must also continue to serve the fundamental purpose of reliably bonding the insulated metal substrate construction together under potentially severe thermal-cycling conditions.

The thermal conductivity of glass-reinforced materials is still limited by the nature of the glass, so it is the non-reinforced dielectrics that have the lowest thermal resistance. However, they demand critical control in manufacture to maintain consistency of dielectric thickness, whereas glass fabric provides a natural mechanical spacer.

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