A power board rarely fails because the wrong component was fitted. It fails because the copper was too thin for the current, because the heat had nowhere to go, or because two nets sat closer together than the voltage allowed. All three are decided on the drawing — which means they are decided by you, before anyone quotes.
It is also the number most often copied from a table that does not apply to your board.
Copper weight is the thickness of the copper on one layer, measured in ounces per square foot: 1 oz is about 35 µm. It is the biggest single lever on a power board, because a trace's resistance falls in proportion to its cross-section — and cross-section is width times thickness. When you cannot widen a trace any further, you thicken the copper instead.
The trap is how that trace usually gets sized. The current-carrying charts in IPC-2221, the generic standard on printed board design, assume a trace on a board sitting in still air, and they were drawn for the copper weights and board constructions of their era. For this purpose they were superseded by IPC-2152, which accounts for board thickness, for the copper area around the trace, and for the thermal environment the board actually sits in. The practical consequence is that the same 3 mm trace carries noticeably more current on a six-layer stack-up with a plane beneath it than on a bare two-layer board, because the plane pulls heat out of the trace as it forms.
So the useful question is not "how many amps does 2 oz carry". It is "what temperature rise can my product tolerate, and what does the board look like around this trace". Answer those two and the copper weight follows from them.
| Copper weight | Thickness | What it is for | What it costs you |
|---|---|---|---|
| 0.5 oz | about 17 µm | Dense signal layers where the routing is the problem, not the current. | Very little current capacity. Not for anything that carries real power. |
| 1 oz | about 35 µm | The default for most boards, and the baseline every other weight is measured against. | Nothing — this is the reference point. |
| 2 oz | about 70 µm | The entry point to what the industry calls heavy copper: motor drives, LED drivers, switch-mode supplies. | Line and space start to coarsen, and etching takes longer. |
| 3 – 6 oz | about 105 – 210 µm | High-current paths, busbars formed in the copper itself, planar magnetics. | Coarser minimum features and a higher board cost per panel. |
| 10 – 12 oz | about 350 – 420 µm | Replacing a busbar outright: very high current through a small area. | Coarse features only — at this point the process is closer to machining than to etching, and fewer suppliers build it at all. |
The trade-off in that last column runs against the instinct that more copper is strictly better. Heavier copper means the etchant has to remove two to twelve times as much material to open the same gap, so the finest line and space a board can hold gets coarser as the copper gets thicker. A board specified at 6 oz gives up fine-pitch routing that the same design could have had at 1 oz.
Which is why the first question to put to a supplier is not "can you do 6 oz" but "what line and space do you hold at 6 oz". Capability tables quote line and space at 1 oz, because that is what most boards use; the number at your copper weight is a different one, and it is the number that decides whether your layout can be built. Our own fabrication parameters run from 0.5 oz to 12 oz — roughly 17 µm to 420 µm of copper per layer — and the 0.06 mm line and space listed beside them is a 1 oz figure.
Three ways heat leaves a power board. Most designs rely on two of them at once, and the choice has to be made together with the stack-up.
A thermal via is a plated hole that exists to move heat rather than current. It carries heat from a pad on the outer layer to inner or bottom copper, where there is enough area to shed it. Under a QFN, a power package or the thermal pad of an array, vias are the difference between a part that runs warm and a part that unsolders itself.
The detail that catches people is what happens during reflow. Solder paste on the pad follows an open via barrel and wicks through to the other side, and what is left behind is a starved joint underneath a part that nobody can inspect. Vias under a component pad are therefore plugged, tented or filled — and which one you choose also decides how flat the pad ends up, which decides how the part sits on it. If the via carries current as well as heat, the choice narrows further, because a conductive fill and a non-conductive plug are not interchangeable in a circuit.
When the heat is more than the copper can carry away, the laminate itself changes: the dielectric is bonded to an aluminium plate, and the plate becomes the heatsink. These are the boards under LED drivers, motor controllers and anything where the enclosure is part of the thermal path. Aluminium is one of the base materials listed in our own fabrication parameters, alongside FR4, CEM1 and CEM3, high-Tg laminates, and the Rogers and Taconic materials — so it is worth asking about at the quote stage rather than assuming it is out of scope.
