Busbar: Design Rules and Process Limits

There is a current above which a copper trace stops being a practical conductor. The trace width needed to keep the temperature rise acceptable grows faster than the current, and the board area that the conductor consumes becomes larger than the circuit it feeds. At that point the design moves to a busbar, which is a separate conductor that carries the current and is attached to the board rather than etched into it.

This article looks at the forms a busbar takes, how the current capability is estimated, and the mechanical and electrical details that decide whether the joint and the conductor work.

When Copper Alone Is Not Enough

An inner layer foil is typically one or two ounces, which is about thirty five to seventy micrometres thick, and an outer layer can be plated up to two ounces. A trace of that thickness carrying a hundred amperes would need to be many centimetres wide to keep the temperature rise within a reasonable limit, and the width would have to be split between the layers with vias carrying the current between them.

The alternative is to make the conductor much thicker. A busbar is commonly a fraction of a millimetre to several millimetres thick, which is one to two orders of magnitude more copper than a trace. The same current can then be carried in a strip a few millimetres wide, and the conductor can be routed in three dimensions rather than confined to the plane of the board.

Forms A Busbar Takes

The simplest form is a flat strip that is soldered or bolted to pads on the board, used for a single rail. A laminated busbar stacks several conductors with insulation between them, which gives a very low inductance path and is the standard construction for the direct current link of an inverter. A rigid bar that also provides mechanical support is used where the busbar is part of the chassis structure.

The laminated form is worth understanding because it solves a problem that a single bar does not. Two conductors carrying the same current in opposite directions, separated by a thin insulator, have their magnetic fields largely cancelled, which reduces the inductance and the radiated field. In a converter that switches fast, that reduction is the difference between a design that meets its emission limit and one that does not.

Current Density And Copper Thickness

The current capability of a bar is estimated from a current density figure, which for a naturally cooled copper bar in air is in the range of a few amperes per square millimetre. The figure is not a constant: it depends on the allowed temperature rise, on the ambient, on the orientation of the bar and on whether it is in still air or in a moving stream.

Applying a single density figure to a bar that is enclosed, or that is bolted to a hot surface, produces a conductor that runs hotter than intended. The bar should be treated like any other thermal problem, with the loss calculated from the resistance and the temperature rise estimated from the surface area. Where the bar is plated, the plating is thin and carries none of the current; its purpose is to keep the surface solderable and to resist oxidation.

Voltage Drop And Kelvin Sensing

At a hundred amperes a resistance of one milliohm produces a tenth of a volt, which is a significant fraction of a low voltage rail. The drop has to be included in the regulation budget, and it also produces the loss that heats the bar. Both effects push the design toward a larger cross section than the current density alone would suggest.

Where the current is measured, the measurement is usually made across a defined section of the bar using a Kelvin connection, with the sense wires taken from points inside the current path rather than from the ends. The sense connections must not carry current, and they should be placed at a known spacing so that the resistance between them can be calculated. The temperature coefficient of copper then has to be compensated, because the resistance of the section changes with the temperature of the bar.

Thermal Rise And Cooling

The bar loses heat by convection and radiation from its surface, and the surface area is therefore a design variable. A wide flat bar has more area for its cross section than a round one, which is why busbars are usually rectangular. Where the heat cannot be removed by convection alone, the bar can be bonded to a heat sink or given fins.

The joints are often the hottest points, because a bolted joint has a contact resistance that adds to the resistance of the bar itself. A joint that is not clean, that is not tightened to the specified torque, or that uses a washer of the wrong material can add a milliohm or more, and at a high current that joint becomes the limiting component. Measuring the temperature of the joints under full load, rather than the temperature of the bar in the middle, is the check that catches a bad connection.

Mechanical And Assembly Considerations

The bar has to be attached to the board in a way that survives thermal expansion. A rigid bar bolted at both ends to a board that expands differently will load the joints as the assembly heats, and a soldered attachment carries that load into the solder. A flexible section, a slotted mounting hole, or a clamped joint that permits a small amount of movement relieves the stress.

The attachment method also determines the assembly sequence, because a bar that is soldered has to be attached before or after the reflow depending on its mass. A heavy bar soldered to the board acts as a heat sink and makes the joint difficult to form, and it may require a selective soldering step or a mechanical fastener instead. The current capability of the copper on the board itself is checked separately, using a trace width calculation, and the overall fabrication route is described under PCB design and fabrication. The manufacturability questions that a busbar raises belong with the general guidelines for manufacture.

Process Control and Verification

On a design of this kind, busbar is the item that decides how the rest of the board is arranged. 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.

FAQ

Can a thick copper inner layer replace a busbar? Up to a point. Heavy copper boards carry several hundred micrometres, which handles a few tens of amperes. Beyond that the busbar is the practical answer.

Why is a laminated busbar better than a single bar? Because the opposing currents in adjacent conductors cancel much of the field, which lowers the inductance and the radiated emission. That matters in a fast switching converter.

Where does a busbar usually fail? At the joints. A contact resistance that adds a milliohm becomes the hottest point in the circuit and eventually fails, so the joint temperature should be measured under load.