Flex PCB Bend Radius: Design Rules and How to Specify It

What the bend radius actually measures

The bend radius is the inner radius of the curve the circuit is formed around - not the outer radius, and not the diameter. Because a thicker stack-up is harder to bend than a thin one, the useful figure is the radius relative to the total thickness, usually written as R/t.

Total thickness includes the base film, copper, adhesive and coverlay - plus anything bonded on top in the bend region. Two designs with the same nominal “0.5 mm radius” can behave very differently if one has a stiffener or an extra layer in the bend zone.

Static vs dynamic bending

These are two different engineering problems and should not share the same rule of thumb:

• Static bend. The circuit is bent once during assembly and then fixed in position - for example folded into a housing. The copper is formed once, so the limit is mainly cracking and delamination at the bend.
• Dynamic bend. The circuit flexes repeatedly in service - hinges, moving heads, wearable joints. Here the copper is fatigue-loaded, so radius, copper type and trace orientation matter much more.

If your application moves even occasionally, treat it as dynamic. Designing it as static is one of the most common causes of field failures.

Common industry starting points

Industry guidance often starts around 10x total thickness for a static bend and 20-25x total thickness for a dynamic bend. These figures are widely used as a first check, not as a specification - the achievable radius depends on the actual stack-up, copper type and process, so confirm it with the fabricator against your drawing.

Use the numbers to challenge your design early: if your stack-up is 0.2 mm thick and the housing forces a 1 mm inner radius, you are at R/t = 5, which is tight for anything other than a single static bend.

What determines how well a circuit survives bending

• Copper type. Rolled annealed copper generally tolerates repeated bending better than electrodeposited copper, which is why dynamic designs often specify it.
• Copper weight. Thicker copper carries more current but is harder to bend; a heavy copper layer raises the minimum radius.
• Layer count and stack-up symmetry. More layers and asymmetric builds shift the neutral axis and concentrate stress.
• Coverlay. A continuous coverlay through the bend protects the traces; coverlay openings in the bend area create weak points.
• Stiffeners. Needed at connectors, but they must stop before the bend zone and transition gradually.
• Plating and finish. Hard gold on fingers is normal; heavy plating inside a bending area is not helpful.

Design rules that protect the bend area

• Run conductors perpendicular to the bend line, so the traces cross the bend at a right angle rather than running along it.
• Keep vias and plated holes out of the bend zone. A hole is a stress riser; route the connection elsewhere.
• Keep stiffeners and coverlay openings out of the bend. A stiffener belongs at the connector, not in the hinge.
• Keep the coverlay continuous through the bend and avoid window edges inside it.
• Round the transitions. Sharp corners and sudden width changes concentrate stress; teardrops and fillets spread it.
• Add bend relief where a stiffened section meets a flexible one, so the stiffness change is gradual.
• Widen traces slightly in the bend area where current allows, and avoid the minimum trace width there.

How to specify the bend on your drawing

A fabricator cannot protect a bend area that is not identified. Put these on the drawing:

• The bend line, marked clearly on the outline
• The bend direction (which way the circuit folds)
• The inner radius, with the unit
• The bend angle
• Static or cyclic, and if cyclic, the expected number of cycles
• Any area that must stay free of stiffener, vias or coverlay openings

If the radius is fixed by the housing, say so - it is a design constraint, not a preference, and it may change which stack-up is appropriate.

What a too-tight bend looks like in failure

• Cracked or fatigued copper - often invisible under the coverlay until the circuit goes open
• Coverlay lift or delamination at the bend edge
• Adhesive squeeze-out and local thickness change
• Intermittent opens that appear only when the assembly is flexed

Because these failures often appear after assembly or in the field rather than at incoming inspection, the bend radius should be reviewed at design stage - not discovered during a reliability test.

Where this fits in the project

For the product range the design will be built on, see our flexible PCB (FPC) page. If you are still at sample stage, the FPC prototype service covers how samples are reviewed. To understand what data a quotation needs, read how to prepare Gerber and stack-up, and for what to check on the finished part see how to verify flexible PCB quality. Send drawings through the contact page.