Why Trace Width Is a Thermal Calculation
A PCB copper trace carrying current generates heat proportional to its resistance and the square of the current — too narrow a trace for its current load can overheat, potentially damaging the board or nearby components, long before it would ever visibly melt. The IPC-2221 standard (the widely used industry reference for this) provides empirical formulas relating trace width, copper thickness (weight, in ounces), current, and an acceptable temperature rise above ambient.
Internal traces (buried between PCB layers) need to be wider than external traces for the same current, since internal layers don't have direct air contact to dissipate heat — a detail that's easy to overlook when routing a board and assuming trace requirements are uniform regardless of layer position.
Getting trace width wrong is a common source of subtle PCB failures: a board that works fine at low current but overheats, drops voltage, or fails intermittently under full load — problems that don't show up until the board is actually stressed near its real operating current.