High Current Connections: Busbars, Terminals and Heavy Copper

Above a certain current, a trace on a board is the wrong conductor. A busbar, a terminal or a heavy copper layer is the answer, and each brings its own mechanical and thermal requirements.

Where the Trace Stops Working

A trace becomes impractical when the copper area needed for the current exceeds what the layout can provide, or when the temperature rise cannot be kept within the limits of the laminate.

The limit is not a single current. A narrow trace on an inner layer with copper on both sides can carry more than a wide trace on an outer layer, and the environment determines the acceptable rise.

Where the current is high and the duty is intermittent, the average heating is lower and the trace can be smaller. The calculation then uses the root mean square current over the duty cycle. Our current capacity notes describe the calculation.

Heavy Copper

A board built with a heavy copper layer, several times the standard weight, carries a much larger current in the same area. The extra copper also spreads heat and stiffens the board.

The process limits change with the copper weight: the minimum feature size increases, the etch tolerance widens and the registration becomes more difficult.

The heavy layer is usually placed in the middle of the stack where it is laminated under pressure, and the outer layers carry the fine routing. Our plating thickness notes describe how the copper is specified and measured.

Busbars and Their Attachment

A busbar carries the current off the board and provides a large cross section in a small volume. It is attached by soldering, by a press fit or by a mechanical fastener.

A soldered busbar is attached with a large pad and a generous solder fillet. The thermal mass of the busbar makes soldering difficult, and the joint requires either a high power iron or a reflow with a long soak.

The joint must also survive the mechanical load of the busbar, which acts as a lever on the pad. A busbar should be supported by the enclosure or by a bracket rather than relying on the solder. Our component reliability notes describe how the load is estimated.

Terminals and Screw Connections

A screw terminal on the board carries the current through a threaded stud or a clamping bracket, and the board provides the insulation and the location.

The terminal must be held so that the torque of the screw is taken by the board or by a bracket. A terminal held only by its solder joints will crack the joints when the screw is tightened.

The pad under the terminal must carry the current from the trace to the terminal, and its area must be sufficient for the contact resistance and for the heat. The plating on the pad must be compatible with the terminal material to avoid galvanic corrosion.

Thermal Behaviour of a High Current Path

The heat is generated in the conductor and in every joint along the path. A joint with a higher resistance than the conductor becomes the hottest point and eventually the failure point.

The temperature of the connections should be measured at the maximum current, since a joint that looks adequate can be the limiting element. A thermal camera on an energised board shows the joints as hot spots.

The measurement should be made at the maximum ambient and at the maximum current simultaneously, since the two effects add. Our thermal design notes describe the copper distribution around the connection.

Verification and Testing

The verification is a temperature rise test at the maximum current, with the rise measured on the conductor and on each joint.

The contact resistance of a screw terminal should be measured before and after the temperature test, and after a thermal cycling test, since the clamping force relaxes with time and temperature.

The mechanical test is a torque test on the terminal and a pull test on the busbar, both of which should not damage the joints.

Practical Rules

Choose the conductor from the current and the allowable rise, and check the joints as carefully as the conductor. A chain is limited by its weakest joint.

Support any heavy conductor mechanically so that the solder joints carry no bending load. Provide the pad area that the joint needs for both the current and the heat.

Measure the temperature of the finished assembly at the maximum load, and record it with the configuration so that a later change can be compared. Our thermal design notes describe how the measurements are used.

Process Control and Verification

On a design of this kind, heavy copper is the item that decides how the rest of the board is arranged. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

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.

Process Control and Verification

On a design of this kind, heavy copper is the item that decides how the rest of the board is arranged. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

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.

Process Control and Verification

On a design of this kind, heavy copper is the item that decides how the rest of the board is arranged. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

FAQ

Can a busbar be soldered by hand? It can, and it requires a high power iron and a large pad. The thermal mass makes an incomplete joint a real risk.

How much copper is enough for a hundred amps? The answer is a temperature rise for a given cross section and environment, and the calculation should be made rather than estimated from a rule of thumb.

What does gopcb provide for high current assemblies? We provide conductor sizing with the environment accounted for, heavy copper stack-ups with their process limits, busbar and terminal attachment design with mechanical support, temperature measurements at the maximum load and ambient, and torque and pull tests on the mechanical connections.