Flex Rigid: Preparation, Placement and Process Control

A flex rigid board is two constructions joined into one. Part of the board is a stack of rigid laminates; part of it is a thin flexible layer that can be bent; and the region where the two meet is the transition zone. That zone is where most flex rigid failures begin, because it is the point at which a stiff material ends abruptly and a thin one continues, and every bending cycle concentrates its stress at the boundary.

This article describes what happens at the transition, how the zone is anchored and stiffened, how conductors are routed across it, and how the result is verified.

What The Transition Zone Is

The zone is the length over which the rigid material stops and only the flexible layers continue. Mechanically it is a change of thickness, stiffness and coefficient of expansion; electrically it is the point at which the reference planes that the signal layers were designed against are removed. Both changes happen at the same place, which is why the region is treated as a separate design problem rather than as the end of the rigid board.

Its length is not arbitrary. The rigid material ends at a defined line, and the flexible layer continues for some distance before it is allowed to bend. That unbent length is the transition zone, and it has to be long enough for the adhesive and the stiffener arrangement to be placed and for the bending to start away from any rigid edge.

Mechanical Stress At The Boundary

A bend concentrates at the stiffest point that can resist it, and in a flex rigid board that point is the end of the rigid material. If the bend is allowed to begin immediately at that edge, the copper at the boundary sees the full strain of the bend on every cycle, and the failure appears as a cracked conductor rather than as a broken laminate, because copper is the least ductile element in the stack at that point.

The usual remedy is to separate the two functions. The rigid edge is kept away from the bend by a length of unbent flex, and the bend is placed where the flexible material is uniform and free of stiffeners, plating and vias. The bend radius is set by the number of layers and their thickness, and a commonly used minimum is ten times the total thickness of the flex section, which is larger than the figure for a plain flexible circuit.

Anchoring, Stiffeners And Adhesives

A stiffener is a piece of rigid material bonded to the flexible layer to give it local rigidity, to support a connector or to provide a surface for a component. At the transition zone a stiffener is used to spread the stress rather than to create a hard edge, so it is normally placed with a tapered or a stepped end rather than a square one, and it stops short of the bend area.

The adhesive that bonds the rigid and flexible sections is a design element in its own right. It has to fill the step without flowing into the bend, it has to survive the lamination temperature and it has to remain flexible enough not to crack when the board is bent. A no flow prepreg is commonly used, because a resin that flows will build a fillet at the edge of the rigid section and that fillet becomes the point at which the bend starts.

Routing Across The Boundary

Conductors should cross the boundary perpendicular to it and should be continuous rather than stitched through vias at that point. A trace that runs at a shallow angle to the rigid edge is the worst case, because the stress along the bend line is applied to it over a long distance. Where a signal has to change layers, the via is placed inside the rigid section or inside the flat flexible section, never in the transition zone.

The reference plane has to be considered at the same time, since the removal of the rigid material also removes the plane that the signal was routed against. The signal can cross a gap in its reference if the return path is provided by stitching capacitors or by a continuous plane on the flexible layers. Where the return path is broken, the impedance changes at the boundary and the reflection appears in the channel, and the impedance structures involved are described under microstrip and stripline routing.

The Manufacturing Sequence

The flexible and rigid sections are laminated together in one press cycle, and the sequence decides what the transition looks like. The flexible core is built first with its own cover layers, the rigid sections are added on both sides with no flow prepreg and with the layers that have been routed away where the flex will bend, and the whole assembly is pressed once. The removal of the rigid material is done by routing after lamination, or by leaving those areas free of rigid laminate in the first place.

The choice affects the surface finish, the soldermask and the plating, because the flexible area may need a different treatment from the rigid area. A design that needs a cover layer on the flex and a mask on the rigid has to state both, and the outline and the mechanical constraints of the whole assembly are described under board outline and mounting design and under layer stackup from one to eight layers.

Verification

Verification is mechanical and electrical at the same time. The board is bent through the range it will see in service, over the number of cycles the product requires, and the continuity of the conductors across the transition is monitored during the test rather than only afterwards. A conductor that cracks gradually will show a rising resistance long before it opens, and that change is the earliest warning available.

A section through the transition zone shows the geometry of the layers, the fillet of adhesive and the position of the conductors relative to the end of the rigid material. Taken together with the bend test, it is the evidence that the design and the process produce a zone that will survive the life of the product rather than a first article that happens to pass.

Connectors, Mounting And The Enclosure

A connector that is mounted on the rigid section is the usual reason the flexible section has to move at all, and its position decides how much of the flex has to bend. A connector placed at the edge of the rigid area leaves very little flat length before the bend, while one placed further in leaves more, and the geometry of the enclosure sets the distance between the two ends. The three constraints are resolved together, and a flex rigid design that is drawn without the enclosure is usually the one that has to be changed after the first mechanical build.

The mounting holes and the stiffeners belong to the same problem, because a board that is fixed at both ends and bent in the middle behaves differently from one that is fixed at one end and free at the other. A fixed-fixed arrangement concentrates the stress at both boundaries and needs a larger bend radius for the same travel, while a free end allows the flex to move as a whole. Stating which of the two applies on the drawing prevents the mechanical designer from assuming the more forgiving case.

FAQ

Can components be placed in the transition zone? They should not be. The zone is designed to bend and to absorb stress, and a component placed there adds a stiff point that concentrates the stress and may crack the part as well as the board.

How many bend cycles will a flex rigid board survive? That depends on the radius and on the construction, and the manufacturer will quote a figure for a specific radius. The number is used as a design limit rather than as a guarantee, and the test is repeated on the production stackup.

Should the flex section be shielded? Shielding is applied where the flexible section runs outside the enclosure or near a noise source. A shield on a flexible section has to bend with it, so the material is chosen for flexibility rather than for the best electrical performance.