Rigid-Flex Circuit Boards and Multi-Board Systems: Differences Between Assembly

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?

With rigid-flex PCB assembly, you’ll be able to achieve difficult form factors or enclosure specifications with your designs, without the traditional hassle. Instead, there are certainly a handful of assembly considerations at work from the design standpoint you’ll want to be prepared for before jumping into the worlds of rigid-flex and multi-board assembly. 

What is a Rigid-Flex Printed Circuit Board 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.

Flex Layers

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.

Rigid Layers

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 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.

Smart watches and other IoT wearables are heavy users of flexible board materials.

Common Rigid-Flex Board 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.

Differences Between Rigid-Flex Circuit Board Assembly and Multi-Board Assembly

The main differences to consider between multi-board system assembly and rigid-flex assembly will be in your signal integrity domains, and in system connectivity. Whereas a rigid-flex assembly poses unique impedance challenges in its designs, multi-board assemblies pose challenges in the form of analog-to-digital signal contamination, frequency mixing, and in managing various other forms of EMC from component interaction or with the chosen connectors. 

Assembly is not always just a plug-and-play process and does require foresight.

Most multi-board systems will involve mixed-signal designs, and as such will require careful attention to not cross signals and avoiding crosstalk. Keep in mind, proper analysis and simulation software will give you a strong idea of the expected model of your design and prepare you for design pitfalls before you run across them in the road. Additionally, individual boards may pass your EMI determinations with ease; however, when the system runs as a whole, EMI coupling can decimate the EMC of a design. 

With multi-board systems, you also have to consider board temperatures and thermal design much more substantially as, depending on the enclosure, having a high speed system without properly temperature-controlled components could mean disaster for your prototype. Utilizing proper heat sink and thermal via placement will enable multi-board design to flourish in a challenging environment. 

The introduction of rigid-flex assemblies to multi-board systems 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. 

With Cadence, you won’t go wrong in their consortium of tools and strategies to tackle any form of design problem from simple 2-layer designs to the most finicky of multi-board and rigid-flex PCB designs. Furthermore, Allegro PCB Designer is the layout tool capable of performing advanced component placement and net classifications perfect for your design needs. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.