Ultra Thick Copper PCB: Design and Manufacturing

Where Ordinary Heavy Copper Stops

Copper weight is quoted in ounces per square foot, where one ounce corresponds to about 35 micrometres. Ordinary boards use one or two ounces, heavy copper power boards use four, and the class described as ultra thick starts above three ounces and extends to six, ten and, for special projects, twenty ounces, which is roughly 700 micrometres of copper on a single layer.

At that point the material stops behaving like a plated laminate and starts behaving like a machined metal part with an insulating core. The engineering question changes with it: the designer is no longer asking how wide a trace should be, but how to get the copper in and out of the process without losing the geometry, and how the finished board will be assembled without destroying it thermally.

Why Anyone Would Go That Far

Current. The cross sectional area of a trace is its width times its thickness, and the copper also acts as the heat sink that carries the loss away. Where the current is measured in hundreds of amperes and the space is fixed, widening the trace is not an option, so the thickness is increased instead.

Heat spreading. A layer of twenty ounce copper is a metal plate inside the board. It spreads heat from the switching devices across the whole assembly, which lowers the junction temperature more effectively than any heat sink that is bolted on, because it is directly bonded to the thermal pad.

Mechanical structure. The copper carries a large part of the stiffness and the strength of the board, and it resists the deformation and the vibration that a thin board cannot. In equipment that sees shock and continuous vibration, that is a functional requirement rather than a bonus.

Inductance. A thick conductor has a lower resistance and a lower self inductance, which reduces the voltage overshoot when a high current switches. Where the loop from the capacitor bank to the switch is short and made of thick copper, the switching noise falls markedly.

Applications

• Power distribution units and bus bars. Where the board replaces a copper bus bar and carries the full system current.
• Electric vehicle and battery systems. Battery management, cell interconnect and the main traction path, where the current is high and the space is fixed by the pack.
• Industrial drives and inverters. DC link connections, phase outputs and the sense shunts.
• High power LED drivers. Where the current path also has to carry the heat away from the emitters.
• Defence and aerospace power. Ruggedised distribution and conversion equipment that has to survive shock, vibration and thermal cycling.

How the Copper Gets There

Two processes produce heavy copper, and the choice determines the achievable geometry.

Etching heavy foil. The fabricator starts with a foil of the required weight and etches the pattern. The problem is that the etchant removes material sideways as well as downwards, so a very thick foil undercuts badly and the finished trace is far narrower than the artwork. The compensation is larger, the minimum feature is coarser, and the tolerance on the trace width is wider. It works well up to about four to six ounces.

Pattern plating up. The board starts with thin copper, the pattern is defined in a thick resist, and copper is electroplated up to the required thickness only where the traces will be. The process reaches the heaviest weights with much better feature definition, because no copper has to be removed from between the traces. It costs line time, needs careful current distribution to keep the thickness uniform across the panel, and requires a resist that can survive a long plating cycle.

Which route is chosen depends on the weight, the finest feature in the design and the plant equipment. A design that needs both a thick current path and a fine pitch component footprint is often built by plating the power layer and etching the signal layers separately. Our notes on PCB manufacturing describe the plating and etching controls involved.

The Manufacturing Problems

Etch uniformity. Thick copper etches unevenly across a panel because the etchant is depleted as it works. The fabricator compensates, and the design should allow for a trace width tolerance wider than a standard board.

Drilling. A hole through a thick copper layer demands a drill that can cut the copper without smearing the resin around it and without damaging the barrel, and the plating that follows has to fill the barrel uniformly. High aspect ratio holes in heavy copper are the hardest combination on the line.

Lamination. Thick copper features are tall, and the resin has to flow around them to fill the gap without trapping voids. The prepreg selection, the lay-up and the pressure profile are set for the pattern rather than for the average thickness.

Warpage. A board with a heavy copper layer on one side and a signal layer on the other bends, so the stack is balanced and the copper is distributed evenly. Where the design cannot be balanced, a compensating copper pattern is added.

Inspection. Copper thickness is verified on a microsection rather than assumed, and the finished trace width is measured on the same section, because those two numbers determine the current capacity of the finished product.

Design Rules

Widen the conductor deliberately. High current paths are often several millimetres or more wide, and the layout should give the current path the shortest and widest route the board allows, with generous copper at every corner and transition.

Parallel the vias. A single via cannot carry a heavy current, so the path changes layer through a bank of vias. Where the current is very high, vias are placed in the pad or an oversized pad is used with several barrels.

Design the thermal reliefs carefully. A pad connected to a heavy copper plane needs a larger relief than usual, because the plane conducts the soldering heat away. Where the joint is reflowed rather than hand soldered, the pad is often left solid with a controlled preheat instead.

Plan the soldering. A board with heavy copper has a large thermal mass and needs a reflow profile measured on the board itself, not taken from the paste datasheet. Both sides of a double sided assembly will need different treatment.

Check the mechanical outline. Thick copper is hard to route and to score. Keep the outline simple, place the tooling tabs where the copper is light, and discuss the depanelisation method with the fabricator. Our notes on PCB design and layout cover the layout side of all of these rules.

Cost

Heavy copper is priced on the finished panel rather than per board. Indicative 2025 figures are around 90 to 120 US dollars per square metre for a two layer four ounce FR-4 board at a hundred pieces, 160 to 220 dollars per square metre for a four layer six ounce high Tg board at fifty pieces, and 250 to 300 dollars per square metre for a two layer ten ounce ceramic based board in prototype quantity.

The drivers are the copper weight, the substrate, the hole count and the plating complexity, the panel area and the delivery schedule. An expedited order commonly adds 20 to 30 percent. A prototype panel is worth buying before a production release, because the finished trace width and the copper thickness on a microsection are the numbers that decide whether the design meets its current rating. Our notes on quality management describe the verification that should accompany the first order.

Choosing a Fabricator

Ask for the maximum copper weight the line can plate or etch, the achievable minimum trace width and spacing at that weight, the largest panel size, how the copper thickness is measured, whether a microsection is supplied with the order, and how the plant adjusts the assembly profile for the thermal mass. A plant that quotes heavy copper but has never supplied a microsection is a risk on a program where the current rating has a safety margin.

FAQ

What is the thickest copper available? Around twenty ounces is the practical upper limit with most fabricators, and ten ounces is the usual ceiling for routine production.

Can ultra thick copper be used on a multilayer board? Yes, but the stack has to be balanced and the lamination controlled, and the layer count is usually limited by the total board thickness.

Does heavy copper affect signal performance? On a high frequency signal the thick copper changes the impedance and the loss slightly, but in power applications the reduced resistance and inductance are a benefit.

How long does it take? Seven to fifteen working days for a standard heavy copper board, with complex or plated-up designs taking longer.

Conclusion

Ultra thick copper is used where the current is high, the space is fixed and the conductor also has to spread the heat. It is built either by etching heavy foil, which limits the feature size, or by plating copper up in the pattern, which reaches the heaviest weights with better definition. Either way, the design has to allow for a wider trace tolerance, more thermal mass in assembly and a microsection to prove the finished thickness. Our notes on industrial PCBA describe where this class of board is used, and our notes on PCB assembly cover the soldering that follows.