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