LIBRARYRouting traces: width, current, and vias.

A trace is a wire made of copper printed on the board. Its width sets how much current it can carry before it overheats, and a via is a plated hole that carries a trace to another layer. Size power traces from their current, keep signal traces short and direct, and change layers with a via only when you must.

Trace width comes from the current

The current a trace can carry rises with its cross-section, which for a fixed copper thickness means its width. The IPC-2221 standard turns a target current and an allowed temperature rise into a minimum copper cross-section, from which the width follows once you know the copper weight. The calculator below implements it: give it the current and the temperature rise, and it returns the width.

Here A is the copper cross-section, I the current, and dT the temperature rise you allow; k is a constant that is larger for an outer-layer trace than an inner one. Divide the cross-section by the copper thickness and you have the trace width.

Copper weight and voltage drop

A trace's thickness is its copper weight, usually 1 oz per square foot, about 35 micrometres of copper. Heavier copper carries more current in the same width. A long trace also has resistance, so a big current down a thin track drops voltage along the way and wastes it as heat. Widen a power trace and that drop falls.

Vias move a trace between layers

A via is a plated hole that connects copper on one layer to copper on another. Use one when a trace has to cross to the other side to get past an obstacle. A via has resistance and inductance of its own, and it carries limited current, so a high-current path uses several vias in parallel rather than one. Keep signal detours through vias few, because each one is a small discontinuity (KiCad).