LED Strip FPC: Voltage Drop, Pitch and Bend Design

A flexible circuit behind an LED strip is a power distribution board with a lighting function. It carries a constant current along its length, feeds a large number of light emitting diodes at a regular pitch, and then has to be bent into the shape of the product without changing the current that reaches the last device.

That combination of current, density and flexibility produces its own set of design rules, and they are easiest to follow when the strip is treated as a distributed power network rather than as a long thin circuit board.

What the Strip Has to Do

The electrical requirement is uniform brightness. Each device draws a defined current at a defined forward voltage, and the copper has to deliver that current with a voltage drop small enough that the last device on the strip is indistinguishable from the first. That is a copper cross section problem along the whole length.

The mechanical requirement is to survive bending, both during installation and, in some products, repeatedly in service. The copper must be ductile, the adhesive must tolerate the bend and the coverlay must not crack at the bend line.

Voltage Drop and Copper Width

The current in the strip is highest at the feed end and falls progressively as devices are tapped off. The trace therefore does not need a constant width: it can be tapered, with the widest section at the connector and narrower sections further along, which saves space and reduces the bending stiffness.

The calculation is straightforward but worth doing properly. A strip carrying 2 A with a 0.2 V allowance over a metre needs a copper cross section that a thin flex layer cannot always provide on one layer, so the feed is often duplicated on both sides of the strip and stitched at intervals so that the return current has a short path.

Device Pitch and Thermal Load

Devices are placed at a regular pitch, and the pitch decides the local heat density. A dense strip runs hotter, which reduces the forward voltage of the devices and changes their brightness, so the thermal path through the flex and into the mounting surface is part of the electrical design.

Copper beside each device spreads heat and also provides the thermal path to the substrate behind the strip. Where the strip is bonded to a metal channel, that bond line becomes the dominant thermal resistance, and its quality matters more than the copper area by that point.

Bend Areas and Direction

The strip is often bent where it leaves the connector or turns a corner in the luminaire. Conductors should cross the bend perpendicular to the bend line, the copper should be a rolled annealed foil rather than an electrodeposited one, and the coverlay over the bend should be continuous with no opening.

Where the strip has to fold, the bend radius is defined with the same margin as any dynamic flex. A fold that is tighter than the specification will show as a crack in the copper after a few installation cycles rather than immediately.

Connectors and Terminations

Connections to a LED strip FPC are made with a board to wire connector, a solder pad or a crimped terminal, and each brings its own mechanical load. The pad area and the stiffener behind it have to be sized for that load, because a strip is thin and a pull on the cable is transferred directly to the pads.

Where a strip is cut to length in the field, the termination point is usually marked with a set of pads at a regular interval. Those pads have to be designed for a hand soldering operation, with enough copper to dissipate the iron heat and a mask opening that is large enough to work in.

Coverlay, Mask and Reflectivity

The coverlay over a strip is often white, because a white surface reflects the light that would otherwise be lost behind the device. That changes the process, since a white coverlay is thicker and its optical properties depend on the material rather than on the copper beneath it.

The mask openings for the devices are defined by the placement tolerance and by the solder paste volume. An opening that is too small shadows the deposit and an opening that is too large leaves the mask edge exposed to the light and to the cleaning process.

Assembly and Test

The strip is assembled in a panel and separated afterwards, or supplied on a reel for automated placement. Both routes need fiducials and a defined panel design, and the separation method has to avoid bending the strip at the device positions.

Test is usually functional at the strip level: current at a defined voltage, brightness uniformity across the length and continuity of the feed. A strip that is electrically correct but mechanically marginal will pass that test, which is why the bend and the adhesion tests are performed on samples as part of the qualification rather than on every unit.

Design Rules and Layout Checklist

Size the copper from the voltage drop budget, taper the feed where the current falls, duplicate the supply on both sides for higher currents, keep conductors perpendicular to any bend, and give the termination pads enough copper for the assembly method and enough stiffener for the mechanical load.

gopcb produces flexible circuits with rolled annealed copper, white and black coverlays, stiffeners and the panel designs that strip assembly requires, and can confirm the voltage drop and bend performance of a proposed layout before tooling.

Handling, Packaging and Field Installation

A flexible strip is easily damaged before it is installed, and the damage is usually invisible. A crease across the copper may not break it immediately, but it creates a stress concentration that fails after a few thermal cycles in the luminaire. Consequently the strip is supplied on a carrier or in a reel with a defined minimum bend radius for the packaging, and the installation instructions repeat that limit rather than assuming the installer will observe it.

The adhesive system is part of the same problem. A pressure sensitive adhesive holds well on a clean, flat surface and poorly on a dusty one, so the surface preparation and the application pressure belong in the instructions. Where the strip is bonded into an aluminium channel, the bond line also transfers heat, and a strip that is only partly adhered will run hotter at the unadhered sections and change colour across the length.

Reliability Testing for Strip Products

Qualification of a strip assembly typically includes a current and brightness measurement before and after thermal cycling, a humidity soak, and a bend test at the specified radius repeated to the expected installation count. The measurement that matters is the change in forward voltage and in brightness at the end of the strip, because that is where the copper loss and the thermal history show first.

A cut-and-terminate test is worth adding for a product that is installed in the field. It confirms that the termination pads can be soldered by hand without damaging the adjacent coverlay, that the cut line produces a clean edge, and that the strip still meets its electrical specification afterwards. Finding that problem in the qualification laboratory is considerably cheaper than finding it on a project site.

FAQ

Why does the end of a long strip look dimmer? Because the copper resistance drops the voltage along the feed. Widening the copper, feeding from both ends or reducing the current per device are the available answers.

Can a strip be cut anywhere? Only where the layout provides a termination point. Cutting between device positions without pads leaves no way to connect the feed.

Does the coverlay colour affect performance? A white coverlay reflects light that would otherwise be absorbed, which improves the apparent efficiency, but it also changes the thickness and the adhesion behaviour that the process has to accommodate.

Related reading: trace width and current calculation, FPC stiffener comparison, conformal coating and board protection, and high temperature PCB materials.