Rigid Flex Stackup Design Guide

A rigid flex board carries the electronics on rigid sections and connects them with a flexible region that folds in three dimensions. It removes connectors and cables from a product, and it brings a set of mechanical rules that a conventional board does not have.

What the Construction Contains

The flexible layer is a thin polyimide film with copper on one or both sides. The rigid sections are built up around it with prepreg and copper, so the same base material serves both parts of the board.

The flexible region is unprotected by the rigid laminate, so it needs a coverlay: a layer of polyimide film with an adhesive that is laminated over the copper. The coverlay is what protects the conductors in the bend area and it is part of the mechanical design.

A stiffener is often added where a connector or a component sits close to a bend. The stiffener prevents the flex from bending at the wrong place and it supports the solder joint.

stackup Choices

A single layer flex with copper on one side bends most easily and has the thinnest profile. Copper on both sides doubles the conductor count and reduces the flexibility unless the copper is kept thin and the traces are run in the neutral axis.

A multi layer flex with an adhesive between layers is more common in a rigid flex because the rigid part needs several layers. The adhesive thickness contributes to the overall thickness and to the bend behaviour.

Adhesiveless constructions use a cast film that is directly bonded to the copper, which gives a thinner stackup and a better dimensional stability. The cost is higher and the availability is narrower.

bend radius and Mechanics

Bend radius is the inside radius of the curve, and the minimum value is set by the thickness and by the number of bend cycles the product will see. A static bend can be much tighter than one that flexes repeatedly.

The practical rule for a static bend is a radius of about ten times the total thickness, and for a dynamic bend many times more. The copper in the bend should be rolled annealed rather than electrodeposited, because its grain structure tolerates flexing far better.

Traces crossing a bend should run perpendicular to the bend line, so that each conductor is bent along its length rather than across its width. A wide trace crossing a bend concentrates the strain at its edges and cracks first.

adhesive and coverlay Selection

The coverlay adhesive has to bond to the polyimide and survive the soldering temperature of the assembly. An acrylic adhesive is the common choice, and an epoxy based one is used where the temperature is higher.

The coverlay window is the opening through which the rigid sections and the pads are exposed. Its edges must not fall inside the bend area, because the adhesive edge is a stress concentration and a place where delamination starts.

The bond between the coverlay and the copper is what keeps the conductor in place during bending. A poor bond shows as a lift at the edge of the window, and the effect of a lifted coverlay in a dynamic bend is a broken trace within a few thousand cycles.

rigid Flex Interface and Layer Transition

The place where the flex enters the rigid section is the most critical area of the whole stackup. The rigid laminate ends there, and the copper that runs from the flex into the rigid part must be free to move without being pinched by the laminate edge.

The coverlay usually ends some distance before the rigid material begins, and the gap between them is a stress relief. That gap must be wide enough that the bend does not start inside the rigid section, and narrow enough that the exposed copper is still supported.

The traces that cross the transition should be continuous and unbranched, and the copper should not be thinned by a plating process that is uneven at the edge. A board that fails at the transition usually cracks where the coverlay ends.

stiffener and Support Placement

A stiffener is a piece of FR4 or polyimide that is bonded to the flex in a region that must not bend. It is used under a connector, under a component, and around a hole that takes a screw or a locating pin.

The stiffener must be placed so that its edge does not coincide with the bend area, for the same reason as the coverlay: the edge is a rigid step and the copper will fatigue against it. Keep a clear distance of at least the bend radius.

Where a stiffener meets the rigid section, the two are often at different heights. The assembly fixture and the housing must account for that step, otherwise the board is stressed when the product is closed.

impedance and Electrical Consistency

A flexible material has a different dielectric constant, as covered in the laminate guide, and a different loss than an FR4 laminate, so a controlled impedance line changes value as it crosses from the rigid part into the flex. The design has to treat the transition as a discontinuity.

The practical approach is to simulate or measure the flex section separately and to keep the impedance sensitive nets out of the bend area. A line that must cross the transition should be widened or narrowed by calculation, not by trial and error.

The copper thickness on a flex layer is usually one ounce or less, which raises the resistance of a power trace. A supply that runs through the flex has to be sized with that higher resistance, and the drop across a long flex trace is often the real limit.

manufacturing and Assembly Notes

The flex material is supplied in panels and the rigid sections are laminated around it in a press cycle that is longer than a conventional multilayer. The additional heat history has to be considered when the number of laminations is chosen.

Process planning such as multilayer flex processing has to allow the flexible region to hang free: a flex area that is clamped flat during assembly is a source of damage. The carrier and the breakaway tabs should support the rigid sections and leave the bend area untouched.

A component placed on the flex must be kept away from the bend, because even a small part stiffens the local area and moves the bend line. If a part is unavoidable there, the bend radius has to be increased and the copper reinforced.

An inspection after assembly should include a bend test on a sample, since the failure mode is a fatigue crack that an optical check will not reveal. The test can be as simple as flexing the sample to the specified radius for the specified number of cycles.

Material and Process Selection

Polyimide is the standard base film because it keeps its mechanical properties at soldering temperature and it has a high dielectric strength. Other films exist for cost or for a specific dielectric behaviour, but the choice affects the whole stackup.

The bond ply or adhesive that joins the layers can be acrylic or epoxy based, and the two have different temperature limits and different flow. A high flow adhesive can fill a blind via; a low flow one may leave a void.

A flex that is fabricated with an adhesiveless bond ply is thinner and more dimensionally stable, and it is the usual choice where the layer count is high. The trade is a higher material cost and a longer lead time.

Whichever material is chosen, the fabrication notes have to state the bend radius, the number of bend cycles, and the direction of the bend, because the shop will build the stackup to those numbers. A drawing that omits them will be built to a generic standard.

Design reviews should include the mechanical engineer as well as the electrical one, because the failing mode of a rigid flex board is usually mechanical rather than electrical. The two disciplines have to agree on where the board is allowed to bend before the artwork is released.

FAQ

How thick should the flexible region be? As thin as the electrical requirements allow. The bend radius is a multiple of the total thickness, so every layer added to the flex lifts the radius or shortens the flex life.

Can a rigid flex board be reflow soldered like a normal board? Yes, provided the flex area is supported and the material is rated for the temperature. The coverlay and the adhesive are the parts that set the limit.

What causes a crack at the flex to rigid transition? Almost always a rigid edge, a coverlay edge, or a stiffener edge that sits inside the bend area. Moving the edge away or increasing the bend radius removes the failure.