Ventec Thermal Management Book Excerpt: Chapter 1

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The Printed Circuit Designer's Guide to...Thermal Management with Insulated Metal Substrates, Vol. 2

Chapter 1: High Emissivity

Regarding basic principles of thermal dissipation there are three ways of dissipating energy:

• Conduction
• Convection
• Radiation 

The integrated metal substrate (IMS) printed circuit boards rely predominantly on heat conduction all the way through the different layers of the substrates from a hot point (the base of the component) to a cold point (the furthest surface of the metal base) and, usually, thereafter, through a dissipator. Ideally, the thermal engineering of the package, soldered connection(s) to the substrate, dielectric layer, IMS metal base, and dissipator should conduct heat as effectively as possible away from the die and into the ambient. That is, they should have the highest possible thermal conductivity (or low thermal resistance, the one being the inverse of the other). However, various other properties are needed, and the entire system cannot be designed solely around thermal performance. The different elements of the “stackup” (noting that inside the package there is typically some combination of metallic parts such as bondwires and/or leadframe, ceramics or plastics, FR4 or similar substrate in the case of a system-in-package device or high-performance MPU, and adhesives) can be modeled as a series of dissimilar thermal resistances. 

Ultimately, thermal energy conducted into the metal base is transferred into the ambient by radiation. The effectiveness of this transfer is influenced by factors such as the emissivity of the surface and the radiating area in contact with the surrounding air. The emissivity is dependent on the material as well as the surface finish (basic physics tells us that a matte black surface radiates heat more effectively than a shiny surface–and also absorbs heat radiated from nearby sources more effectively than a polished surface).

After being transferred into the surrounding air by radiation, the heat dissipates throughout the atmosphere by a process of convection.

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