Rigid-Flex PCB Layout: Basic Guidelines

How do you design a rigid-flex PCB layout that holds up to real-world bending?

Rigid-flex circuit boards combine the best qualities of rigid and flex PCBs into a single assembly, cutting down on connectors, cabling, and overall assembly complexity while still holding up to real-world bending and flexing. That versatility comes at a cost: rigid-flex designs introduce layout requirements that don’t apply to a standard rigid board, and getting them wrong can mean a design that either can’t be manufactured as drawn or that fails prematurely once it’s in service. Aerospace, medical, automotive, military, and consumer electronics manufacturers increasingly rely on rigid-flex PCBs for exactly this reason, which makes getting the layout right from the start even more important. Here are the basic layout guidelines we follow on every rigid-flex project to keep quality consistent without derailing production timelines.

1. Insulate the Flex Layers with Bondply

The rolled copper cladding used in the flex layers of a rigid-flex board becomes brittle over time if it’s left unprotected, which can eventually lead to cracked traces in the areas that see the most repeated bending. Bondply, an adhesive insulating layer, is applied over that copper cladding specifically to prevent this kind of degradation. Skipping or skimping on bondply coverage is one of the more common reasons a rigid-flex board that performs fine in initial testing develops reliability issues after months of real-world flexing.

2. Keep Vias Clear of the Rigid-Flex Transition

Vias placed too close to the transition between a rigid section and a flex ribbon put stress on an area of the board that’s already mechanically vulnerable. As a general guideline, vias should sit roughly 30 millimeters from the edge of the rigid area, adjacent to the flex ribbon, a substantially greater clearance than what’s typically used on a rigid-only stackup. That extra distance gives the transition zone room to flex without concentrating stress directly at a via barrel, which is where cracking is most likely to start.

3. Stagger Traces Vertically Across Flex Layers

Wiring in the flexible sections of a rigid-flex board has to keep working through repeated bending, and how the traces are arranged from layer to layer has a direct effect on how well it holds up. Stacking traces directly on top of each other, layer after layer, concentrates mechanical stress at the same point through the thickness of the ribbon. Staggering the traces vertically, so they don’t line up directly above or below one another, spreads that stress across the ribbon instead and meaningfully extends the flex life of the design.

4. Limit Adhesive on High-Layer-Count Flex Areas

Adhesive is a normal part of building up a multi-layer rigid-flex stackup, but applying it to the wrong areas can undermine the flexibility the design depends on. Avoid applying adhesive to the bendable sections of a circuit that has more than four flex layers, since adhesive restricts movement in exactly the areas that need to move the most. On thinner, lower-layer-count flex sections this is less of a concern, but as flex layer count increases, adhesive placement needs to be planned around where the board will actually be bending in service.

These four guidelines cover the layout decisions that most often determine whether a rigid-flex design holds up to real-world use or develops problems after it’s already in the field. If you’re planning a rigid-flex project and want an experienced team involved from the layout stage onward, BESTProto’s PCB layout and design services are built around exactly that kind of upfront review.