Laminated busbars: construction, benefits and design rules

What a laminated busbar is

A laminated busbar is an assembly of two or more conductors bonded together with a thin insulating layer between them. The conductors are usually flat copper sheets cut to shape. The insulation is a dielectric film or a prepreg that is pressed and cured under heat and pressure, so the stack becomes a single rigid part.

The result is different from a set of separate bars. The spacing between conductors is fixed by the laminate, not by the installer. The current path is short and repeatable, the whole assembly can be fitted as one component and the layers can be arranged so that the outgoing and return currents run close together.

That last point is the main electrical reason to laminate. When two conductors carrying opposite currents are close, their magnetic fields partly cancel, and the loop inductance falls. In a switching circuit, lower inductance means a smaller voltage overshoot when the current changes quickly.

The layer build

A typical laminated busbar is a sandwich: conductor, dielectric, conductor, dielectric, and so on, with the outer surfaces covered by a protective film or a coating. The conductor thickness is chosen for current and fault duty, usually between 0.5 mm and several millimetres. The dielectric thickness is chosen for the voltage class and the insulation co-ordination.

The layers are not arbitrary. The order decides which conductors are adjacent, and therefore how much inductance the stack has and how much capacitance sits between layers. Put the outgoing and return paths next to each other for the lowest loop inductance. Put a layer that carries a sensitive signal away from a layer with a high dv/dt, or add a screen if the coupling matters.

Why low inductance matters

Inductance opposes a change in current. When a semiconductor switches, the current changes in a short time, and the busbar inductance produces a voltage spike in proportion to the current change rate. A lower inductance busbar reduces that spike, which gives the switch more margin against its voltage rating.

The inductance of a pair of conductors depends on the area of the loop they form. Separating the outgoing and return paths by a large distance makes a large loop and a high inductance. Laminating them with a thin dielectric makes a small loop and a low inductance. The effect is strongest when the two paths are wide and close, and when the current is forced to flow in opposite directions.

The same close spacing creates capacitance between the layers. That can be useful as a local decoupling path, but it also couples noise. The designer has to decide whether the capacitance is wanted, and whether a screen layer is needed between a power layer and a control layer.

Insulation and dielectric design

The dielectric has to withstand the working voltage, the transient overvoltage and the pollution that the assembly will see in service. The design follows the insulation co-ordination rules for the equipment: the voltage class, the pollution degree, the altitude and the overvoltage category set the required clearance and creepage.

Thin dielectric gives low inductance, but it also gives a high electric field. Partial discharge becomes the limiting factor. A laminate that passes a short dielectric strength test can still fail in service if it has voids, delamination or a sharp conductor edge that concentrates the field. The manufacturing process, the edge quality and the curing cycle matter as much as the film thickness.

The edges are the weakest path. A conductor that reaches the edge of the laminate leaves only a thin creepage path through air, and a cut edge can expose a sharp burr. Good designs keep the conductor buried, encapsulate the edge or add a margin of dielectric around the perimeter. The hole walls need the same attention, because a plated or cut hole can reduce the distance between layers.

• Specify the working voltage, transient level, pollution degree and altitude.
• Set the dielectric thickness from the insulation co-ordination, not from the lowest inductance target.
• Keep conductor edges buried or encapsulated.
• Control burrs and hole quality, because a sharp edge concentrates the field.
• Ask for a partial discharge test on a sample if the voltage class is high.

Design rules for the stack

Start with the current path. Draw the outgoing and return conductors so that they overlap as much as possible. Keep the loop between a switch and its capacitor short and wide. Every millimetre of separation adds inductance, and every narrow neck adds resistance and heat.

Then set the insulation. The dielectric thickness has to satisfy the voltage class with margin for the manufacturing tolerance and the edge quality. The conductor-to-edge margin has to satisfy the creepage requirement for the pollution degree. If the margin is tight, encapsulation can help, but it has to be qualified for the environment.

Then design the terminations. A laminated busbar usually ends in a set of pads, studs, threaded inserts or press-fit pins. The termination has to carry the current, withstand the bolt torque and keep the layers sealed. A stud that is pressed into a laminate can crack the dielectric if the hole and the interference are wrong, and a bolted joint that crushes the laminate can reduce the insulation thickness.

1. 01

   Map the current

   Decide which conductors carry outgoing and return current and overlap them for the lowest loop area.

2. 02

   Set the voltage

   Define the working voltage, transient, pollution degree and altitude before choosing the dielectric.

3. 03

   Fix the margins

   Set conductor-to-edge and hole-to-edge distances from the creepage and clearance rules.

4. 04

   Design the terminations

   Choose pads, studs or press-fit pins that suit the current, the torque and the laminate.

5. 05

   Plan the mounting

   Support the stack, avoid point loads and keep the assembly flat against the mounting surface.

6. 06

   Define the tests

   State the dielectric, partial discharge, continuity and dimensional checks for the first article.

Terminations and assembly

Most field failures in laminated busbars start at a termination. The joint has to be flat, clean and correctly torqued, and the laminate has to be protected from the clamp load. A washer that is too small can crush the film. A bolt that is too long can bottom out and leave the joint loose. A stud that is not sealed can let moisture into the layer stack.

The assembly order matters too. If the busbar is bolted to a device before the device is aligned, the laminate can be twisted. If the stack is used to pull two components together, the layers take a bending load they were not designed for. The mounting should locate the busbar and let the fasteners clamp it without distorting it.

Inspection and qualification

A production laminated busbar should be checked for continuity, layer-to-layer insulation, dimensions, hole positions, plating and marking. The first article should add a dielectric withstand test, a partial discharge measurement where the voltage class requires it and a cross-section to confirm the layer build.

For a new design, a thermal cycle or a power cycle test is more useful than a single dielectric test. The laminate is a composite of copper and polymer with different expansion rates, and the interface is where a void or a delamination will start. A sample that survives thermal cycling gives more confidence than a sample that passed one short voltage test.

When lamination pays

Lamination pays when the product will be built in volume, the layout repeats and the inductance or space budget is tight. It also pays when the assembly must be tested as one unit before it leaves the works, because the laminate removes a set of loose bars and joints from the build.

It does not pay for a one-off panel or a design that is still changing. The tooling and the qualification cost are real, and a change to the layer order or a termination means a new laminate. For low volume, separate bars or a flexible link are usually cheaper and faster to change.