Standard MCPCB Thickness: How to Choose

Two Thicknesses, One Confusion

A metal core board has a metal base and a dielectric layer, and both have a thickness. When a specification says 1.6 millimetres it usually means the total, but the number that governs the thermal performance is the dielectric, and the number that governs the stiffness is the base. Confusing the two is the most common error in specifying a metal core board.

A typical metal core board totals 1.6 millimetres, made of a 1.5 millimetre aluminium base and a 0.1 millimetre dielectric, with copper foil on top. Change the dielectric to 0.075 millimetres and the total becomes 1.575, which is outside a 1.6 millimetre tolerance while the thermal resistance has improved by a quarter. The specification has to name both, and it has to name which one the tolerance applies to.

Standard Base Thicknesses

• 0.8 millimetres. The thinnest common base, used for LED strips, backlight modules and any product where the profile and the weight matter more than the stiffness.
• 1.0 millimetre. A general purpose base for low and medium power LED modules and driver boards.
• 1.2 millimetres. Common for LED lighting bars and drivers, a small step in stiffness over 1.0.
• 1.5 millimetres. The thickness that, with a 0.1 millimetre dielectric, gives the familiar 1.6 millimetre total.
• 1.6 millimetres. The industry default for power boards and high power LED arrays, strong enough to carry connectors and heavy components.
• 2.0 millimetres. Used in automotive and industrial products where vibration and thermal cycling impose mechanical loads.
• 3.0 millimetres and above. Structural bases for outdoor lighting, large arrays and products where the board is part of the mechanical structure.

Alloys are usually 5052 or 6061 in a temper chosen for flatness and formability. Where the board will be bent or coined, a softer temper is specified; where it must stay flat through assembly, a harder one is used and the flatness tolerance is stated.

Dielectric Thickness

The dielectric is the thermal bottleneck and the electrical isolation at the same time, and its thickness is chosen from both requirements.

Thermal resistance. For a given material, the thermal resistance falls in proportion to the thickness. Halving the dielectric halves the temperature rise across it, which on a high power LED array is worth more than any change to the base. Typical thicknesses run from about 0.05 to 0.2 millimetres, with 0.075 to 0.15 being the common range.

Breakdown voltage. The isolation is the dielectric alone, so the working voltage and the required withstand voltage set a minimum thickness. A mains powered LED driver needs more dielectric than a 24 volt lighting module, and the thickness is chosen from the safety standard rather than from the thermal wish.

Capacitance. A thin dielectric makes a large capacitor between the copper and the base. On a switching board with fast edges that capacitance couples noise into the base, which then radiates unless it is grounded. A thicker dielectric reduces the coupling and the radiated emissions, so the two requirements pull in opposite directions.

Thermal conductivity of the material. The dielectric film is available in grades from about 1 to 12 watts per metre kelvin. Upgrading the grade often reduces the temperature more than thinning the film, and it does so without reducing the isolation. It is usually the first thing to change when a metal core design runs hot.

Choosing by Application

• LED strips and backlights: 0.8 to 1.0 millimetre base, thin dielectric, low power per unit area.
• General LED lighting: 1.0 to 1.2 millimetre base, dielectric chosen from the power density.
• High power LED arrays and street lighting: 1.5 to 2.0 millimetres, with a high conductivity dielectric and a heavy copper layer.
• Power supplies and motor drives: 1.6 to 2.0 millimetres, driven by the weight of the magnetics and the thermal load.
• Automotive and industrial: 2.0 millimetres and above, for vibration, thermal cycling and mechanical fixing.
• Structural and outdoor: 3.0 millimetres and above where the board carries the assembly.

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

Stiffness and Flatness

Stiffness rises with the cube of the thickness, so a 1.5 millimetre base is far more than twice as stiff as a 0.8 millimetre base of the same outline. Long narrow boards and boards carrying heavy components need the thicker base, and the cost of the additional aluminium is trivial compared with a field failure caused by a warped board.

Flatness is a separate specification from thickness, and it matters more than it appears. A metal core board is often clamped against a heat sink or a housing, and a board that is not flat does not make contact, which destroys the thermal path the design was built around. The flatness tolerance should be stated, and the mounting and the clamping force designed to accommodate it.

The base also affects the mechanical process. A thicker base needs more force to punch, is cut with a router or a saw rather than a punch die, and has a larger thermal mass in the reflow oven, which changes the assembly profile. Our notes on PCB manufacturing describe the tooling and the assembly consequences, and our notes on PCB assembly cover the soldering.

Copper Thickness

The copper on a metal core board carries the current and spreads the heat along the surface. One ounce is the standard, two ounces is used for medium currents, and three to six ounces appears in high current LED drivers and power boards. Heavier copper also spreads the heat more evenly, which reduces the peak temperature under the emitters, so it is often specified for thermal reasons rather than for current.

The practical limit is the etching process. Heavy copper has a coarser minimum feature, which constrains the fine pitch components that can share the board, and it needs a wider thermal relief at every pad that is soldered.

Cost

The base thickness contributes less to the price than most designers expect; the dielectric grade, the copper weight, the board area and the quantity dominate. As a rough guide, changing from a 1.0 to a 1.6 millimetre base adds a small amount to the board price, while upgrading the dielectric from a standard to a high conductivity grade adds 20 to 60 percent, which is the cheaper way to improve thermal performance.

What a thicker base does cost is the flatness control and the tooling, and where the board is punched rather than routed the tooling is a one-time charge amortised over the order. Our notes on quality management describe the incoming inspection that confirms the thickness and the flatness the supplier delivered, and our notes on PCB design and layout cover the layout rules that go with the construction.

FAQ

What is the standard thickness of an MCPCB? 1.6 millimetres total is the most common, usually a 1.5 millimetre aluminium base with a 0.1 millimetre dielectric.

Does a thicker base run cooler? It spreads heat over a larger area, but the dielectric is the bottleneck in the through path. Upgrading the dielectric grade often helps more than adding aluminium.

How thin can a metal core board be? 0.8 millimetres total is the usual practical minimum for a standard construction, and thinner boards are fragile and need mechanical support.

How do I specify the thickness? State the base thickness, the dielectric thickness and the total, and state which of them the tolerance applies to.

Conclusion

A metal core board has two thicknesses and both matter. Choose the base from the stiffness, the mounting and the area over which the heat must spread; choose the dielectric from the thermal resistance and the required isolation; and choose the copper from the current and the heat it has to spread. State all three, state the flatness, and remember that the cheapest way to make a hot metal core design cooler is usually to change the dielectric, not the aluminium.