Trace Width and Current: Sizing for Temperature Rise

A trace carrying current dissipates heat, and the heat leaves through the laminate and the air. The temperature rise that results is the quantity that sets the trace width, and it depends on the environment as much as on the current.

What Determines the Temperature Rise

The heat generated is the square of the current times the resistance. The resistance depends on the copper thickness, the trace width and the temperature, since copper resistance rises with temperature.

The heat leaves by conduction into the laminate and into adjacent copper, and by convection and radiation from the surface. Of these, conduction into the copper is the dominant path on a board.

This is why the same trace carries much more current on an inner layer, where the laminate on both sides conducts heat away, than on an outer layer where one side is exposed to air. The difference is often a factor of two. Our thermal design notes describe the copper distribution.

Using the Charts Correctly

The published charts express the relationship between the trace cross section, the current and the temperature rise, for a set of assumptions about the board and the environment.

The assumptions include the ambient temperature, the copper weight, the presence of adjacent copper and whether the trace is on an inner or an outer layer. Changing any of them changes the answer.

The charts are also based on a single isolated trace. A bundle of traces running together heats each other, and the effective rise is higher than the chart suggests. Our current capacity notes describe how a bundle is handled.

Copper Thickness and Its Cost

Doubling the copper thickness halves the resistance and the heat generated for a given current, which allows a narrower trace for the same temperature rise.

The thicker copper costs more, requires a wider minimum feature and affects the etch tolerance. The impedance of a controlled impedance trace also changes, since the same width over thicker copper has a different impedance.

Where the current is high and the space is limited, the alternatives to thicker copper are a wider trace, a bus bar, or a cable. The comparison is between the cost of the copper and the cost of the space. Our plating thickness notes describe how the copper is specified.

Vias and Connectors in the Path

A via has less copper than the trace it connects, and a single via can carry less current than the trace. Where a trace must change layers, several vias are used in parallel, and the number is determined by the current.

The current capacity of a via depends on the barrel copper thickness, the via diameter and the length, and the resistance increases with the length. A long via in a thick board carries less current than a short one of the same diameter.

Connectors and their contacts have their own ratings and derating, and the trace may be limited by the connector rather than by its own width. Our etching process notes describe the tolerance on the finished width.

Fusing Current

A trace will eventually act as a fuse and open. The fusing current is far above the design current, typically an order of magnitude, and it depends on the same geometry and environment.

The fusing current is not a protection mechanism. A trace that is expected to open under fault current will do so unreliably and with damage to the surrounding material.

Where protection is required, it is provided by a fuse or a protection device rather than by the trace. The trace is sized so that it survives the fault current until the protection operates.

Verification

The verification is a temperature measurement on the trace at the maximum current and the maximum ambient, using a thermal camera or a thermocouple on a test coupon.

The measurement should be made on the production board rather than on a coupon, because the copper distribution affects the result. Where a coupon is used, it should include the same copper area around the trace.

The measured rise is compared against the limit derived from the material’s maximum temperature and the ambient. Our quality notes describe how the result is recorded.

Practical Rules

Start from the current and the allowable rise, choose the copper weight from the space available, and calculate the width. Then check the vias and the connector against the same current.

Where the current is high and the duty is low, the average heating is lower than the peak and the trace can be narrower. The calculation then uses the root mean square current rather than the peak.

Leave margin. A trace that is exactly at the limit at the maximum ambient has no margin for a higher ambient, a blocked airflow path or a component that adds heat nearby. The margin is cheap at the layout stage.

Process Control and Verification

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

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.

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.

Verification and Records

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.

FAQ

How much current can a trace carry? The answer is a temperature rise for a given geometry and environment, and the acceptable rise depends on the material and the ambient.

Does a wider trace always run cooler? Yes, for the same current. It also occupies space and changes the impedance of a controlled trace.

What does gopcb provide for current capacity? We provide trace width and copper weight calculations for the required rise and the actual environment, via count for layer changes, thermal measurement on the production board, and a review of the connector and fuse against the same current.