Busbar Design for High Current PCB Assemblies

At some point a copper trace becomes an impractical way to carry current. The width needed to keep the temperature rise acceptable exceeds the space available, the voltage drop becomes significant, and the layer count required to share the current makes the board expensive. A busbar solves all three problems by moving the conductor off the board and into a separate piece of metal.

When a Busbar Beats Copper

A busbar wins when the current is high enough that the copper cross section becomes unreasonable, typically above 50 to 100 amperes on a typical board, and when the conductor has to travel any distance. It also wins when the mechanical interface matters, because a bolted busbar can be assembled and serviced in the field in a way that a soldered connection cannot. High current battery, inverter, and motor drive assemblies are the usual applications.

The alternative is to use more copper on the board, and that remains a legitimate choice for moderate currents and short distances. Multiple layers of heavy copper in parallel, or a metal core substrate, can carry very high currents without a discrete busbar. The decision usually comes down to the available space, the thermal environment, and whether the connection must be removable.

Current Rating and Temperature Rise

A busbar is rated by the current it carries for a given temperature rise, and the figure depends on the material, the cross section, the ambient temperature, the airflow, and the surface finish. A bare copper bar in still air dissipates heat less effectively than one that is blackened or that has a larger surface area, and the difference in rating can be 20 to 30 percent for the same cross section.

The calculation should be verified by test rather than trusted alone. A busbar that looks generous in the calculation can run hot because the heat cannot escape from the enclosure, or because the contact resistance at the joint adds more loss than expected. Measuring the temperature at the hottest point of the bar, not at the end, and comparing several current levels, gives a rating curve that is specific to the installation.

Material and Plating Choices

Copper is the default material because its conductivity is high and it is easy to form. Aluminium is lighter and cheaper but has lower conductivity and a difficult oxide that must be removed before a joint is made. Where weight matters, aluminium is used with a bimetallic transition or a plated joint to avoid a galvanic couple with copper.

Plating protects the surface and stabilises the contact resistance. Tin plating is common for bolted joints that are assembled once, while silver plating gives a lower contact resistance and better performance at high temperature. A nickel underlayer prevents diffusion between the plating and the copper base. Where silver is used, the surface must be protected from sulphur in the environment, because silver sulphide is an insulator.

Attachment Methods

A bolted joint is the most common attachment for a heavy busbar. The joint must have a defined contact area and a specified torque, and the surfaces should be flat and clean. A joint that is tightened without a torque specification will have a contact resistance that varies from unit to unit, which shows up as a variation in temperature between supposedly identical assemblies.

Soldered and welded attachments suit smaller busbars and automated production. Soldering a thick bar requires more heat than a board can tolerate locally, so the joint is often made by a selective process with preheating, or the bar is designed with a tab that can be reflowed with the rest of the assembly. Welding, including ultrasonic and resistance welding, gives a metallurgical joint without the thermal exposure of soldering.

Layout and Creepage

A busbar changes the electric field around the board. The clearance between a busbar and an adjacent conductor is measured through air and along surfaces, and the requirement rises with the working voltage. Where a busbar carries a high voltage, the spacing to the enclosure, to the mounting hardware, and to other conductors must be checked, and an insulating sleeve or a barrier may be necessary.

Mechanical layout matters as much as electrical. The bar should be supported so that it does not transmit vibration to the solder joints at its ends, and it should be routed so that it does not obstruct the airflow that cools the board. A busbar that runs across the top of a hot component will make that component hotter, which is rarely the intended effect.

Busbar design should also consider the assembly sequence. A bar that must be fitted after the board is populated needs access for a tool, and a bar that is fitted before reflow must survive the oven temperature without distorting. Where the bar is large, its weight during handling can stress the joints at its ends, so a support bracket or a standoff is often needed to carry the mass rather than the solder.

Thermal Interface to the Busbar

A busbar is often the most effective heat spreader on the assembly, and its thermal role can be as important as its electrical one. Connecting a hot device directly to a large bar gives the heat an easy path out of the board, provided the joint between them has a low thermal resistance. Where the bar also carries current, the same joint must be good for both purposes.

Thermal interface materials are used where the surfaces are not perfectly flat, and their thickness should be controlled because their thermal conductivity is much lower than that of the metal. A joint with a thick layer of a low conductivity material can add more thermal resistance than the bar removes, which is why the flatness of both surfaces is worth specifying.

Inspection and Reliability

Busbar joints fail for mechanical reasons more often than electrical ones. A joint that loosens with thermal cycling develops a higher resistance, which raises the temperature, which accelerates the loosening, and the process continues until the connection overheats. A specified torque, a locking feature such as a serrated washer or a thread locking compound, and a re-torque interval address the mechanism directly.

Verification combines a micro-ohm measurement at the joint with a thermal image under load. Both should be recorded at the first article and repeated after thermal cycling, because a joint that passes when it is new may not pass after a hundred excursions. The relevant assembly process controls and quality checks apply to busbar work as much as to conventional soldering.

FAQ

At what current does a busbar become necessary? There is no fixed figure, but above roughly 50 to 100 amperes on a typical board the copper cross section required becomes difficult to accommodate. The decision should come from a thermal calculation rather than from habit.

Should the joint be bolted or soldered? Bolted joints suit serviceable high current connections and tolerate thermal cycling well when the torque is specified. Soldered or welded joints suit automated production and smaller bars.

Why does a busbar joint get hot over time? Usually because the joint loosens and the contact resistance rises, which increases the local heating and accelerates the loosening. A defined torque and a locking feature prevent the cycle.