Braided copper vs laminated copper: movement and current

Two answers to the same problem

A busbar expands when it warms and contracts when it cools. A door swings, a drawer slides, a plug-in unit is withdrawn. In each case the current has to cross a gap that is not rigid, and a solid bar would load the joints at both ends.

Two conductor families are used for that job. The first is braided copper: many fine wires woven into a flat or tubular rope. The second is laminated copper: a stack of thin copper foils or sheets, consolidated into a solid block at each end and left free in the middle. Both are flexible connectors, and both are terminated with pads, ferrules or welded blocks.

The word laminated needs care. Here it means a stack of copper foils, not an epoxy-cast rigid busbar with conductors bonded between insulation. A cast laminated bar is rigid, and its design rules belong with busbar, not with flexible links.

How each one is built

A braid starts as drawn copper wire, often between 0.10 mm and 0.30 mm in diameter. Groups of wires are bunched into ends, the ends are carried on a braiding machine, and the carriers move in opposing circles to interlace the strands. The result can be a flat tape or a tube, depending on the mandrel and the take-off.

A laminated link starts as copper foil or thin sheet, commonly between 0.05 mm and 0.5 mm thick. The foil is slit to width, stacked to the required copper section and consolidated at the ends by welding, pressing or sintering. Holes and pad shapes are usually punched or machined after consolidation.

Current path and copper density

The copper area of a braid is the number of wires multiplied by the area of one wire. A construction of 24 carriers, 8 ends per carrier and 0.20 mm wire contains 192 wires and about 6.0 mm² of copper. That figure is arithmetic, not a rating, and the envelope around it is much larger because the weave is mostly air.

Inside a braid, strand to strand contact is a series of small contact points. The path is longer than the straight length, and current sharing between carriers is uneven near the consolidated end. The result is a conductor that runs warmer than a solid bar of the same copper area, so a derating factor is normal.

A laminated link has the opposite profile. The foils touch over their full face at the consolidated ends, the current path is short and direct, and sharing between foils is set by geometry rather than by chance contact. For a given envelope, a laminate usually carries more current and runs cooler, provided the termination is fully fused.

Movement, bend radius and fatigue

A braid distributes a bend across hundreds of wires. Each strand sees a small strain, and the strands slide to share the movement. That is why a braid can be routed around a corner, bent in two planes or twisted through a small angle without concentrating damage in one place.

A laminate bends as a pack. The outer foil sees a strain close to the foil thickness divided by twice the bend radius. For illustration, a 0.2 mm foil on a 10 mm inside radius sees about 1 per cent strain, and a 0.5 mm foil on the same radius sees about 2.5 per cent. The foils also slide over each other, so the free length has to allow it.

The practical limit follows. A laminate is good at in-plane bending with a generous radius and a modest stroke. It is poor at torsion, because twisting separates the foils and loads the edges. A braid is the better choice where the movement is multi-axis, or where the bend radius is tight relative to the section.

Terminations and the joint

A braid can be terminated with a crimped ferrule, a welded or sintered pad, or a lug that is crimped first and bolted second. The ferrule has to capture every strand, because a strand that escapes the crimp carries no current and can work loose. The transition from the clamp to the free weave is a stress point and should be supported.

A laminate is terminated by consolidating the foil ends into a block. If the outer foils are not fully fused, they carry less current and the middle of the stack runs hotter. A block that is too short concentrates the current at the weld line and shortens the fatigue life at the same time.

In both cases the joint is the weak point. A flexible link often has two joints in series, and each one needs its own torque, surface preparation and resistance check. A millivolt drop test tells you more than a visual check of the conductor.

Cost, tooling and lead time

Braid is slow to make per kilogram. Wire drawing, bunching and braiding are sequential operations, and the braider produces length rather than parts. Tooling is modest: mandrels, take-off guides and crimp or weld tooling. That makes braid economical at low and medium volume, and easy to change when the length or the pad shape moves.

A laminate spends its cost in foil slitting, stacking, fixture design and welding. The pad is usually punched, which needs a tool. At low volume the set-up dominates, and a design change to the pad or the hole pattern can mean a new tool. At high volume, with a fixed geometry, the laminate spreads that cost and becomes competitive.

Material yield also differs. A braid produces little offcut because it is made to length. A laminate cuts pads from a stack and can leave skeleton scrap. The scrap rate belongs in the should-cost, not in a footnote.

How to choose

Start with the movement, not the current. Write down the stroke, the direction, the number of cycles and the space available for the loop. If the link has to twist, or the movement is in more than one plane, braid is usually the safer default. If the movement is small and in one plane, a laminate will give more current in less space.

Then check the termination. A laminate needs a consolidated block with enough length for the current and the fatigue duty. A braid needs a ferrule or weld that captures every strand. If the joint has to be removable, both products end in a bolted lug, and the bolt interface becomes the shared constraint.

Finally, price the whole assembly at the real volume. A braid with a crimped ferrule and a laminate with a punched pad are not compared by conductor cost alone. Include tooling, scrap, assembly labour and test.