When vias aren’t enough: coins, inlays, and embedded heat pipes.
TL;DR
- There’s an escalation ladder for board-level cooling: vias → heavy copper → copper coin/inlay → embedded heat pipe / IMS. Climb it only as far as the power density forces you.
- A copper coin (a solid slug pressed into the board under a hot part) cuts the junction-to-heatsink resistance dramatically — often the best $/°C once you pass ~5 W/cm².
- Embedded heat pipes move heat *laterally* to a cooler edge or sink; they shine when the hotspot and the heatsink can’t be co-located. They’re expensive and fab-specific — model the benefit in TRM before committing.
The escalation ladder
Don’t jump to exotic solutions. Each rung is cheaper and simpler than the next; stop at the first that meets margin:
- Thermal vias + plane area — the default; good to ~2–3 W with disciplined copper. (rules)
- Heavy copper (2–4 oz) — more spreading and current capacity, modest cost adder.
- Copper coin / inlay — a solid copper slug through the board under the hot device; step change in vertical conduction.
- IMS / aluminum-core (MCPCB) — the whole board becomes a heat spreader; great for LED/power, limited routing.
- Embedded heat pipe / vapor chamber — for lateral transport when the sink is elsewhere; highest cost and complexity.
Copper coins & inlays
A coin replaces the via array under a hot pad with solid copper — eliminating the air gaps and plating limits of barrels. Two styles: a pressed-fit coin (a machined slug pressed into a milled cavity) and a plated copper-filled cavity. The win is the vertical θ: a coin can be 3–5× lower resistance than a dense via array of the same footprint, because it’s solid metal, not a hollow lattice. The cost is a fab process step and a heavier board.
Embedded heat pipes
When the hot device and the available heatsink/chassis surface are far apart on the board, vertical conduction doesn’t help — you need lateral transport. An embedded heat pipe (or vapor chamber laminated into the stack) moves heat sideways at an effective conductivity many times that of copper. Use cases: a power stage that must dump to a board-edge bracket; a tightly-packed module with no room for a sink directly above the hotspot.
Caveats: heat pipes have an orientation dependence (wick vs gravity), a maximum transport capacity, and they make the board fab a specialty job. The benefit is real but situational — quantify it in TRM (as an anisotropic high-k region) against the simpler “just move the hot part nearer the sink” option, which is often cheaper.
Climb the cooling ladder one rung at a time. The expensive rungs are real tools, not defaults — earn them with a TRM number.
Checklist
- ☐ Exhaust vias + plane + heavy copper before exotics
- ☐ Consider a copper coin above ~5 W/cm² point power density
- ☐ For coins: choose pressed-fit vs plated-fill with your fab early
- ☐ Heat pipe only when hotspot and sink can’t be co-located
- ☐ Model every option in TRM (coin = solid-copper region; pipe = anisotropic high-k) before committing
- ☐ Confirm the construction with the fab — these are specialty processes
Need the analysis?
Extreme-power-density boards are our high-power practice, and every option gets a TRM number before you spend on tooling. Scope a project.
References
- IPC-2152 thermal characterization; IPC-7093 (bottom-termination thermal).
- Copper-coin / inlay fabrication app notes (Ventec, AT&S).
- Embedded heat pipe / vapor chamber integration literature (thermal-management journals).