Trace Fusing and Current Crowding in Power Nets

A conductor fails when its temperature rises far enough that the copper no longer carries the load or the laminate degrades around it. The current at which that happens is set by the cross section at the narrowest point rather than by the average width.

How a Conductor Fails

Copper has a positive temperature coefficient, so resistance rises as the trace heats. Where the current is held constant, the heating and the resistance reinforce each other until either the laminate gives way or the trace opens.

The failure is therefore a thermal event rather than a purely electrical one. Our current capacity notes work through the arithmetic.

Current Crowding

Current does not distribute evenly through a conductor that changes shape. It concentrates at the inside of a corner and at the entry to a narrow neck, so the local current density is higher than the average.

The crowding is worst where a wide plane feeds a narrow trace. Our copper balance notes describe the geometry that surrounds it.

The Narrowest Point Sets the Limit

A net that is wide for most of its length and narrow at one via or one thermal relief carries the current of the narrow point, not the wide one. Those points should be enumerated during layout review.

Thermal relief spokes are a common example, because they are sized for soldering rather than for current. Our power layout notes describe the trade.

Temperature Rise Budget

The allowable rise depends on the laminate, the ambient and what else shares the board. A figure of ten degrees is a common default, but it is a default rather than a limit.

Where the board runs hot already, the budget is smaller. Our thermal design notes describe the surrounding conditions.

Copper Thickness and Width

Doubling the copper thickness halves the resistance, but it also changes the minimum width that can be etched. Where the budget is tight, the two have to be chosen together.

The finished thickness is what matters, not the starting foil. Our copper thickness notes describe the difference.

Measurement and Margin

The practical measurement is the temperature of the conductor under the worst case load, taken with a thermocouple or an infrared camera at a stated emissivity. The margin is then the difference between that reading and the laminate limit.

A reading taken at half load does not establish the margin. Our thermal measurement notes describe the technique.

Verification

The verification is an enumeration of the narrow points on the net, a calculation at each, and a temperature measurement under the worst case load rather than a nominal one.

Our design rules notes describe how the enumeration is recorded.

Process Control and Verification

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

Checks Before Release

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.

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 tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Verification and Records

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

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. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

FAQ

Is a fuse needed as well as a wide trace? They do different jobs. The trace carries the working current with an acceptable rise, and a fuse interrupts a fault current. One does not replace the other.

Does a plane remove the concern? No. A plane removes the crowding in the middle of the net but not at the neck where the current leaves it.

What does gopcb provide for power nets? We provide an enumeration of the narrow points on the net, a width calculation at each against a stated temperature rise budget, finished copper thickness stated separately from the starting foil, thermal relief spokes sized for their current, and a measured conductor temperature under the worst case load.