FR4 and Aluminum Substrates: Where the Process Differs

An aluminum substrate and a glass reinforced epoxy board are both called printed circuit boards, but the process that makes them differs from the first operation. The metal has to be drilled differently, and the insulation between the conductor and the base is a different material with different limits.

What FR4 Means

FR4 is a flammability rating rather than a material name. It describes a laminate that self extinguishes when the flame is removed, and the boards that carry the designation are made from a range of epoxy resins with glass reinforcement and fillers.

Within the grade there is a wide range of behaviour. The glass transition temperature, the dielectric constant and the thickness tolerance all vary, which is why a drawing that specifies only FR4 leaves the electrical result to the supplier.

What an Aluminum Substrate Is

A metal core board has three layers: the copper that forms the circuit, an insulating layer, and the aluminum base. The insulation is the electrical barrier and the thermal path at the same time, which is the compromise the whole construction rests on.

The single sided version is the common one. A double sided version adds a second insulation layer and a second copper layer, and a multilayer version can be built by bonding a conventional stack to the metal base.

The Insulation Layer

The insulation is a filled polymer, usually an epoxy with a ceramic filler to raise its thermal conductivity. Its thickness is a compromise between the thermal resistance, which falls as the layer gets thinner, and the breakdown voltage, which rises.

The layer is also the limiting component for the temperature. A metal core board keeps the conductor cool by spreading heat, but the insulation has a maximum operating temperature that the design must respect even when the metal base is cooler.

Drilling the Metal

Aluminum is soft and conducts heat away from the cutting edge, which makes it difficult to drill. A controlled depth operation is used on a single sided board, because the drill must reach the insulation without damaging the base.

The tooling wears faster and the parameters differ from those used on a laminate. The cost difference between a metal core board and an FR4 board begins here, in the drill life and the machine time rather than in the material.

Single Sided Construction

A single sided metal board has a simple process. The conductor is imaged and etched on the copper foil, the insulation is already bonded to the base, and the remaining steps are the mask, the finish and the profile.

The absence of a plating step is the notable difference. There is no plated through hole on a single sided board, so the routing has to be completed on one layer and the connections to the outside world are made by terminals or by connectors.

Double Sided and Multilayer Metal Boards

A double sided metal board requires the two copper layers to be bonded to the base and to be connected. The process is closer to a conventional one, but the metal base makes the lamination and the drilling slower and the yield lower.

The thermal benefit is shared by both layers, which is why the construction is used for assemblies that dissipate a lot of heat from a small area. The cost is correspondingly higher than either a single sided metal board or a conventional double sided one.

The Thermal Path

The heat path runs from the device, through the solder joint, into the copper, through the insulation and into the metal base. The insulation is usually the largest term, which is why its thickness and its filler content are the variables that matter most.

The base then has to get rid of the heat. In a luminaire it conducts into the housing, while in a sealed module it may need an external surface. A metal core only helps if the heat has somewhere to go.

Where Each Material Is Used

A metal core board is used where the power density is high and the space is small: light emitting diode arrays, motor drives in a confined housing and power modules. The base also provides mechanical stiffness, which suits a thin assembly.

A conventional laminate is used everywhere else. It is cheaper, it supports plated through holes and multilayer routing, and its electrical properties are better understood for high frequency work.

Assembly Differences

Soldering to a metal core board requires more heat, because the base conducts it away from the joint. The profile has to be adjusted, and hand soldering with a small iron may not be able to reach temperature at all.

The large thermal mass also affects reflow. The board takes longer to reach the soak temperature and cools more slowly, so the profile is developed on the actual assembly rather than copied from a similar project.

Finish, Mask and Legend

The mask and the finish are similar to those used on a laminate, but the surface is less flat and the insulation is softer. The mask adhesion depends on the preparation of the insulation rather than on the copper.

The legend is printed on the mask as usual. Where the board is used as a visible part of the product, the legend and the mask colour are often specified by the mechanical designer rather than by the electrical one.

Cost and Where It Belongs

An aluminum substrate board costs more than a single sided laminate, and the difference grows with the layer count and the finish. The comparison should be made against the total thermal solution, not against the board alone.

Where a metal core removes a heatsink, a fan or a metal bracket, it can be cheaper overall. Where the heat can be spread by copper alone, the laminate is the better answer. The alternative approach using copper is covered under metal core constructions.

Design Rules and Testing

Design rules follow from the process. Annular rings are generous, the minimum trace is wider than on a laminate, and the insulation layer thickness is stated on the drawing because it controls both the thermal and the electrical behaviour.

Testing covers the dielectric strength of the insulation, the thermal resistance of a sample and the adhesion of the mask. The laminate guide and the dimensional stability considerations still apply, although the metal base changes the numbers.

Working With a Fabricator

A metal core board is produced by fewer shops than a laminate, and the process parameters differ between them. Sending the drawing to a fabricator that runs the construction every week produces a better result than sending it to one that will learn on the order.

The drawing should state the insulation thickness, the base alloy and the finish, because each of them changes the thermal and the electrical behaviour. A metal core board specified only by its dimensions will be quoted at the cheapest configuration and may not meet the requirement.

Where the Two Materials Meet

Some products use both. A control section is built on a laminate and a power section on a metal core, joined by a connector or by a press fit terminal. The two materials have different expansion, so the joint between them is a design item.

The alternative is a single board with a localized thermal solution: a thick copper area, a metal inlay or a heatsink bonded to the laminate. Each of those is a different answer to the same question, and the choice depends on the power and on the volume.

FAQ

Is FR4 a material? No. It is a flammability grade, and many different laminates meet it. The electrical and thermal parameters have to be specified separately.

Why is drilling an aluminum board more expensive? Because the metal is soft, abrasive and thermally conductive. Tool life is shorter and the parameters differ from those used on a laminate.

Does a metal core board remove the need for a heatsink? Not always. It reduces the thermal resistance between the device and the base, but the base still has to reject the heat to the environment.