Rigid-Flex

6 Aug 2020 • 5 minute read

Rigid-flex sounds like something that might be a Crossfit workout-of-the-day. But it is actually a way of doing electronic design for small form factors using flexible PCBs (typically along with some normal rigid PCBs too). But they require additional design approaches to ensure correct-by-construction design. If you have a smart watch, or a phone, or earbuds, you probably have something that qualifies.

Inside a variety of small electronic devices—from earphones to smartphones, tablets, and laptops—are rigid-flex PCBs comprised of rigid and flexible substrates laminated together. Such circuits are considered reliable, versatile, and space-efficient. As designs continue to shrink for a variety of applications, this type of flexible substrate for electronic circuitry keeps growing in popularity, particularly in consumer electronics. Because of the bending possible with rigid-flex circuits, designers can place much more circuitry into the space available, even stacking the board layers in a 3D format on the rigid side. Multiple stack-up zones contribute to lower cost.

Traditionally, designers would integrate the flexible portion of their circuitry as a connector from one rigid board to another. However, flexible technology has matured substantially in recent years. Now, because of more stringent area requirements, designers are placing components on the flexible circuit area, using this area like a rigid substrate. PCB design technology to address rigid- flex design has been available for some time. However, utilizing both the rigid and the flexible areas for components introduces new fabrication challenges that require more sophisticated PCB design technology.

Rigid-flex PCBs consist of zones with different layer counts and materials, making for more complex analysis. Stiffeners bring rigidity to these PCBs, and are placed near or on the opposite side of components or near connector areas. They usually consist of a metal, such as stainless steel or aluminum, with the addition of dielectric material like a polymide build-up. The flexible portion of the design typically consists of a dielectric material with bend areas. The bend area must restrict the placement of components and vias; otherwise, these elements contribute to stress and cracking. Routing must cross perpendicular to the bend line to minimize material stress at this location. Adjacent layer routing through the bend area should be offset to prevent what is called the I-BEAM effect. Traces routed in this manner can add stiffness to an area designed to be flexible. There’s also a transition zone—an intersection between the rigid and flex zones that may require overlap of material and also special spacing for holes and conductive materials. Consider the transition zone a stress-relief area. As a simple example, a design might have a four-layer rigid connected to a two-layer flex, which terminates on a four-layer rigid. More complex configurations are now common, and there are many possibilities.

The diagram below shows an example of signals going from the PCB, through the transition zone, to the flexible part of the design.

To meet customers’ requirements, the fabrication industry continues to innovate, increasing the number of conductive and non-conductive layers on flex and rigid-flex designs. There’s also been an increase in different types of materials and associated rules required in rigid-flex PCB design. As a result, designers have to do many more manual checks in order to benefit from the advantages of this technology—and to ensure that their designs can be fabricated according to their intent. To ensure correct-by-construction design, designers need in-design inter-layer checks to flag errors right when they are created. After all, fixing errors after the design is somewhat complete takes a lot longer than finding and then fixing the errors as they occur.

Allegro can perform inter-layer checks so that the mechanical and electrical design can be done together. In particular:

Layer-to-layer checks to assess stack-up mask layers
Coverlay to pad
Mask to pad
Precious metal to coverlay
Bend area/line to stiffener, component, pin, and via
Gaps, such as edge-to-edge spacing in areas such as the bend line to the component, the via to the bend line, and the stiffener to the bend area
Inside areas, such as gold mask to coverlay, pin to coverlay, and stiffener adhesive to stiffener
Overlaps when two geometries overlay by a minimum or more, such as soldermask overlay into the transition zone

Rigid-flex designs have additional complexity on the mechanical design side, and how this interacts with the electrical design

Do not place vias in bend areas to avoid cracking the substrate over time
Do not put pads too close to the bend area, as the pads can eventually peel off
Avoid overlapping bend areas with stiffeners, or else there could be peeling or restriction of the full bend
Avoid placing stiffeners too close to vias or pins to avoid shorting

Routing flex vs. rigid generally comes down to one word: arcs. The nature of all geometry residing in a flex zone, whether it’s the board outline, teardrops, or routing, involves arcs and tapered transitions. CAD tools need to support group routing functions to carry a bus across the flex while locking to the contour of the board outline. Line-width transitions should be tapered and all pin/via junctions should be tear-dropped to reduce stress at the solder joints. Advances in CAD tools over the years have resulted in a better ability to push and shove traces during the edit commands. However, this has, for the most part, been a challenge with arc routes. Change, even daily change, is a given in PCB design. But adding an additional signal to a routed bus structure should not require designers to delete routes followed by the group reroute.

Cadence’s Allegro PCB design portfolio automates inter-layer, in-design checks in rigid-flex PCBs, providing the capabilities discussed in this section. By allowing you to perform DRCs for various non-electrical flex layers, the tool helps to save time and avoid respins. The tool also supports real-time concurrent team design, so multiple PCB designers can work on the same PCB design database.

Design houses and fabricators agree on a stack-up for a design that has impedance control or flex/rigid-flex designs because of the complexities involved. Traditionally, design houses and their fabrication partners use spreadsheets, presentations, and other such tools to communicate build intent. These methods are both time-consuming and error-prone. To avoid such problems and save time, advanced PCB designers now use IPC-2581 to exchange stack-up data electronically. IPC-2581 is an open, intelligent, neutral design data exchange format that is supported by over 85 PCB design and supply-chain companies worldwide. IPC-2581 revision B now supports bi-directional exchange of stack-up data to eliminate the discovery of problems late in the design hand-off cycle.

Learn More

Read the white paper Automating Inter-Layer In-Design Checks in Rigid-Flex PCBs.

Sign up for Sunday Brunch, the weekly Breakfast Bytes email.