Embedded Copper Coins: How Heat Leaves a Board

Coin insertion, also called embedded copper, is the practice of pressing a solid piece of copper into a cavity in the board so that its face is flush with the surface. The coin becomes part of the thermal path, and in some designs part of the electrical path as well. It is used where a component dissipates more heat than the laminate can carry away even with a metal backing, and where the heat has to move in the z direction rather than being spread across the board.

Why a Coin Rather Than More Copper

A normal board conducts heat away from a component through the copper pad, through thermal vias and into the planes, and then laterally across the board. Each of those steps has a resistance, and in a laminate the vertical path is the limiting one because the dielectric is a poor conductor and the vias have a small cross section. Adding more vias reduces the resistance but it does so slowly, because the copper in a via barrel is thin.

A coin is a solid block of copper a few millimetres across and as thick as the board. Its thermal resistance is a fraction of that of a via array with the same footprint, and its thermal mass is far higher, which is what matters for a component that dissipates in pulses rather than continuously. The face of the coin is exposed on both sides of the board, so the component sits on copper and the other end of the copper touches the heat sink.

Construction and Manufacturing

The cavity is formed by routing or by a punch before lamination, and the coin is inserted and then laminated into the stack. Two approaches dominate. In the first, the coin is bonded with the same prepreg that bonds the other layers, and the resin flows around it to fill the gap. In the second, the coin is a press fit into a machined cavity and relies on the interference to hold it in place and to make the thermal contact.

Both approaches have a common problem: the coin and the laminate expand by different amounts when the board is heated, and the interface between them is where a void will form. A void at that interface is an air gap in the thermal path as well as a mechanical weak point, so the cavity tolerance, the coin tolerance and the lamination pressure all have to be controlled together. Our thermal management article describes how the resulting resistance is estimated.

Electrical Use and Insulated Versions

Where the coin is connected to a net, it can carry current as well as heat, which is useful for a high current device that also needs cooling. That requires the coin to be electrically connected to the pads above and below it, and it means the coin is at the potential of that net, which in turn constrains what can be placed near it.

Where the coin must be electrically isolated, an insulated coin is used: the copper is embedded in a thermally conductive but electrically insulating layer, so heat passes through while current does not. That version is used where the component’s mounting face is not at ground potential and the heat sink is, and it costs more because the insulating layer adds thermal resistance and the manufacturing process is more demanding.

Design Rules for Embedded Coins

The coin has to be placed so that it is surrounded by enough laminate to hold the stack together. The clearance to the nearest hole, to the board edge and to other copper features is larger than for an ordinary feature, because the coin is a rigid inclusion in a material that moves. Placing a coin close to a plated hole creates a stress concentration that will crack during thermal cycling.

The pads above and below the coin must be at least as large as the coin face, so that the component and the heat sink both make contact with copper rather than with the gap around it. The surface must be flat, which usually means the coin is machined flush after lamination or is specified with a flatness tolerance that the lamination process can meet. Both requirements affect the fabrication drawing rather than the layout alone.

Reliability and Failure Modes

The failure modes are mechanical. A void at the interface reduces the thermal performance and can grow with thermal cycling, eventually producing a delamination around the coin. A coin that is not flush leaves a step that prevents the component from seating, and a step under a large package produces a solder joint of variable thickness that fails at the thinnest point.

Where the coin carries current, the joint between the coin and the copper pad is a resistance in the current path, and if it is not made properly it becomes a hot spot. That is why the electrical version is usually tested for continuity and resistance as part of the normal electrical test, while the purely thermal version is verified by cross section.

When Embedded Copper Is Worth It

The process adds cost and lead time, and it restricts the layout around the coin, so it is reserved for cases where nothing else will do. The usual case is a component dissipating tens of watts in a package a few millimetres across, mounted on a board that must also carry dense routing. Under those conditions the alternative is a much larger board, a much more expensive thermal solution or a lower power rating.

The decision should be made by comparing the thermal resistance the design needs with what a via array can deliver. If a reasonable via array with a generous pad brings the junction temperature within limits, the coin is unnecessary. If the calculation shows that no practical array will do it, the coin is the answer and the layout should be planned around it from the beginning. Our design release checklist places that comparison in the review sequence.

Assembly and Thermal Interface

The benefit of a coin is realised only if the interface between the board and the heat sink is as good as the interface inside the board. A coin that conducts heat perfectly into the bottom face of the board achieves nothing if that face is bolted to a heat sink with an air gap, so the mechanical design and the thermal interface material are part of the same solution. Our thermal management article describes how those interfaces are specified.

FAQ

Is a coin the same as a thermal pad on a component? No. The thermal pad is part of the component package; the coin is embedded in the board beneath it to move heat through the laminate.

Can a coin be connected to a signal net? Yes, if the design requires it, but then the coin is at that potential and the layout around it has to respect the clearance that implies. An insulated coin avoids that restriction.

Why does a void under a coin matter so much? Because air is a far worse conductor than copper. A small void can multiply the thermal resistance of the path and turn a design that was within limits into one that is not.