Double Sided and Sandwich Metal Core Board Construction

A metal core board conducts heat from a component through a thin dielectric into an aluminium base, and the base spreads it sideways into a heatsink or a chassis. The limitation of the simplest construction is that the circuit is on one face only, so a design that needs components on both sides, or that needs a ground plane beneath the signal layer, cannot be built on it.

Double sided and sandwich constructions answer that limitation by putting copper on both faces of the dielectric, or by bonding metal on both sides of the core. The result is a board with more routing freedom and, in the sandwich case, a thermal path that runs in both directions, at the cost of additional process steps and a different set of design rules.

The Double Sided Metal Core Board

In a double sided construction the dielectric is applied to both faces of the aluminium base, and copper is laminated or bonded on each side. The two copper layers are connected by plated through holes that pass through the metal base, which requires the hole to be insulated from the base before plating. That insulation is the critical process step, because a breach in it creates a short between the circuit and the chassis.

Because the holes have to be insulated, the drilling and plating sequence is more involved than on a laminate board. The hole is drilled through the aluminium, an insulating layer is formed or inserted, and the barrel is then metallised. The dielectric strength of the insulation has to withstand the working voltage of the circuit with margin, and it has to survive the thermal cycling that the assembly will see.

The Sandwich Construction

A sandwich board places metal on both outer faces with the circuit layers between them, or places a metal core in the centre with circuits on both sides. The first arrangement provides shielding and mechanical stiffness as well as heat spreading, and it is used where electromagnetic compatibility and rigidity are both required. The second gives a symmetric thermal path, so a component on either face can dissipate into the core.

Symmetry is the main advantage for thermal design. In a single sided construction the heat has to travel from the component through the dielectric into the base plate, and the thermal resistance of that layer dominates. In a sandwich the same path exists on both sides, and a design that places power devices on both faces can use the whole base as a spreader rather than only the region beneath the components. The price is additional lamination and the difficulty of making connections through the structure.

Thermal Path and Its Calculation

The thermal resistance of a metal core board is the sum of the resistance of the component attach, the copper pad, the dielectric and the base, plus the interface to the heatsink. The dielectric is normally the largest term, at one to three watts per metre kelvin over a thickness of 75 to 150 micrometres. The copper and the aluminium are far more conductive, and they contribute mostly by spreading heat laterally.

Spreading is what makes the metal core useful. A small component on a large base can dissipate far more power than the same component on a laminate, because the base has enough area to transfer the heat to the air or to a chassis. The calculation therefore has to include the spreading area rather than only the area directly under the device, and a thermal simulation is the practical way to size the base thickness and the board outline. This is the same reasoning that governs any thermal management decision on a dense board.

Design Rules That Differ From a Laminate

Minimum trace width and spacing are usually larger than on a laminate, because the dielectric is bonded rather than laminated from prepreg and the etching is done on a surface that may not be perfectly flat. Component placement has to avoid the edges of the panel, where the dielectric is thinner and the mechanical protection is reduced. Vias that pass through the base require additional clearance for the insulation layer.

Assembly also has constraints. The base cannot be drilled and tapped in the same way as a thick aluminium plate without care, because the drill may break through into the dielectric. Reflow is usually single sided, since a double sided assembly on a metal base requires the board to survive two passes with a large thermal mass. Where components must be on both sides, the second side is normally attached with a low temperature alloy or with an adhesive.

Where These Constructions Are Used

The single sided metal core board dominates high power LED modules, where a single layer of circuitry is enough and the thermal path is the only real requirement. Double sided boards appear where control circuitry has to share the substrate with the power stage, and where the control signals need a ground plane or a second routing layer. Sandwich boards appear in automotive and industrial equipment where shielding, rigidity and heat spreading are all needed at once.

Choosing between them follows from the requirement. If the circuit fits on one layer and the thermal path is the only driver, the simplest construction is the cheapest. If a second layer is needed for routing or for a plane, the double sided version adds it, with the insulation of the through holes as the process risk to manage. If the board is part of a shielded enclosure and carries power on both faces, the sandwich construction earns its extra cost. The decision should be made with the supplier, because the process capability for the insulation step varies more between fabricators than the laminate process does, and the pcb manufacturing processes involved are less standardised.

Mechanical Integration and Fixing

The aluminium base is usually the mounting surface as well as the heat spreader, so the fixing method has to be designed with it. A screw that passes through the board can crush the dielectric if it is over tightened, and a screw in a blind hole can bottom out and crack the base. The practical approach is to use a defined torque, a washer with a controlled bearing area and, where possible, a fixing point that is away from the circuitry. Threaded inserts pressed into the base are common where the joint will be opened repeatedly.

Flatness matters because the base has to make contact with a heatsink or a chassis over a large area. A base that bows after lamination leaves an air gap that dominates the thermal path, whatever the dielectric resistance is. Thicker bases resist bowing better, and some suppliers specify a flatness tolerance as part of the board specification rather than as a mechanical drawing note. Measuring the flatness on a sample, and using a compliant thermal interface material with a defined thickness range, is what keeps the assembled thermal resistance close to the calculated value.

Panel Utilisation and Cost

Metal core panels are more expensive per unit area than laminate, and the dielectric is usually applied in fixed widths, so the outline should be designed to fit those widths rather than to use the smallest possible area. A board that is a few millimetres narrower may fit two across a standard panel, halving the material cost, while an outline that is slightly too wide wastes a strip. Discussing the panel layout with the supplier at the drawing stage is worth more than a small reduction in board area.

Assembly cost follows the same logic. A board that requires two reflow passes with a large thermal mass is more expensive to assemble than one that is single sided, and the second pass also risks damaging the first. Where a design genuinely needs components on both faces, the sequence should be planned so that the higher temperature process comes first, and the second attachment should use an alloy or an adhesive with a lower processing temperature. Planning that sequence early keeps the manufacturable design guidelines achievable in practice.

Additional Considerations for This Build

Practical attention to insulated through hole pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating insulated through hole explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

FAQ

Can a metal core board have more than two copper layers? It can, but each additional layer adds lamination steps and reduces the thermal performance, because the heat has to cross more dielectric. Beyond four layers the advantage over a ceramic substrate becomes small.

How is the through hole insulated? By an insulating sleeve, a coated wall or an anodised surface, depending on the supplier. The method determines the dielectric strength and the thermal resistance of the hole.

Is a sandwich board heavier? Yes, and that matters in portable products. The base thickness is chosen from the thermal and mechanical requirement rather than from habit.