Bend Radius: Preparation, Placement and Process Control
A flex circuit bent around a sharp corner will crack its copper, and the radius at which it survives is a design parameter rather than a manufacturing preference. The bend radius is set by the thickness of the copper, the thickness of the adhesive and coverlay, and by whether the bend happens once during assembly or a million times in service.
This article explains why flex bends at all, what sets the minimum radius, how coverlay shifts the neutral axis, and how the rules change between a static and a dynamic bend.
Why Flex Needs A Radius
A flex circuit is a laminate of copper foil and polymer film, and it is thin enough that it can be folded. When it is folded, the outer surface of the bend is stretched and the inner surface is compressed. Somewhere between them there is a plane that is neither, and that plane is the neutral axis. The strain in the copper is proportional to its distance from that axis divided by the bend radius, so a small radius and a thick stack both raise the strain, and the copper cracks when the strain exceeds what the foil can take.
The important consequence is that the copper carrying the signal is usually not on the neutral axis. It sits on one side of the polyimide core, so it is either stretched or compressed by every bend. Design work on flex is therefore about placing the copper as close to the neutral axis as possible and giving the bend a radius large enough that the strain stays in a safe band. The same dimensional reasoning that governs a rigid board is described under PCB dimensional stability and expansion.
What Sets The Minimum
The classic rule of thumb quotes a minimum bend radius as a multiple of the total flex thickness, commonly six to ten times for a single sided flex and somewhat more for a double sided or multilayer construction. The multiplier is not arbitrary: it is a restatement of the strain limit, and it changes with the copper thickness. A rolled annealed foil, which has a grain structure elongated in the plane of the sheet, tolerates far more bending than an electrodeposited foil of the same thickness.
Adhesive thickness matters because it moves the copper away from the neutral axis. A construction with two layers of thick adhesive places the copper further from the axis than a thin adhesive-less construction, so it needs a larger radius for the same strain. For a bend that must be as tight as possible, the design uses an adhesive-less laminate, one copper layer, and a thin coverlay; adding layers or adhesive immediately increases the radius the design must allow.
Coverlay And Its Effect On The Stack
The coverlay protects the copper and it also changes the mechanical balance of the section. A coverlay on one side only, and a bare film on the other, puts the copper off centre and increases the strain on the outer surface. This is why a flex is often built with copper and coverlay on both sides even when only one layer carries signal: the build is symmetrical, the neutral axis passes through the middle of the copper, and the bend radius can be smaller.
Coverlay openings also matter. An opening that exposes a pad creates a step in the section, and a bend placed across that step concentrates strain at the edge of the opening. Bends should be located in a region of uniform construction with no pads, no stiffeners and no plated holes. The way protection is applied to a finished assembly is a related but distinct question, covered under conformal coating as board protection.
Static And Dynamic Flexing
A static bend is formed once, during assembly into the product, and then held. The copper is strained once and remains at that strain for life, so the allowable radius is set by the elastic limit and by the risk of a crack during the bend operation itself. A dynamic bend flexes repeatedly, and now fatigue matters: the copper accumulates damage with each cycle, and the number of cycles to failure falls steeply as the strain rises.
Dynamic applications, such as a hinge or a moving head, need a much larger radius, and the design usually allows the flex to form a rolling loop rather than a sharp fold so that the material is never stretched to its limit. The arc length is set by the travel, and the loop is arranged so that the bend moves along the flex rather than concentrating at one point. Where a dynamic flex is unavoidable in a tight space, the copper is reduced in thickness in the bend region and the trace is run perpendicular to the bend line, not across it. Qualification of such a construction is part of the general programme described under multilayer prototype requirements.
Layout Rules Inside The Bend
Traces run through a bend should be perpendicular to the bend line, so that each trace is stretched along its length rather than across its width. A trace that crosses at an angle, and especially one that runs parallel to the bend, is loaded across its width and lifted from the film, and it usually fails before the perpendicular traces around it. Where the mechanical requirement forces a trace to run along the bend, it is kept wide and given as much spacing as the region allows.
Solid copper planes are avoided in a bend region. A plane is stiff, it raises the strain across the whole section, and it does not conform to the curve; a cross hatched or broken plane behaves better. Vias and plated holes are kept out of the bend entirely, because the barrel is a rigid inclusion in a compliant film and it concentrates stress at the plated rim. Where a bend region must carry a generous conductor, it is split into several parallel traces rather than one wide one.
Qualification And Handling
A flex design is qualified with a bend test rather than with a calculation alone. A sample of the same construction is bent around a mandrel of the proposed radius, held at temperature, and inspected for cracks in the copper, for lifted coverlay and for a rise in resistance through the traces. Dynamic designs are cycled to a specified number of bends and the resistance is monitored, with failure defined as a rise above a stated percentage.
Handling is part of the specification as well. A flex that is bent to its limit by an operator during assembly has been tested by the operation rather than by the drawing, and a single over-bend can crack a trace that then passes continuity but fails later. Assembly drawings state the bend radius, the direction of the bend and the position of the bend line, and fixtures are used where the radius cannot be judged by eye. The quality questions that apply to any board apply equally here, and they are gathered under the quality characteristics of a board design.
FAQ
What is the minimum bend radius for a flex circuit? It is normally quoted as a multiple of the total flex thickness, commonly six to ten times for a static single sided bend and considerably more for a dynamic one. Copper type, copper thickness and adhesive thickness all shift the practical figure.
Can a bend cross a via? It should not. A plated barrel is rigid and it concentrates strain at the rim, so vias are placed outside the bend region and the traces are routed around it.
Does a stiffener help in a bend? A stiffener is used to make a region rigid for a connector or a component, and it must stop before the bend begins. A stiffener that extends into the bend region removes the flexibility the design depends on.
