Fusing Current: Design Rules and Process Limits

A copper trace is a fuse that nobody specified. It has a resistance, it heats when current flows, and above some current it will open. Most of the time that behaviour is undesirable, but there are cases where the trace is deliberately relied on to clear a fault, and cases where the designer must be sure it never does. Both require a number rather than an intuition.

Two Different Limits

The first limit is the current a trace can carry continuously without exceeding an acceptable temperature rise. The second is the current at which it will melt and open, which is far higher and depends on how long the current lasts.

Confusing the two leads to opposite errors. A designer who sizes a trace on the fusing figure produces a conductor that runs hot in normal service, while one who sizes on the continuous figure may assume protection that does not exist.

Temperature Rise in Continuous Operation

Steady state heating is a balance between the power dissipated in the trace and the heat removed by conduction into the board, convection from the surface and radiation. On an inner layer the board conducts well, so the temperature rise for a given current is lower than on an outer layer.

The usual approach uses a chart or a calculation that relates trace width, copper thickness and temperature rise. The result should be treated as an estimate, because the real board has neighbouring traces that also heat and a solder mask that changes the radiative behaviour. The practical methods are described in our article on high current capacity.

Fusing Current and Duration

Fusing is a transient process. A current well above the continuous rating will not open a trace instantly, and a current slightly above it may never open it because the board conducts the heat away. The relevant quantity is the energy delivered against the energy needed to raise the copper to its melting point, which is why short pulses tolerate far higher currents.

Manufacturers publish fusing curves for standard trace geometries, usually derived from tests on isolated traces in still air. A trace in a board surrounded by copper behaves differently, and the difference is large enough that the published curve is a starting point rather than an answer.

Copper Thickness and Its Effect

Both limits scale with cross sectional area, so copper weight matters directly. Doubling the weight halves the resistance per unit length and roughly doubles the current for the same temperature rise, and it increases the energy needed to fuse the trace.

The finished thickness is not the nominal weight, because plating adds copper and the etching process removes some. The value that matters is the finished figure, and it should be verified on a coupon rather than assumed. The measurement is described in our article on plating thickness.

Deliberate Fusing and Protection

A trace can be deliberately narrowed to act as a fuse, and this is done in some low cost designs to avoid a separate component. It is a poor practice for anything safety related, because the fusing current varies with the laminate, the copper thickness and the environment, and the resulting tolerance is far wider than any component fuse.

Where the trace is intended to survive a fault, its fusing current has to exceed the fault current for the duration the protection takes to operate. That comparison belongs with the protection design, and the interactions are described in our article on fuse selection.

Via and Connector Contributions

A current path includes vias and connectors, and both add resistance and heat. A via has a copper cross section that may be smaller than the trace feeding it, which makes it the hottest point in the path and the place where a fault is most likely to open.

Connector contacts have their own rating, and a path designed around the trace rating may be limited by the contact instead. Each element should be checked against the same current, and the weakest one sets the limit for the chain.

Verification

Verification means measuring the temperature rise rather than trusting the calculation. A prototype run at the maximum continuous current with an infrared camera or thermocouples bonded to the trace shows the real figure, including the effect of neighbouring copper.

Where a fault condition must be survived, a test at the fault current for the protection operating time confirms the margin. The investigation methods for a path that fails under stress are the same as any overstress analysis, described in our article on electrical overstress.

Checks Before Release

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected. Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

Verification and Records

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

FAQ

Is a wider trace always better? Electrically yes, but it uses area and changes impedance. Where the current is modest, the wider trace costs routing space for no benefit.

Can a trace replace a fuse? It can, and it is done, but the fusing current tolerance is wide. It is unsuitable where the protection has to be predictable.

Does solder mask affect the temperature rise? It changes the radiative and convective behaviour slightly, and it also changes the appearance of an overheated trace, which matters for inspection.