Thermal Conductive Laminate: A Metal Core Alternative

The Gap Between FR-4 and Metal Core

An ordinary FR-4 board conducts heat at around 0.3 watts per metre kelvin, which is why a hot component on a FR-4 board heats only the copper and the few square millimetres around it. A metal core board solves that by putting a solid aluminium or copper base under the circuit, but it also restricts the design: most constructions are single sided with limited plated through holes, and the metal base complicates the mechanical outline and the electrical isolation.

A thermal conductive laminate fills the space between them. It is a resin based laminate, made like FR-4 and processed like FR-4, with a ceramic filler that raises its thermal conductivity to between about 1 and 3 watts per metre kelvin. It supports multiple layers, plated through holes, fine geometry and controlled impedance, while spreading heat far better than standard FR-4. It is the answer when the board is too hot for FR-4 but too complex for a metal core.

How the Material Works

The filler is the whole point. Ceramic particles, usually alumina or boron nitride, are dispersed through the resin at a high loading, forming a continuous path for heat through the dielectric. The copper on either side of the laminate then spreads the heat laterally, exactly as it does on a standard board, but the heat can also pass through the laminate to the other side, where a copper plane or the enclosure carries it away.

The consequences of the filler are what an engineer has to plan around. A heavily filled laminate is more brittle than FR-4, its dielectric constant is higher and less uniform, its bonding strength to copper is lower, and its drilling behaviour is different. It is a real engineering material, not a drop-in replacement, and the fabricator that runs it regularly will set the process to suit it.

Thermal Conductive Laminate Against Metal Core

• Layer count. TCL supports four, six or more layers; a metal core board is usually single sided with limited plated holes.
• Thermal performance. A metal core board is better in the direction through the board, because aluminium conducts far better than any filled resin. A TCL is competitive laterally, because the copper does the spreading in both cases.
• Isolation. A metal core board needs the dielectric to hold off the full working voltage; a TCL is an ordinary dielectric and the isolation is designed the same way as on FR-4.
• Mechanical. A metal core board is stiff, flat and heavy, and it can be used as a structural element. A TCL is a normal board and needs the housing for stiffness.
• Processing. TCL goes through the standard multilayer line. Metal core boards need special routing and punching tooling.
• Cost. Both carry a premium over FR-4. The metal core premium is driven by the aluminium and the special tooling, and the TCL premium by the filled laminate.

The practical rule is that a metal core board wins where the heat must go through the board into a chassis, and a TCL wins where the circuit needs layers, plated holes and ordinary design rules but the components must run cooler than FR-4 allows.

Where It Is Used

• LED lighting. Driver boards and light engines where the LED current and the control circuit share one board, which a metal core construction cannot do.
• Power conversion. DC to DC converters and power supplies where the switching devices are placed directly on the control board.
• Motor drives. Small drives that combine the logic, the gate drive and the power stage.
• Automotive electronics. Modules that sit in a warm location and cannot justify a metal core board.
• Compact power modules. Where the whole system has to be on one multilayer board inside a sealed enclosure.

Our notes on energy PCBA describe how these boards are used in power conversion.

Design Rules

Spread the heat in copper first. The laminate is a bottleneck compared with copper, so the area under and around a hot component should be as large as the layout allows, with thermal vias to the other side where a plane can take the heat further.

Use thermal vias deliberately. A via under a pad connects the copper on both sides and roughly halves the thermal resistance through the board. Where a via is placed inside a solder pad, it must be filled and plated over, or the paste wicks into the hole.

Expect a higher dielectric constant. The filled laminate typically has a Dk of about 4.5 to 5, and it varies more between batches than FR-4, so impedance targets should be quoted with a tolerance and verified on a coupon.

Plan for brittleness. Keep score lines and depanelisation tabs away from component areas, avoid small internal corners in the outline, and use a router rather than a punch where the outline is complex.

Set the drill programme for the material. A filled laminate is abrasive and its resin is more prone to smear, so the drill parameters and the desmear process are chosen for it. Our notes on PCB manufacturing describe the process controls, and our notes on PCB design and layout cover the layout side.

Measuring Whether It Helped

A thermal design is only worth the material premium if it can be shown to work. The measurement is straightforward: build two versions of the board, one in FR-4 and one in TCL, place a thermocouple or an infrared camera on the hot component, and run both at the same power in the same enclosure. The difference in the steady state temperature is the answer, and it is often larger than the laminate datasheet suggests, because the copper spreading improves as well.

Two cautions apply. A bench measurement in open air flatters both materials and understates the difference in a sealed product, so the test should be run in the real enclosure. And the measurement should be taken after the temperature has stabilised, which on a board inside a sealed box can take much longer than expected. Our notes on quality management describe how the verification is documented.

What It Costs

The laminate is the premium. A filled thermal conductive laminate typically costs two to four times as much as a standard FR-4 of the same thickness, and the effect on the finished board price depends on the area that uses it and on whether the whole board or one section is built on it.

For a small power board in moderate quantity, the board price commonly rises by 30 to 100 percent over an equivalent FR-4 board, which is usually less than the cost of the metal core alternative and far less than the cost of adding a heat sink. The comparison that matters is against the whole thermal solution, including the heat sink, the thermal interface material, the fasteners and the assembly labour. Our notes on PCB assembly describe how the assembly process is adjusted for the material.

Mixing Materials in One Board

Where only part of the board runs hot, a hybrid stack can use the thermal laminate under the power section and ordinary FR-4 elsewhere. The two materials are bonded in one lamination, and the same rules apply as in any hybrid: the stack must be symmetrical enough to stay flat, and the tooling, the drill programme and the desmear process have to suit the harder of the two materials.

FAQ

Is a thermal conductive laminate the same as a metal core board? No. It is a filled resin laminate with no metal base, processed like FR-4, which is why it supports multiple layers and plated through holes.

How much better is it than FR-4? Its thermal conductivity is typically three to ten times that of standard FR-4, depending on the grade, though copper still conducts far better than either.

Can it replace a heat sink? Often it removes the need for one on a small power board, because it spreads the heat into the copper and the housing. Where the dissipation is high, a heat sink is still required.

Does it support controlled impedance? Yes, but the dielectric constant is higher and varies more between batches, so the impedance should be specified with a tolerance and verified on a coupon.

Conclusion

A thermal conductive laminate is the middle option between ordinary FR-4 and a metal core board: it spreads heat much better than FR-4 while keeping the multilayer, plated hole and fine geometry capability that a metal core construction gives up. Spread the heat in copper first, use thermal vias to reach the other side, expect a higher and less uniform dielectric constant, and prove the benefit with a measurement in the real enclosure before committing the whole design to the material.