Via Treatment for a Rigid Flex Board: Design Guide

A rigid flex board combines stiff sections that carry connectors and heavy components with flexible sections that fold into the shape of the product. The vias are the same in principle as on any other board, but the flexible region changes the rules. A via that would be unremarkable on a rigid board becomes a stress raiser when the material around it bends.

What Makes Rigid Flex Different

The flexible layers use polyimide film with thin copper and a flexible adhesive system, while the rigid sections use a conventional laminate bonded on either side of the flexible core. The two are laminated together, and the transition between them is the region where most manufacturing difficulty lives.

The flexible section has to bend, often many times, and it has to do so without cracking the copper or delaminating the interface. Every feature in that region, including every via, is loaded when the board is folded or flexed in service.

Via Treatment and Anchor Pads

Via treatment is the practice of adding a design feature around the via to hold it in place. The most common is an anchor pad or a teardrop, which is a small extension of copper beyond the annular ring that spreads the load and gives the plated barrel a mechanical grip on the layers around it.

On a rigid flex board this matters more than usual. The polyimide film is more compliant than a rigid laminate, so the barrel carries more of the strain, and a via without an anchor can separate from the pad during bending or during thermal cycling. The anchor is not decorative; it changes the failure mode.

Plating Quality Through Polyimide

Polyimide is harder to drill cleanly than epoxy glass. The material smears when drilled, and the smear has to be removed before plating or the barrel will not adhere to the copper of the inner layers. A poor desmear process produces a via that passes electrical test and fails later.

The plating itself must be continuous and of adequate thickness through the full length of the barrel, which is a function of the aspect ratio and of the bath. Aspect ratio limits are more conservative on flexible materials, and the process requirements are closer to those of a high reliability board than to a consumer one, as discussed in the material on blind and buried via stack selection.

Placement Rules in the Bend Region

The most important rule is to keep vias out of the area that will bend. If a via must be in the flexible section, it should be placed away from the bend line, and the bend radius should be large enough that the strain at the via location is small. A design that puts a row of vias exactly on the fold line is asking for a crack.

Where vias are unavoidable in a flexible area, they are usually arranged in a line perpendicular to the bend rather than along it, so that the material bends between them rather than through them. The bend radius itself should be specified on the drawing and controlled in the assembly fixture, because a design that assumes a generous radius fails when a production operator folds the board tightly by hand.

Copper and Coverlay in the Flex Area

Copper in a bending region is best left as a single layer in the neutral axis of the stack, so that it is neither stretched nor compressed. Where two copper layers are needed, they should be thin and the stackup should position them symmetrically about the neutral axis.

A coverlay, which is a polyimide film with adhesive rather than a printed solder mask, protects the flexible section. Openings in the coverlay expose the pads that must be soldered, and the edges of those openings have to be considered because an abrupt change in stiffness concentrates stress. The practice around masking and protective layers is described in conformal coating and board protection.

Transitions Between Rigid and Flex

The transition is where the rigid laminate ends and the flexible material is exposed. It is the highest stress location on the board, and the design should ensure that no via, no pad and no abrupt copper edge sits in that band. A gradual transition with a small taper, and a strip of unsupported flexible material, keeps the stress lower.

Adhesive squeeze out during lamination can also intrude into the flexible region and make it stiff, which changes the bend radius and can crack the board. The supplier should confirm that the flexible area is free of excess adhesive after the process, since the fault is invisible after assembly.

Electrical Considerations

A via in a flexible section still has an inductance, and its stub still resonates. Where the flexible section carries high speed signals, the same rules apply as on a rigid board, and vias should be minimised because each one is both an electrical discontinuity and a mechanical weak point.

Clearance to neighbouring features also matters. A via close to a trace on another layer couples to it and changes its impedance, and the spacing should be checked on every layer the via passes through, following the guidance in via to trace clearance in multilayer boards.

Assembly and Handling

The flexible sections are handled during assembly, and careless handling is a common cause of damage. Boards should be supported during reflow, held by the rigid sections rather than the flex, and kept flat until they are installed, because a flexible board that is stored folded takes a set that affects the fold later.

Forming is done with a fixture that defines the radius, not by hand. Where a board is folded for installation, the fixture should be used for every unit, so that the strain seen by each board is the same. Manual folding produces variation that shows up as a small number of early failures rather than as an obvious manufacturing fault.

Reliability Testing

Qualification for a rigid flex board includes bend testing, usually cycling the flexible section through a defined radius a specified number of times, followed by a continuity check and a microsection. Thermal cycling is run as well, and the combination of the two is what reveals a marginal via or a delaminated interface.

The test should reflect the actual assembly and use. A board that passes a bend test at a generous radius tells nothing about a product that folds the flex sharply during installation, and the drawing should state the radius the design was qualified for so the assembly instructions can be written to match it.

FAQ

Can vias be placed in the flexible area? They can, but they should be kept away from the bend and given an anchor pad. Where the design allows it, keeping all vias in the rigid sections is the more reliable choice.

Does the coverlay replace the solder mask? In the flexible area it does, because a printed mask cracks when the board bends. The coverlay openings define the solderable areas.

How is a plated via in polyimide inspected? By microsection on a sample, since the barrel cannot be inspected optically. Coupons from the same panel provide the evidence for the process.