“Usually when we think about multi-layer circuit board PCB design, we often think of circuit board racks or gaming platform combinations in a server environment. But what if our typical rigid circuit boards don’t fit into the physical enclosures that multi-layer circuit boards use? Would we be willing to pay extra to use a flex circuit board? What if we could have the best of both worlds?

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Usually when we think about multi-layer circuit board PCB design, we often think of circuit board racks or gaming platform combinations in a server environment. But what if our typical rigid circuit boards don’t fit into the physical enclosures that multi-layer circuit boards use? Would we be willing to pay extra to use a flex circuit board? What if we could have the best of both worlds?

In this article, we will introduce the advantages and properties of rigid-flex combinations and how to better meet the PCB design needs of multi-layer circuit boards.

What is a rigid-flex PCB?

In standard multilayer circuit board PCB design, we adopt the circuit board concept, divide different functional circuits into smaller circuit boards, and use various interconnecting lines to put the system into a housing.

The problem with this standard approach is that there is no guarantee of interconnect reliability (especially when EMI/EMC issues are taken into account). Standard card edge connectors with good conductivity that meet our size requirements don’t always exist; the best alternative is cables, but cables aren’t practical and they don’t quite fit the space requirements of the enclosure.

If a multilayer circuit board design requires us to interconnect several rigid circuit boards within a compact enclosure with high layer count and high-speed connectivity requirements, a rigid-flex combination is the best solution.

What is rigid-flexible bonding? Simply put, it is to connect two or more rigid circuit boards through flexible parts.

A single flexible layer typically consists of the following materials:

● Flexible polyimide core;
● Conductive copper layer;
●Adhesive

A conductive copper layer is sandwiched between flexible polyimides on both sides by an adhesive. The polyimide layer and the adhesive layer are usually treated as one unit (called a cover layer), which can be laminated on the copper layer by heat and pressure. There can be multiple flexible layers in any given design.

The rigid section is added to the flex layer via the standard PCB material rigid layer:

●Prepreg material infused with glass fiber with resin, which will flow and bond when heated;
●Non-conductive fiberglass base (usually FR-4);
●Traditional green solder mask;
●Silk-screened markings and identification information

The flexible polyimide layer and conductive copper layer are usually continuous throughout the board (both rigid and flexible). But some designs limit the amount of flexible polyimide, filling the rigid layer portion with prepreg.

In terms of design, rigid-flex is considered a circuit board that can be folded. This reduces the total number of interconnects required in the system and avoids manual steps such as soldering flat ribbon cables to rigid circuit boards.

Image Credit: Cadence Allegro Design Tools

Common rigid-flex configuration

Standard configuration: Symmetrical structure with flexible layer on the center of the stack. It typically employs a uniform layer count similar to standard multilayer PCB designs.

Odd Layer Count Configurations: While not common in traditional PCB designs, odd layer counts provide EMI shielding on both sides of the flex layer for line impedance control and electromagnetic compatibility requirements.

Asymmetric configuration: A configuration is considered asymmetric if the flex layer is not in the center of the stack. Sometimes impedance and dielectric thickness requirements vary widely, resulting in a “top-heavy” design. Other times the blind via aspect ratio can be reduced by an asymmetric structure. Since this tends to deform and twist the design, it may be necessary to hold down the clamps.

Blind and buried vias: Rigid-flex circuits support blind vias, which connect the outer layer of the PCB to one or more inner layers without passing through the entire board; while buried vias connect one or more inner layers without through the outer layer. When dealing with flexible layers, complex via structures often require asymmetric structures.

Shielded flex layer: Special shielding films (eg Tatsuta and APlus) laminated on the flex layer. Special cover openings with conductive adhesive allow the shielding film to make contact with the ground. These films can shield flexible areas without significantly increasing thickness.

There are many different configuration possibilities for rigid-flex. The number of layers between rigid and flexible sections does not need to be matched, allowing us to achieve complete customization to fit the PCB design into a sealed enclosure; just make sure the design follows the quality standards specified in IPC 2223C.

Summarize

Rigid-flex can help us meet complex geometric or EMI requirements, allowing us to employ flexible circuits or robust and reliable rigid circuit boards when necessary, with minimal manufacturing and assembly costs.

Since rigid designs often deal with complex three-dimensional requirements, it is imperative to have robust PCB design software that supports a holistic design approach to bridge the gap between the electromechanical domains.