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Thermal design decisions made at layout review are usually too late. Here's what to lock in before you route.
Every generation of power electronics asks the same board to do more in less space. EV traction inverters, aerospace power supplies, motor drives, they're all pushing more current through boards that haven't gotten any bigger. The copper has to work harder, and FR4 doesn't help much. Standard laminate conducts heat at roughly 0.3 W/m·K. Copper conducts at 401 W/m·K. That gap is the entire thermal design problem in one number.
What actually moves heat, and when each one makes sense
Thermal vias: The baseline solution, and still the right one for a lot of designs. A single via under a component has limited thermal capacity, but via arrays under QFNs, DFNs, and other exposed-pad packages meaningfully cut junction temperature by giving heat multiple parallel paths down through the stackup. The design decisions that matter: via size and spacing, whether they're filled or capped, and getting them tied into a real copper plane on the other end instead of an isolated pour. This is where most designs should start.Heavy copper: When current density is the driver, not just component-level heat, heavy copper (generally 4 oz/ft² and up, sometimes to 20 oz/ft²) does two jobs at once: it carries more current with less resistive heating, and it spreads whatever heat is generated across a wider area instead of letting it concentrate. It's a staple in military, automotive, and industrial control boards for exactly this reason. The tradeoff is etching tolerance and spacing, which changes your minimum trace/space rules and needs to be flagged early.Copper coin and copper inlay: For components that don't have a die-attach pad suited to a via array, like larger BGA or LGA packages, or for genuinely concentrated heat loads, an embedded or inlaid copper coin gives you a direct vertical path from the component straight through the board to a heatsink or chassis, bypassing the dielectric layers almost entirely. Press-fit, embedded (laminated in), and inlay (machined and flush-mounted) constructions all behave differently under thermal cycling, and the right one depends on your CTE mismatch tolerance and mechanical loading, not just thermal performance on paper.Embedded heat spreaders: For boards with multiple hot components or heat sources that aren't co-located, a thick internal copper plane acts as a spreader, moving heat laterally before it ever needs to go vertical. This is often the difference between a design that needs three localized copper coins and one that needs a single well-placed internal plane.
None of these are mutually exclusive. Most high-power-density boards we build combine two or three of them, thermal vias under the hot components, a heavy copper plane to spread and carry current, and a coin only where the local heat load genuinely needs it.
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