Flex Rigid Transition: Key Checks Before Release

A flex rigid transition is the region where the flexible part of a board meets the rigid part. The laminate ends, the coverlay ends, the stiffener usually ends as well, and three mechanical discontinuities sit within a few millimetres of each other. The transition is where most rigid flex boards fail, and the cause is almost always a rigid edge that sits inside the bend area.

What the Transition Contains

The rigid section is a stack of prepreg and copper built around the flex layer, while the flexible section is the bare flex with a coverlay on both faces. The two meet at the edge of the rigid laminate.

The coverlay is the protective film over the flex conductors, and its edge is a step in the surface. The step has a thickness of tens of microns and it is a stress concentration for the copper underneath.

The stiffener, where it is used, is another edge with a much larger step. It is bonded to the flex to stop it bending at the wrong place, and its own edge becomes a new bending line.

The three features have to be positioned relative to each other and to the bend, and that positioning is the whole of the transition design.

Where the Bend Is Allowed

The bend has to happen in the flexible section, away from every rigid edge. The distance from the edge of the rigid laminate to the start of the bend is the clearance that keeps the transition out of the strained region.

A practical rule is to keep the bend at least one bend radius away from the rigid edge and from the coverlay edge. Anything closer puts the copper at the transition into the strained area.

The bend radius itself is measured on the inside of the curve, and the minimum value comes from the total thickness of the flexible section. A static bend can be tighter than a dynamic one by a large factor.

Where the transition is also the place where the board folds into a housing, the mechanical design has to provide the support that defines the bend line rather than leaving it to the flex.

Trace Routing at the Transition

The traces that cross the transition should run perpendicular to the rigid edge, so that each conductor is bent along its length rather than across its width.

The traces should also be continuous and unbranched through the transition, and the copper should not be thinned by an uneven plating at the edge. A plated through hole placed at the transition adds a rigid point in the bend path.

A wide trace crossing a bend concentrates the strain at its edges, so a supply that has to cross the transition is often split into several narrower traces that are spread across the flex.

The copper type matters as well, since a rolled annealed foil tolerates flexing far better than an electrodeposited one. The choice belongs to the stackup rather than to the layout, and the comparison in the flex materials guide covers the options.

The panelisation contributes as well, because a flex section that is clamped flat during assembly is a source of damage. The carrier and the tabs should support the rigid sections and leave the bend area untouched, as described in the multilayer flex process material.

Coverlay and Stiffener Placement

The coverlay ends short of the rigid laminate, and the gap between them is a stress relief. The gap has to be wide enough that the bend does not start inside the rigid section and narrow enough that the exposed copper is still supported.

The coverlay window, which is the opening over the pads, must not fall inside the bend area. The adhesive edge of the window is a delamination site, and a lifted coverlay cracks the trace within a few thousand cycles.

A stiffener is placed under a connector or a component and its edge becomes a bend line. It should be positioned so that the intended bend is elsewhere, and its edge should be at least a bend radius away from the flex area that moves.

Where a stiffener and the rigid section are close together, the two steps are often at different heights, and the assembly fixture has to account for that or the board is stressed when the housing is closed.

Strain Relief and Support

A transition that is clamped or bonded to a housing becomes a supported edge, and the movement is transferred to the flex beyond it. That is usually desirable, because it defines where the bend occurs.

A transition that is free to move collects the strain at the coverlay edge, which is the weakest point. A small adhesive bead across the transition is a common way to spread the load.

The support should be continuous across the width of the flex, because a point support creates a local bend that the traces cannot tolerate. A clamp with a soft pad is better than a screw through a hole.

The coating applied after assembly can also help mechanically, since it holds the coverlay edge and the exposed copper in place.

Design and Verification

The transition should be drawn as a mechanical feature with dimensions for the rigid edge, the coverlay edge and the bend line. A drawing that shows only the outline leaves the three positions to the shop.

The fabrication notes should state the bend radius, the number of cycles and the direction of the bend, because the shop builds the stackup to those numbers. A note that omits them is built to a generic default.

The verification is a bend test on a sample, and the failure mode is a fatigue crack that an optical inspection will not reveal. The test is a flexing to the specified radius for the specified number of cycles.

A cross section of the transition is worth as much as the bend test, because it shows the coverlay end, the laminate edge and the copper in a single image. The cross section should be photographed and kept with the quality record.

Practical Rules

Keep the bend away from every rigid edge by at least one bend radius, and check the distance on the drawing rather than in the layout tool.

Run the traces perpendicular to the edge, keep them unbranched, and split a wide conductor into several narrow ones where it has to cross.

End the coverlay and the stiffener at a defined distance from the bend, and give both a tolerance on the drawing.

Verify with a bend test and a cross section, and record the result with the design so that the next revision starts from the same numbers.

FAQ

Why does a rigid flex board fail at the transition? Because the rigid edge, the coverlay edge and the stiffener edge are all stress concentrations. A crack starts where one of them falls inside the bend area.

How far should the bend be from the rigid edge? At least one bend radius, and more where the flex cycles. The clearance keeps the strained region in the flexible part of the board.

Should traces cross the transition at an angle? They should cross perpendicular to the edge, so that each trace is bent along its length rather than across its width.