Enter current and allowed temperature rise. You get the IPC-2221 answer that almost every free calculator gives, alongside an IPC-2152-informed estimate — and an explanation of why they differ, which on internal layers is by a lot.
| Current | IPC-2221 width | IPC-2152 estimate | Difference |
|---|
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The equation almost every online calculator uses —
I = k · ΔT0.44 · A0.725 — comes from
IPC-2221, and its data is older than that. IPC-2152 superseded it in 2009 after a
far more thorough test programme, and the results were different enough that designing to the old
formula now routinely gives the wrong answer.
Most calculators do not tell you which standard they are using. This one shows both, and says where they disagree.
This is the big one. IPC-2221 applies a factor of two derating to internal traces: a buried trace is assumed to carry roughly half the current of an identical surface trace, on the reasoning that it cannot convect heat to the air.
IPC-2152's testing found that backwards. A buried trace is surrounded by laminate, and FR-4 conducts heat considerably better than still air does. In many configurations an internal trace runs cooler than the same trace on the surface. The old 2× derating is not conservative in an interesting way — it is simply wrong, and it costs real copper and real routing space on every inner layer.
Switch the layer selector and watch the two columns diverge. That gap is the cost of using an obsolete standard.
IPC-2221 knows about width, thickness, current and temperature rise. It does not know whether your trace runs alone in the middle of nowhere or beside a solid ground plane — and that changes the answer substantially, because the plane spreads heat away from the conductor. IPC-2152 addresses this with correction factors for exactly that situation.
On a real board almost every trace has copper near it, which is one reason IPC-2221 tends to be conservative on outer layers even before the internal-layer problem is considered.
A 10 °C rise is the usual default and it is a design decision rather than a physical boundary. What matters is where the trace ends up: 10 °C on a board already sitting at 85 °C inside an enclosure is a very different proposition from 10 °C on the bench. And copper's resistance rises about 0.39% per degree, so a hot trace also drops more voltage — which is frequently the real constraint on a low-voltage rail. The trace IR drop calculator works that side of it.
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