The third path leaves the board entirely. A heatsink bolted or bonded to the surface turns flatness into a thermal parameter: if the board warps in reflow, the interface between board and heatsink becomes an air gap, and the thermal design that worked on paper stops working. On boards like that, asking about the flatness spec and about how evenly the copper is balanced across the panel is more useful than asking about the surface finish.
One is measured through the air, one along the surface — and only one of them is affected by how clean the board stays.
Clearance is the shortest distance between two conductors through air. Creepage is the shortest distance between them along the surface of the board. They have different minimum values for the same voltage, and creepage is the larger of the two wherever the surface can get dirty or damp, because contamination along a surface builds a conductive path that air alone does not.
Both distances grow with the voltage, with the pollution degree of the environment the product sits in, and with altitude — thinner air insulates less. This is the subject of IEC 60664-1, the insulation-coordination standard that most low-voltage product standards point back to, and of the spacing tables in IPC-2221.
The material property underneath the creepage distance is the comparative tracking index (CTI), measured under IEC 60112. It describes how readily a surface forms a conductive track when contamination and voltage act on it together, and it sorts materials into four groups: group I at 600 V and above, group II from 400 to 599, group IIIa from 175 to 399, and group IIIb from 100 to 174. A higher group lets the same voltage be held with a shorter creepage distance — which is board area you do not have to pay for.
The same copper that solves your current problem makes the assembly harder in three specific ways.
A board carrying 6 oz copper holds far more metal than a 1 oz board of the same size, and metal takes time to heat. The reflow profile has to be stretched for the board as a whole, which means parts sitting on a ground pour see a different thermal history from parts at the edge of the panel. On a mixed board, that difference is where tombstoning and cold joints come from.
Copper and laminate expand at different rates, and the effect scales with how much copper is present and how evenly it is spread. Asymmetric copper on a heavy board warps during reflow — enough to shift a fine-pitch part, or to break the flatness a heatsink interface depends on.
Power boards are rarely all surface-mount. Screw terminals, relays, transformers and bulk capacitors are through-hole, and a large copper mass changes what happens at those joints: the barrel pulls heat away from the iron while the solder is trying to melt into it. It is worth asking directly how your through-hole parts will be soldered and what the resulting joint is inspected against, because on a heavy board this is the step where a person reaches the board.
There is no single number, and a supplier who gives you one without asking about your stack-up is guessing. Copper weight and trace width together set the cross-section that carries the current; the copper around the trace decides how quickly the heat leaves it. The same trace runs cooler on a six-layer board with an internal plane than on a bare two-layer board, which is exactly why the current charts in IPC-2221 were superseded by IPC-2152. Start from the temperature rise your product can tolerate and from your stack-up, and the copper weight follows from those two.
Usually yes. Solder paste on the pad wicks down an open via barrel during reflow and comes out the other side, leaving a starved joint underneath a part nobody can inspect. Plugged, tented and filled vias all stop that, and each leaves a different pad surface — which matters when the part sitting on top is a QFN or a power package with a large thermal pad. The right choice also depends on whether the via carries current as well as heat, because a conductive fill and a non-conductive plug are not interchangeable once the via is part of the circuit.
Not after the fact. Creepage is a dimension of the artwork: it either exists on the drawing or it does not exist at all. What a coating changes is the assumption underneath the creepage tables — the pollution degree the assembly is designed to survive. IEC 60664-3 covers coated, potted and moulded assemblies and does allow reduced distances where the coating is part of the design. That is a decision taken while designing, with the coating specified, not a repair for a board that was laid out without it.
ENIG, HASL, OSP, immersion silver, immersion tin and ENEPIG — cost, shelf life and failure modes, side by side.
The passive package spectrum, what the small end demands, and why 0.3 mm BGA pitch is a different problem.
X-ray is what sees the voids under a thermal pad; functional test is what proves the board works under load.
Send us the stack-up and the working voltage. Those two numbers tell us more than a BOM does, and if the copper weight, the thermal path or the spacings are still open, we will say so at the quote stage — while the artwork can still change, rather than after the boards come off the line.
