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How is a Multi-board PCB System Assembly Different from Rigid-Flex Assembly

Published Date
Author OrCAD PCB Solutions

Gaming rig

CC BY-SA 3.0 Lukealderton

When people typically think of multi-board PCB design, they tend to picture racks of boards in server farms or the components of a gaming rig. But what if your typical rigid boards don’t fit within the physical envelope of your multi-board application? Do you pay a premium for flexible circuitry? What if you could have the best of both worlds?

In this post, we’ll cover the benefits of rigid-flex assemblies, what they are, and how they can better serve your multi-board PCB design needs.

What is a Rigid-Flex PCB Assembly?

In your standard multi-board PCB design, you take a board concept, partition out the different functional circuits onto smaller boards, and use a variety of interconnects to fit your system into an enclosure.

The problem with this standard approach, is that you can’t always count on the reliability of your interconnects, especially after factoring in EMI/EMC concerns. Standard card edge connectors which come with good conductivity, aren’t always available in the sizes you require. Cables are your next best bet, but even these can feel unwieldy and are not quite suitable for the space requirements of your envelope.

If you find yourself with a multi-board design that requires several rigid boards to be interconnected within a compact enclosure, with a high layer count and a need for high speed connections, a rigid-flex assembly might be the solution you’re looking for.

What is a rigid-flex assembly? Simply put it’s two or more rigid boards electrically connected to each other via flexible sections.

A single flex layer generally consists of the following materials:

  • Flexible polyimide core

  • Conductive copper layer

  • Adhesive

The conductive copper layer is sandwiched between two flexible polyimides on both sides with an adhesive. Often, the polyimide and adhesive layer are treated as one unit called the coverlay which can be laminated onto the copper layer through heat and pressure. You can have multiple flex layers in any given design.

The rigid section adds on to the flex layer with a rigid layer of standard PCB materials:

  • Prepreg, which is fiberglass injected with resin that flows and sticks when heated

  • Non-conductive fiberglass substrate (typically FR-4)

  • Classic green soldermask

  • Silkscreen markings and identifying information

The flexible polyimide layer and the conductive copper layers are generally continuous throughout the entire board, including both the rigid and flexible layers. However some designs limit the amount of flexible polyimide used, filling that layer in the rigid section with prepreg.

For design purposes a rigid-flex assembly is treated as one board that can fold in on itself. This reduces the total number of interconnects required in a system and avoids labor intensive steps such as soldering flat ribbon cables onto rigid boards.

Common Rigid-Flex Configurations

Now that you know what goes into a typical rigid-flex assembly layer, let’s take a look at some common configurations.

  • Standard Configuration: Symmetrical construction with the flex layer at the center of the stack. It generally uses an even layer count like your standard multilayer PCB design.

  • Odd Layer Count Configuration: While uncommon in traditional PCB designs, the ability to provide EMI shielding to both sides of a flex layer encourages the use of odd layer counts to meet stripline impedance control and EMC requirements.

  • Asymmetrical Configuration: If the flex layer is not at the center of the stack, it’s considered asymmetrical. Sometimes widely varying impedance and dielectric thickness requirements result in “top heavy” designs. Other times, the blind via aspect ratio can be reduced through an asymmetrical construction. Since this makes the design prone to warping and twisting, a hold down fixture may need to be used.

  • Blind & Buried Vias: Rigid-Flex boards support blind vias, which connect an outer layer of a PCB to one or more inner layers without passing through the entire board, and buried vias, which connect one or more inner layers without passing to an outer layer. Complex via structures often lead to asymmetrical construction to deal with the flex layer.

  • Shielded Flex Layers: Specialized shielding films such as Tatsuta and APlus, laminate to the flex layer coverlays. Special coverlay openings with electrically conductive adhesive allow the shielding films to contact ground. These films make it possible to shield flex areas without significantly increasing thickness.

There are many different configurations possible with a rigid-flex assembly. The number of layers between rigid and flex sections do not have to match giving you full customizability to fit a PCB design into a tight enclosure. Just be sure to follow the standards outlined in IPC 2223C for quality.

Conclusion

Rigid-flex assemblies allow you to meet complex geometric or EMI requirements by allowing you to use flexible circuitry when necessary; and solid, reliable rigid circuit boards where possible to keep manufacturing and assembly costs down.

Because rigid designs often deal with complex 3D requirements, it can be useful to have powerful PCB design software that supports a holistic approach to design that bridges the gap between electrical and mechanical domains. Check out Cadence’s suite of PCB design and analysis tools today.