Flex Circuit 3D Folding And Bend Verification

A flat layout is a poor representation of a folded flex circuit. The board may be drawn as a rectangle on the screen, while in the product it wraps around a battery, bends through a hinge or folds behind a display. The parts that will collide are not visible in two dimensions, and the fold region that will crack is just another area of copper on the screen.

Checking the folded shape therefore belongs in the design flow rather than in the mechanical review that happens after the artwork is released. This article covers how a fold is defined, what limits the bend, and what a three dimensional check can and cannot confirm.

Why A Folded Flex Is Hard To Check On A Flat Layout

The flat layout shows the conductor routing and the component placement, and it hides the relationship between the parts once the circuit is folded. A connector that sits on the far side of a fold may end up facing the wrong way, and a component that is clear of a fold line on the screen may sit exactly where the material has to curve.

The conventional way to check the folded shape has been to export the board outline as a mechanical model and assemble it in a computer aided design tool. That works, but it is slow and it separates the electrical design from the mechanical one, so the check happens late and changes are expensive. Bringing the fold into the layout environment keeps the two views together.

Working inside the layout tool also keeps the fold definition with the board rather than in a separate document. A fold that exists only as a note on a drawing is easily lost when the outline is revised, while a fold that is part of the board data moves with it. On a design that is revised several times before release, that difference is what stops a panel from being built to an old folded geometry.

Defining A Fold Line

A fold is defined by a line drawn on the board, together with an angle and the width of the bend region. The line marks where the material curves, and the width defines the band over which the curve is distributed. A narrow band produces a tight bend and a wide band produces a gentle one, and the two are not interchangeable even at the same angle.

The fold line has to be placed so that the bend region contains no copper, no stiffener and no component. Copper that is bent repeatedly work hardens and eventually cracks, and a stiffener prevents the material from curving at all, which transfers the stress to the conductor beside it. Drawing the fold in the layout makes it possible to check those conditions while the routing can still be changed.

Bend Radius And Material Limits

The bend radius is the parameter that connects the mechanical requirement to the material. A laminate with a rolled annealed copper foil tolerates a much tighter bend than one with an electrodeposited foil, because the grain structure of the rolled foil accommodates the strain. The number of cycles the bend will see matters just as much: a fold that is made once during assembly is a different requirement from a hinge that moves every time the product is used.

The radius should be measured to the inside surface of the bend and stated explicitly, and the value should come from the material supplier rather than from a rule of thumb. Where the space forces a radius tighter than the material allows, the answer is usually a thinner construction, a different copper type, or a change in the mechanical design rather than a change in the routing.

Interference And Component Keepouts

Once the circuit is folded, the clearance between the two halves becomes the constraint. A component that is tall can touch the facing surface, and a connector can press against a housing wall. These are the failures that a flat check cannot see, and they are common in products where the flex wraps around a pack.

A three dimensional check lets the designer rotate the model and measure the gap, and a warning can be raised automatically where a component falls inside a bend region or where a stiffener overlaps a fold. The check should be run with the components modelled at their real height rather than as flat outlines, because a flat outline hides exactly the interference that matters.

Exporting A 3D Model For Mechanical Review

The folded geometry can be exported as a solid model and handed to the mechanical team, which removes the need for them to rebuild the shape from a drawing. The export should include the board outline, the fold lines and the component heights, so that the mechanical model is a realistic representation rather than a plane with a few blocks on it.

Keeping both views in step is the real benefit. When the electrical design changes, the folded model changes with it, and the mechanical team is working from the current geometry rather than from a version that was exported weeks earlier. That is particularly valuable on a rigid flex design, where the rigid sections are the parts that carry the connectors and the flex sections are the parts that move, and where a change in one usually affects the other. The stack up rules that govern those sections are described in layer stack up, and the mechanical envelope constraints follow the same thinking as board outline and mounting design.

What The Check Still Cannot Tell You

A geometric check confirms the shape, not the durability. It cannot tell whether the copper in a bend region will survive the number of cycles the product requires, or whether the adhesive between the coverlay and the base film will delaminate. Those questions are answered by a bend test on a real sample, with the radius and the cycle count matching the application.

It also cannot confirm the electrical behaviour once the circuit is folded. Bending a flex changes the geometry of the traces, and a differential pair routed across a fold will have a different impedance in the bent state than in the flat state. Where a high speed signal crosses a fold, the transition should be measured on a folded sample, and the routing should be arranged so that the pair crosses the bend perpendicular to the fold line. The routing techniques for differential pairs are the starting point for that arrangement.

FAQ

Can a fold line be placed over copper? It can be drawn there, but it should not be. Repeated bending work hardens the copper and eventually cracks it, so the bend region is normally kept free of conductors and plated through holes.

How tight can a bend radius be? It depends on the copper type, the thickness and the number of cycles. A rolled annealed foil allows a much tighter radius than an electrodeposited one, and the supplier figure should be used rather than an assumed rule.

Does a three dimensional check replace a bend test? No. The geometric check confirms that the folded shape fits, while the bend test confirms that the material and the construction survive the cycles the product requires.