At 1 oz copper, a trace carrying 20 A is a heater. The resistance is easy to calculate, the temperature rise is not, and by the time the board is in a chassis the margin has gone. Moving to heavier copper is the usual answer, but the copper weight is only the first decision. The process windows, the thermal path and the assembly profile all change with it.
This article covers what counts as heavy copper, where the real limits sit, how heat is managed on such a board, and what a quotation needs to state.
What counts as heavy copper
The weight refers to the finished copper on a layer, not to the plating thickness added during processing. Roughly half of the industry treats 4 oz and above as heavy copper, and power work often starts at 2 oz.
| Finished copper | Typical current and role | Typical use |
|---|---|---|
| 1 oz (35 um) | Signal and light power | Logic, control, low current rails |
| 2 oz (70 um) | Moderate current, short runs | Power supply rails, motor drive stages |
| 3 to 4 oz | Heavier current with thermal spread | DC-DC outputs, battery paths, RF amplifier bias |
| 6 oz and above | High current with heat spreading by copper | Busbars, high current backplanes, power combining networks |
Where the real limits sit
Copper weight is a number a buyer can state. What limits the build is what happens to it in processing.
- Line width and spacing. Etching heavy copper removes more material sideways, so a heavy layer cannot carry the same fine geometry as a 1 oz layer. Minimum lines and gaps move up with the weight, and the tolerance on them widens.
- Annular ring and holes. Pads and rings need more copper around a hole once the layer is thick, and drill to copper clearance grows with it.
- Plating in the barrel. A 6 oz layer multiplies the copper the plating has to cover, and high aspect ratio holes are the hardest place to reach the minimum.
- Resin flow and fill. Thick copper leaves less room for resin between layers. Blind and buried structures, and heavy copper combined with thin dielectrics, need a stack up that can actually be laminated.
- Lamination cycles. Each sequential press adds registration and stress, so the number of press cycles allowed on a heavy stack is a real constraint rather than a preference.
- Flatness. Uneven copper distribution warps a board. A heavy layer on one side and a light layer on the other is a distortion problem before it is an assembly problem.
Thermal path: spreading against sinking
On a power stage there are two ways to move heat, and they are not interchangeable. Copper spreading carries heat away from the joint into the board, which helps a surface mount device with a large pad and a thermal via field underneath. Sinking takes the heat out of the board into a chassis or a heat sink, which needs a controlled interface and a flat surface.
Thermal vias do both jobs at once, and their quality decides the result. A field of vias under a power device transfers heat through the board, so barrel plating thickness, fill and voiding matter more than the count. Where the current is high and the dielectric must still insulate, the stack uses thick copper for the conductors and a defined dielectric for the isolation.
Solder and assembly on heavy copper
Heavy copper changes the reflow profile, because the board behaves as a heat sink. Large planes pull heat away from the joint, so soak time and peak time have to be set on the actual board rather than on a generic profile for the paste. Where the power devices are through hole, selective or wave soldering fills the barrels from the other side, and the thermal relief on those pads decides whether the joint forms at all.
Placement and inspection follow the same logic. Voiding under a power pad reduces the thermal path, so X-ray with void control on the thermal pad and on the vias is part of the build record, and flatness is measured before assembly because a warped heavy board will not sit on a paste stencil.
What the RFQ has to state
- Finished copper weight per layer, not only the plating thickness.
- Current per net and the temperature rise the product accepts at that current.
- The stack up, including dielectric thickness and any blind or buried structure.
- Surface finish, and whether wire bonding or press fit is involved.
- Flatness requirement after assembly, and any thermal interface the unit depends on.
- Reports expected with the shipment, such as microsection, impedance and X-ray void data.
How SUPERB PCB supports heavy copper work
Superb Automation has supplied components and PCB assembly since 2000, and power boards are handled across the same flow as the rest of the programme. Inner layer base copper runs from 0.5 oz to 4 oz as standard, with a maximum of 6 oz on inner layers and 10 oz on outer layers, plating at 20 um or more, line width tolerance at plus or minus 20 percent, and fine lines down to 0.05 mm on the high layer line. Stack ups go to 68 layers with an aspect ratio up to 25 to 1, warpage held to 0.75 percent for surface mount work, and each order can ship with microsection and impedance reports.
Assembly uses twelve zone reflow on SAC305 with nitrogen below 500 ppm, selective and wave soldering for through hole power pins, CT capable X-ray with void control, and in-circuit, flying probe and functional test on product fixtures. Insulation and high voltage areas are checked to the drawing, and the quality system is ISO 9001 with solder workmanship to IPC-A-610 Class 2 or Class 3 as the assembly drawing states.
- Heavy copper boards: current carrying layers and thermal spread.
- PCB capability: layer counts, copper weights and tolerances.
- Process parameters: the numbers behind the build.
- PCB testing: microsection, impedance and the reports that travel with the order.
Frequently asked questions
Do I have to state the copper weight layer by layer?
Yes, for anything above 2 oz. A single number for the board hides the layer where the current actually flows, and the outer layers usually need less than the inner ones.
Can heavy copper be combined with fine pitch components?
Often yes, by keeping the fine geometry on layers that carry the signal and the heavy copper on the layers that carry the current. The mix has to be shown on the stack up so the impedance and the drill programme match it.
What report should come with a heavy copper order?
A microsection for plating and dielectric, an impedance report where controlled impedance is called out, and X-ray void data for thermal pads and vias. Flatness is checked against the drawing.
Next step
Send the Gerber data, the stack up with the copper weight per layer, the current per net and the temperature rise the product allows, plus the assembly data if the boards are built into a unit. The reply confirms which layers carry the heavy copper, states the process limits that apply, and quotes the build with the test and reporting scope attached.
