Coin Insertion PCB: Embedded Copper for Heat and Current

Why a Board Needs Metal Inside It

Power density is the constraint that shapes modern boards. A MOSFET, an insulated gate bipolar transistor module or a power management IC can dissipate tens of watts from a package a few millimetres on a side, and the laminate underneath it is a thermal insulator. FR-4 conducts heat at roughly 0.3 to 0.4 W per metre kelvin. Copper conducts at around 400. That ratio of about a thousand to one is the whole argument for coin insertion.

Coin insertion places a machined block of solid copper into the body of the board, directly beneath the device that produces the heat. The coin becomes part of the finished laminate, connected to the copper layers, and provides a path for heat and for current that does not depend on the thermal properties of the resin.

What a Copper Coin Actually Is

The coin is not a plated feature and not a filled via. It is a solid piece of copper, usually one to six millimetres thick, cut and ground to a controlled thickness, then seated in a cavity routed or laser cut through part of the stack. The stack is laminated around it, so the resin bonds to the sides of the coin and the copper layers connect to its faces.

A typical construction has five elements: the base laminate, which may be a high glass transition temperature FR-4, an automotive grade material or a low loss high frequency laminate; the machined copper coin; the cavity that receives it; the multilayer stack that surrounds the assembly; and the plated copper that joins the coin to the circuit layers. Because the coin is solid metal rather than a plated barrel, both its thermal resistance and its electrical resistance are far lower than any via structure of the same footprint.

For boards that combine this approach with thick outer layers, our notes on high current PCB design cover the current density side of the same problem.

What the Coin Does for Heat and Current

Heat leaves a device through a chain of interfaces: the die, the package, the solder joint, the copper pad, the laminate, and finally the heatsink or enclosure. The laminate is normally the worst link. Replacing it locally with solid copper collapses that part of the chain and spreads the heat sideways into a much larger area before it has to cross the resin again.

The practical results are consistent across applications. Junction temperatures fall because the thermal resistance from the device to the far side of the board drops by roughly 30 to 70 percent compared with a conventional stack. Hot spots spread out, so the neighbouring components run cooler. And because the coin is also a conductor, the resistance of the high current path falls, the current carrying capacity rises, and the current crowding that would otherwise concentrate at a narrow neck is distributed through bulk copper instead.

That last point is easy to overlook. A coin is often specified for thermal reasons and then delivers most of its benefit in the electrical path, particularly in motor drive and battery circuits where hundreds of amperes pass through a small area of the board.

Choosing the Coin and the Material Set

The coin itself is normally C1100 high purity copper or a high conductivity copper alloy. Thickness follows the power level rather than the available space:

• Medium power. One to two millimetres of copper, typical of converter stages and power management circuits.
• High power. Three to four millimetres, typical of motor controllers and inverter phases.
• Very high power. Five to six millimetres, used where the device is a large module or where the duty cycle approaches continuous.

The laminate around the coin has to survive the lamination cycle and the thermal cycling of the application. High glass transition temperature FR-4 is the economical choice, automotive grade material is used where the board must pass qualification testing, and low loss laminates appear when the same board also carries a radio frequency or fast switching section. Surface finish is chosen for solderability and oxidation resistance: electroless nickel immersion gold, immersion tin and organic solderability preservative are all common, with the finish applied after the coin is in place.

How the Board Is Fabricated

Coin insertion adds mechanical operations to a process that is otherwise chemical and photographic. The sequence runs as follows.

• Coin machining. The copper piece is cut and ground to a controlled thickness and a flatness specification, because a coin that is not flat will not sit flush against the copper layers.
• Cavity formation. The cavity is routed with computer numerical control tooling or cut with a laser, to a tolerance that matches the coin and holds the position relative to the component pads.
• Coin placement. The coin is seated in the cavity and checked before the stack is closed. Registration at this stage sets the registration of the finished board.
• Lamination. The stack is pressed at high temperature and pressure so that resin flows around the coin, fills the gaps and bonds the assembly. This is the most sensitive step: too little resin flow leaves voids at the coin edge, and too much shifts the layers around it.
• Plating and finishing. Copper plating connects the coin to the circuit layers, the surface finish is applied, and electrical test confirms connectivity and isolation.

