Rigid Flex Prototype PCB

Why a Prototype Stage Matters More Here

A rigid flex board combines rigid sections that carry the components with flexible sections that fold, bend or twist, all in one laminated structure. The benefits are real: connectors are removed, the assembly can be folded into a shape a flat board cannot reach, the signal path is shorter and the mechanical reliability of the joints improves. The cost is that the design is more complex than either a rigid or a flexible board on its own, and the interactions between the materials only appear once a board has been built.

That is why the prototype stage is more important for rigid flex than for almost any other construction. The questions that matter, how the bend behaves, whether the layers stay bonded and whether the impedance holds across the transition, cannot be answered from a drawing.

What the Structure Contains

The rigid sections use a conventional laminate, usually FR-4, and provide the mechanical strength and the area for the components, the connectors and the heavier devices. The flexible sections use polyimide film, which tolerates heat and repeated bending and provides the electrical connection between the rigid regions. Copper foil carries the conductors, and adhesive and coverlay layers bond the stack together and protect the exposed conductors.

An adhesion promotion layer or a no flow prepreg is used at the transition so that the rigid and flexible materials bond reliably, and the transition itself is designed so that the bending stress is spread rather than concentrated on a single line. Our notes on PCB manufacturing describe the lamination involved.

Advantages

Miniaturisation is the first benefit, because removing connectors and cables reduces both the volume and the weight of the assembly. Reliability improves as a consequence, because every connector removed is a potential failure point and every solder joint not required is a joint that cannot crack. Signal performance improves because the electrical path between the rigid sections is short and continuous, which matters at high frequency.

Design flexibility follows: the assembly can be arranged in three dimensions, which allows a compact product to be built around the available space rather than forcing the space to suit a flat board.

Design Rules That Matter

The bend radius has to be large enough that the conductors on the outside of the bend are not strained beyond their limit, which in practice means keeping it well above the minimum the material allows and orienting the conductors perpendicular to the bend line so that they are loaded along their strongest direction. Vias and components should be kept out of the bend area entirely, because both concentrate stress.

The stack-up has to balance the rigid and flexible regions so that the finished board does not warp, and the thickness of the flexible section has to be consistent so that the bend behaves predictably. Routing should avoid abrupt changes of width and sharp corners in the flexible region, and the polyimide should be of a quality that retains its flexibility and its thermal performance over the life of the product. Our notes on PCB design and layout cover the planning.

The Prototype Build

Fabrication proceeds through design and engineering planning, which fixes the rigid and flexible regions in the data, then material preparation, lamination under heat and pressure, drilling and plating to connect the layers through the rigid and flexible areas, etching to form the conductors, and the application of the coverlay that protects the flexible sections. The transitions between the rigid and flexible regions are formed by removing material from the rigid layers at the bend, which is one of the steps that most often reveals a design problem.

The prototype is then assembled and its behaviour checked. This is where the mechanical questions are answered, because the folding can be performed and the result inspected, and where the electrical questions are answered by measuring the impedance and the continuity across the transition.

Cost

A rigid flex prototype is considerably more expensive per board than a rigid prototype, because the material set is more costly and the process involves more steps with lower yields. The price depends on the layer count, the quality of the polyimide, the complexity of the shape and the tolerances, and it falls as the quantity rises, though rigid flex volumes are generally lower than rigid volumes.

The prototype price should be judged against the saving it produces in the product, which is usually the reduction in connectors, cables and assembly labour, and against the cost of discovering a design problem after the tooling has been committed. Our notes on quality management describe how the build is controlled.

Common Difficulties

Warping and mechanical stress are the classic problems, and they come from an unbalanced stack-up or a bend radius that is too tight. Precision requirements are high, so the fabricator’s capability matters more than it does for a simple rigid board. Testing is more complex because the assembly may not fit the fixtures used for rigid boards, and the flexible sections have to be handled without damage during test and assembly.

Each of these is easier to avoid than to correct, which is the argument for building a prototype and for involving the manufacturer in the design review before the first build. Our PCB assembly group handles the assembly and test of these boards.

Applications

Consumer electronics use rigid flex in phones, tablets, foldable devices and wearables where space is at a premium. Automotive systems use it in driver assistance and infotainment displays. Aerospace and defence use it because it saves weight and survives vibration. Medical devices use it in implants, imaging equipment and surgical instruments where the assembly has to fit a complex shape.

Choosing a Supplier

The criteria are experience with rigid flex specifically, the certifications the manufacturer holds, the prototype lead time, and whether design for manufacture support is available so that the stack-up and the bend are reviewed before the tooling is committed. A supplier that treats a rigid flex board as a rigid board with a bending area will produce a board that fails in the field. Our notes on PCBA testing describe how the electrical verification is carried out.

FAQ

Why build a rigid flex prototype? Because the mechanical and electrical behaviour of a folded assembly cannot be predicted from a drawing, and prototype feedback is far cheaper than a failed production run.

What bend radius is required? It depends on the stack-up and the copper, but the rule is to keep it as generous as the product allows and to orient conductors perpendicular to the bend.

Can components be placed in the flexible area? They should not be, because they concentrate stress and the bend will damage the joints.

How much does a prototype cost? Considerably more than a rigid prototype of the same size, because of the materials and the additional process steps.

What should be checked in the prototype? The fold and its effect on the conductors, the impedance and continuity across the transition, the flatness of the rigid sections and the mechanical fit in the product.

Conclusion

A rigid flex prototype is the stage at which a design stops being a drawing and becomes a physical assembly that folds. A balanced stack-up, a generous bend radius, no components or vias in the bend, quality polyimide and a manufacturer experienced in the construction are what turn the prototype into a design that can go to production.