Estimate the current that melts a copper trace in a given time, or the time a trace survives a short-circuit current. Useful for fault analysis and for sizing traces that must survive until a fuse opens.
Onderdonk's equation. For a short pulse all the energy goes into heating the copper (adiabatic): I = A · √( log10(1 + (Tm − Ta)/(234 + Ta)) / (33·t) ), with A in circular mils, t in seconds, Tm = 1084.6 °C (copper melting point) and Ta the starting temperature. Inverting it gives the time to fuse.
Limits. Valid for pulses up to a few seconds. For longer times heat flows into the board, so the real fusing current is higher than the estimate. Use it to compare designs and to check that a trace outlasts the fuse or breaker that protects it, not as a fuse design rule.
It is not a reliable fuse: the fusing current depends on the board, the copper tolerance and the pulse shape, and a melting trace can damage the board. Use a real fuse.
The energy needed to reach the melting point is fixed, so the current falls as one over the square root of the pulse time.
Width a trace needs for a current, or the current a trace can carry, with IPC-2152 and IPC-2221.
Current capacity, resistance, thermal resistance, capacitance and inductance of a plated via.
Thermal resistance of an array of vias under a pad, with fill options, and the temperature drop through the board.
Impedance of an outer-layer trace over a plane, with solder mask or embedded, and width for a target impedance.
Impedance of an inner-layer trace between two planes, symmetric or offset, and width for a target.
Differential, odd, even and common-mode impedance of edge-coupled microstrip and stripline pairs.