The yield risks are concentrated in the interface between the coin and the resin. Voiding at that boundary, a coin that tilts during pressing, and delamination under thermal cycling are the failures to design and qualify against. A fabricator with experience in the process controls cavity dimensions, coin flatness and the lamination profile together, rather than treating them as independent steps.

Design Rules That Decide the Outcome

Several decisions are made long before the coin is ever machined, and they matter more than the coin thickness.

Place the coin directly under the thermal pad of the device, not offset to one side, because lateral heat spreading through the laminate is weak. Size the coin from the thermal simulation rather than from the component outline. Match the coin to the copper area that connects it on both faces, since a large coin joined to a small pad simply moves the bottleneck. Keep the solder joint under the device void free, because a void in the joint negates the benefit of the coin beneath it.

Thermal simulation, current density analysis and mechanical stress analysis should all run before release. The mechanical check matters because copper and laminate expand at different rates, and the coin is large enough for that difference to load the surrounding layers. A via field around the perimeter of the coin, used as a secondary path, is a common way to soften the transition between the coin and the rest of the board.

Where the board is part of a power system rather than a signal product, the same rules apply as for any energy PCB or industrial PCB built for continuous duty.

How It Compares With Other Approaches

Against heavy copper. Heavy copper plating raises the copper thickness across the whole board, typically to two or six ounces. A coin adds copper only where it is needed and can be far thicker: one millimetre of coin is roughly two hundred times the thickness of a one ounce foil. Heavy copper is a routing solution; a coin is a local thermal solution.

Against metal core boards. A metal core or aluminium substrate spreads heat over the entire board, which suits arrays of small devices, but it constrains the layer count and the ability to build a complex multilayer circuit. The coin delivers its benefit inside an ordinary multilayer stack.

Against thermal vias. A via is a thin plated barrel through a dielectric, and its thermal resistance is dominated by the plating thickness and the fill. A solid coin is a much shorter and much wider path. Where a via field cannot get the junction temperature down, a coin can.

Where It Is Used

Coin insertion appears wherever a small area has to move a large amount of heat or current. Power modules such as DC-DC converters, regulators and power management stages are the classic case. Automotive electronics uses it in motor controllers, battery management systems and on-board inverters. Industrial equipment uses it in motor drives, industrial power supplies and automation controllers. High power LED drivers use it for the same reason, because the light engine concentrates heat in a small footprint.

These are boards that also have to be manufactured as ordinary multilayer circuits, which is why the technique is usually paired with a PCB manufacturing route that can handle the mechanical operations alongside the standard imaging and plating steps. Thermal performance and mechanical robustness are often qualified together, following the same discipline as any thermal management programme.

Limits and Cost

The main limitation is fabrication difficulty. Coin insertion needs precision machining, careful cavity control and a lamination process that tolerates a metal insert, and not every fabricator offers it. The coefficient of thermal expansion difference between copper and the laminate has to be managed in the design. Tighter tolerances and larger coins raise the cost.

As a 2026 reference, prototypes with a single coin typically fall between 120 and 450 US dollars per piece, small batch production between 35 and 120 dollars per piece, and volume production between 8 and 40 dollars per piece. The spread is driven by coin dimensions, layer count, board thickness, order quantity and process complexity, so a quote is only meaningful against a specific stack.

Frequently Asked Questions

Is a copper coin the same as a copper filled via? No. A filled via is a plated hole filled with resin or paste. A coin is a solid machined block of copper set into a cavity, with a far larger cross section.

How much does it reduce thermal resistance? Reported reductions against a conventional stack are in the range of 30 to 70 percent, depending on coin size, thickness and how the rest of the stack is built.

Does it help with current as well as heat? Yes. The coin lowers resistance and spreads current, which is why it is used in high current paths and not only under hot devices.

How is the thickness chosen? From the power level and the thermal simulation. One to two millimetres for medium power, three to four for high power, five to six for very high power.

What fails first? The coin to resin interface. Voids, tilt during lamination and delamination after thermal cycling are the defects to qualify against.

Conclusion

Coin insertion solves a problem that laminate alone cannot: a large amount of heat and current concentrated in a small area of the board. A solid copper block, machined to a controlled thickness, seated in a cavity and laminated into the stack, drops the local thermal resistance, spreads the current and takes the worst link out of the thermal chain. It costs more than a conventional board and demands a fabricator who can hold the mechanical tolerances, but where the junction temperature or the current density is the binding constraint, it is often the only approach that fits inside the space available.