Laminated Busbar: Structure, Materials and Aluminium Conductor Layers

What is a laminated busbar?

A laminated busbar replaces a bundle of cables or separate bars with a single engineered sandwich: conductor, insulation, conductor, insulation, sealed at the edges and fitted with tabs, holes or press-in terminals where devices and capacitors connect.

The reason for building it this way is inductance. Every conductor loop stores magnetic energy, and when a power transistor (IGBT, SiC or GaN MOSFET) switches off a large current in tens of nanoseconds, that stored energy appears as a voltage spike: V = L × di/dt. Switching 1,000 A in 100 ns gives di/dt of 10 kA/µs; with 20 nH of loop inductance the spike is about 200 V, with 5 nH it falls to about 50 V. A smaller spike lets the designer use devices closer to their voltage rating, run faster switching edges and reduce snubber components.

Putting the DC+ and DC− plates face to face, separated only by insulation film, makes currents flow in opposite directions over a wide area. Their magnetic fields largely cancel, which is why laminated busbars are standard in traction inverters, wind and solar converters, industrial drives, UPS systems, rail propulsion and battery energy storage.

Layers of a laminated busbar

Most designs follow the same build-up, whatever the conductor metal. The number of layers depends on the circuit: a two-level DC link needs two conductors, a three-level converter usually needs three or four, and multi-phase assemblies can exceed six.

Inductance of two parallel plates is roughly proportional to the plate separation divided by the plate width (L ≈ μ₀ × length × separation / width). That simple relationship explains the design rules: keep the insulation as thin as voltage and partial-discharge limits allow, make the plates wide, and keep the DC+ and DC− plates overlapping all the way to the device terminals. As an example, a 300 mm long, 100 mm wide plate pair separated by 0.25 mm has a calculated loop inductance of about 1 nH before terminal and via effects are added; real assemblies land higher because of holes, cut-outs and terminal geometry.

Aluminium or copper conductor layers?

Copper has been the default because it packs the most conductivity into the smallest volume. Aluminium has become common in battery packs, energy storage systems and EV drive units because it removes about half the conductor mass for the same resistance, costs less per ampere and is easy to cut, punch and form.

Conductivity minimums: 1350 bus conductor per ASTM B236; 6101 tempers per ASTM B317 (T6 55 %, T61 57 %, T63 56 %, T64 59.5 %, T65 56.5 % IACS). Copper per ASTM B187 / IACS definition. Mass ratios are calculated from conductivity and density.

The practical consequence is that an aluminium layer is thicker or wider than the copper layer it replaces. Because loop inductance depends on separation and width rather than conductor thickness, a wider aluminium plate can actually help inductance; a thicker one increases the overall stack height slightly. Where space is very tight and currents are high, copper still wins; where weight and cost dominate, as in most battery and storage systems, aluminium is usually the better engineering choice.

Choosing the insulation system

Insulation selection balances three things: dielectric strength (and partial-discharge inception voltage), operating temperature, and thickness, because every extra micrometre raises inductance. Voids, wrinkles and sharp plate edges are where failures start, so the lamination process and edge design matter as much as the film itself.

A widely quoted design rule in the industry is about 1 mm of rigid insulation per kilovolt between sub-assemblies, but the governing requirement is the partial-discharge limit: the electric field (voltage divided by distance) at the worst point must stay below the level at which discharges start. Designs for altitude, such as aircraft or high-altitude solar sites, need extra margin because partial-discharge inception voltage falls as air pressure drops. Creepage and clearance at exposed edges follow the insulation-coordination rules in IEC 60664-1 for the relevant pollution degree and overvoltage category.

Design rules engineers use

1. Size the conductor for temperature riseStart from RMS current and allowable temperature rise (often 30 – 50 K above ambient). Aluminium plates need about 1.6 × the cross-section of copper for the same DC resistance; at high frequencies, skin and proximity effects favour wide, thin plates.
2. Maximise overlapKeep DC+ and DC− plates overlapping right up to the device and capacitor terminals. Every gap, slot or detour adds inductance.
3. Minimise insulation thickness safelyUse the thinnest film that meets dielectric, partial-discharge and lifetime targets, with margin for altitude and temperature.
4. Design the edgesUse film overhang, edge fill or encapsulation to achieve creepage. Radius the plate corners and deburr all holes; sharp edges concentrate electric field.
5. Plan terminals and platingAluminium forms an oxide film instantly, so contact areas are normally tin- or nickel-plated, or fitted with bimetallic inserts where they bolt to copper.
6. Control joint pressureAluminium creeps under sustained bolt load more than copper. Use larger washers or conical spring washers and the torque values specified for aluminium joints.
7. Model and verifyExtract inductance with 3D field simulation, then measure it on the prototype with a double-pulse test before releasing the design.

Tests and standards for laminated busbars

Laminated busbars are tested as electrical insulation systems and as conductors. A typical production and qualification programme looks like this:

For the aluminium layers, the most useful document is the mill test certificate. It should show the alloy and temper, chemical analysis and, for conductor grades, the measured electrical conductivity or resistivity. Ask for conductivity to be reported in % IACS or MS/m so it can be compared directly with your thermal model.

Aluminium conductor stock for laminated busbars

Metline supplies the aluminium that goes into the conductor layers; the lamination, insulation, plating and final electrical testing are carried out by the busbar maker. Typical requests we handle:

• EV traction inverters: DC-link busbars between capacitor bank and power modules
• Battery energy storage: rack and string busbars where weight and cost matter
• Solar and wind converters: multi-level DC-link assemblies
• Rail traction and industrial drives: high-current, vibration-tolerant power stages
• UPS and data-centre power: compact low-inductance distribution

Common mistakes when specifying aluminium layers

• Copying copper thicknesses: an aluminium plate of the same size has about 1.6 × the resistance. Re-run the thermal calculation rather than swapping the metal.
• Ignoring the oxide film: bare aluminium contact faces without plating or proper joint preparation lead to rising contact resistance over time.
• Choosing a structural alloy by habit: 6061 or 6082 are strong but have noticeably lower conductivity than 6101 or 1350. Use conductor grades unless strength is the governing requirement.
• Leaving burrs on punched holes: burrs pierce thin insulation films and create partial-discharge sites. Specify deburring on the drawing.
• Forming hard tempers too tightly: 1xxx grades in H14/H18 or 6101-T6 crack on tight bends. Order a softer temper (O or H111 for 1xxx grades; T61 or T64 rather than T6 for 6101) for parts with tight tabs, or allow a larger bend radius.
• Overlooking galvanic contact: bolting bare aluminium directly to copper in a humid environment invites corrosion; use plated surfaces or bimetallic transition pieces.

Related busbar and conductor guides

These pages cover other busbar types and the aluminium conductor grades used in them:

Laminated busbar: frequently asked questions