Embedded Copper Coin PCB: Structure, Process and Design

A copper coin is a solid piece of copper pressed into a cavity in a circuit board so that it passes through the thickness of the laminate beneath a hot component. It carries heat from the surface of the board to the opposite side, where it meets a heat sink, with far less thermal resistance than a group of plated vias.

The technology is used where the heat flux is high and the board is the only path available: a power amplifier, a motor driver or a high brightness LED whose junction temperature cannot be held with a via array alone.

What a Copper Coin Does

The coin provides a continuous metal path through the board. Unlike a thermal via, which is a plated hole with a limited copper cross section, the coin is solid copper with a thermal conductivity roughly twice that of the plating and a cross section that can be made as large as the layout allows.

It also provides a mechanical and electrical connection. A coin can be connected to a ground net or to a power plane, and it can carry current as well as heat, which is useful in a power stage where the device has a thermal pad that is also an electrical node.

Structure of the Insert

The coin is a machined or forged copper piece, usually with a slightly tapered or stepped profile so that it keys into the cavity and cannot fall through during assembly. Its height is matched to the finished board thickness, and in some designs it protrudes slightly so that it makes contact with the heat sink directly.

The copper is usually a high conductivity grade rather than the foil used for the circuit, and its surface is prepared so that the plating and the laminate adhere to it. A poorly prepared coin surface is the source of most delamination in this type of board.

Process: Cavity, Insertion and Lamination

The cavity is formed in the core by routing or by a shaped punch, and its dimensions are held to a tolerance that allows the coin to fit with a controlled gap. The coin is placed in the cavity, often with a bonding film or a prepreg around it, and the stack is laminated under heat and pressure.

The resin flows into the gap between the coin and the cavity wall and bonds the two together. If the gap is too small the coin will not seat and the board will not close; if it is too large the resin cannot fill the space and a void forms, which becomes a path for moisture and a source of delamination during assembly.

Thermal Performance

The thermal advantage over a via array comes from two effects: a larger copper cross section and no dielectric in the path. A typical coin conducts heat vertically with a thermal resistance measured in a fraction of a degree per watt, which is why it is used where a via array would need an impractically large area.

The path continues beyond the coin. The thermal interface between the coin and the heat sink, and the interface between the device and the coin, both add resistance, and a well designed coin with a poor interface material performs no better than a simpler solution. The thermal analysis therefore covers the whole path rather than the coin alone.

<img src="https://www.gopcba.com/wp-content/uploads/2025/08/16-1-1.png" alt="Power device mounted over an embedded copper coin” />

Design Rules Around the Coin

The coin needs a keep-out for components and traces on the layers it passes through, unless the design connects to it deliberately. The clearance to the cavity wall has to allow the resin to flow, and the coin should not be placed so close to a hole or an edge that the remaining laminate cannot support the press force.

The connection to the coin is made by copper that overlaps its surface on the outer layers and, where the layers permit, by an internal connection to a plane. Where the coin is an electrical node, the current path and the isolation from surrounding copper are both designed explicitly.

Electrical Considerations

A coin is a large piece of copper and it behaves as a low inductance connection for a power device, which is a benefit in a switching circuit. Where the coin is connected to a switching node, however, it also becomes a radiator, and the loop area of the associated circuit has to be kept small.

Where the coin is grounded, it provides an excellent return path and a thermal path at the same time. The design has to decide which of the two roles it is playing, because the layout around a grounded coin and around a floating thermal coin are different.

Cost and Application Fit

A copper coin board costs significantly more than a standard board, because it requires a cavity, a machined insert, a controlled lamination and additional inspection. It is justified where the thermal requirement cannot be met another way or where the alternative would be a ceramic substrate or a metal core board.

The volume also matters. A coin is a discrete part with its own tooling and handling, so the economics improve with volume, while a small batch may be better served by a metal core construction or by a large via array.

Inspection and Reliability

Inspection covers the coin position, the gap to the cavity wall, the resin fill and the flatness of the surface after lamination. Cross sectioning a coupon confirms the fill and the bond, and a thermal measurement on a test structure confirms the performance.

Reliability testing follows the failure modes of the construction: thermal cycling to test the bond between the coin and the resin, a moisture soak to test the fill, and a mechanical test on the attached heat sink. gopcb produces boards with embedded copper coins, machined inserts and the coupon data that a thermal design requires.

The coin is not a substitute for a heat spreader, it is the bridge that reaches one. Where the board is mounted on a metal case, the coin conducts the heat to the outer surface and the case spreads it over an area far larger than the device. Where the case is not available, an external heat spreader bonded to the coin does the same job, and the design has to ensure that the mechanical attachment does not load the laminate around the insert. Both arrangements are decided together with the coin size, because a coin that is larger than the spreading area it feeds gains nothing beyond a certain point.

FAQ

How is a copper coin different from a thermal via array? The coin is solid copper through the board, so it carries far more heat in the same area and adds no dielectric to the path. A via array is simpler and cheaper but needs much more area for the same performance.

Can the coin carry current? Yes, and it often does. Where it is used as a power connection, the design has to account for the current path and for the loop area of the switching circuit.

Why does a gap cause problems? Because the resin has to fill the space between the coin and the cavity. A gap that is too small prevents the coin from seating, and one that is too large leaves a void that absorbs moisture.

Related reading: thermal substrate comparison, trace width and current calculation, metal core PCB thickness, and PCB manufacturing processes.