Rigid-Flex PCB Right the First Time--Without Paper Dolls

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The biggest problem with designing rigid-flex hybrid PCBs is making sure everything will fold in the right way, while maintaining good flex-circuit stability and lifespan. The next big problem to solve is the conveyance of the design to a fabricator who will clearly understand the design intent and therefore produce exactly what the designer/engineer intended.

Rigid-flex circuit boards require additional cutting and lamination stages, and more exotic materials in manufacturing; therefore, the cost of re-spins and failures are substantially higher than traditional rigid boards. To reduce the risk and costs associated with rigid-flex design and prototyping, it is desirable to model the flexible parts of the circuit in 3D CAD to ensure correct form and fit. In addition, it is necessary to provide absolutely clear documentation for manufacturing to the fabrication and assembly houses.

The traditional attempt most design teams use to mitigate these risks is to create a “paper doll” of the PCB, by printing out a 1:1 representation of the board and then folding it up to fit a sample enclosure. This presents a number of issues: 

1. The paper doll does not also model the 3D thickness of the rigid and flex sections
2. The paper doll does not include 3D models of the electronic components mounted on the PCB
3. A physical sample of the final enclosure is needed, which may not yet be available
4. If the mechanical enclosure is custom designed, a costly 3D print will be required for testing. This adds much time and expense to the project. As cool as 3D printers are, it's not a sensible use for them if the modeling can be done entirely in software.

This paper discusses practical steps in two approaches to solve these problems, contrasting against the traditional paper doll approach above.

In the first scenario, a 3D MCAD model of the PCB assembly can be created in the MCAD package where a sheet metal model can be generated for the PCB substrate model. This sheet metal model can be bent into shape in the MCAD software to fit the final enclosure and check for clearance violations. This is not the best approach, but it is better than paper dolls.

In the second scenario, a significant part of the enclosure or mechanical assembly model is brought from the MCAD package into the PCB design software, where the rigid-flex board outline can be designed specifically to fit with it. Rigid-flex layer stack sections can be defined and then flexible circuit areas have bending lines added. In the PCB design tool's 3D mode, the folds are then implemented to reveal where potential clearance violations and interference occurs. The PCB design can then be interactively modified to resolve the problems and check right away—without having to build any further mock-ups or translate design databases from one tool to another. 

To read this entire article, which appeared in the June 2015 issue of The PCB Design Magazine, click here.

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