Guidelines and Best Practices for Flexible Printed Circuit Board (PCB) Design

With several industries now increasingly using flexible printed circuit boards (PCBs), it is becoming necessary to focus on the requirements, processes, education on terminology, and best practices for maintaining a high probability of first-pass success for making flex PCBs. As the name suggests, these designs consist of multiple layers of flexible circuit substrates.

Compared to the design for rigid boards, designers have a larger variety of options when working with flexible PCBs containing dense designs conforming to a specific form factor. With flexible boards, product designers can apply greater functionality within smaller volumes of space more cost-efficiently, while also providing better stability and reliability.

Designing with flexible circuits offers some unique direct advantages such as a reduction in the number of connectors, resulting in cost-reduction and gain in physical space. There are indirect advantages as well, such as the reductions in time and cost involved in assembly. However, moving to flexible PCB usage brings its own set of challenges and concerns. Alleviation of these challenges and concerns requires the designer following some tips, best practices, and guidelines.

Benefits and Challenges of Flexible Circuits

Most electronic equipment today use flexible circuits. Such equipment include laptops, digital cameras, LCD televisions and monitors, mobile phones, and many more. Basically, original equipment manufacturers (OEMs) use flexible circuits whenever their products need to be compact, lightweight, and flexible. This is because the flexible technology offers them several benefits such as:

• Improved reliability and cost reduction compared to traditional rigid-board approach.
• Removal of connectors and additional solder joints leading to improved signal integrity, as cross-sectional changes to conductor paths are eliminated.
• Routing is possible in three dimensions, reducing space requirements.
• Improvements in electromechanical functionality of the product. This includes improved shock and vibration tolerance, weight reduction, higher heat resistance, and better dynamic bending.

To allow realization of the above benefits, designers must augment their traditional proficiency in rigid board design and expand their knowledge base to include flexible technology. Unless these designers make use of their individual learning curves and the various challenges of flexible technologies, there is a high potential of expensive design failures, even leading to project derailment. Furthermore, use of flexible circuit technology also demands ECAD design tools be chosen carefully to facilitate and ensure process compliance.

Guidelines and Best Practices

Eminent PCB manufacturers such as Rush PCB Inc. understand the criticality of education on requirements, terminology, processes, guidelines, and best practices for mitigating the challenges associated with flexible circuit design. Therefore, they have formulated some guidelines and best practices for addressing these challenges:

Managing the Stackup — Rigid and flexible circuits invariably have different stackups, and the designer must manage them efficiently. They also need to convey the stackup properly to the fabricator. This complex task is often made easier with an ECAD tool that offers region-specific and board-specific stackup designs.

Managing the Board Outline — it is necessary to properly configure and manage the flexible board throughout the design process. In fact, the actual design of a flexible circuit is more of an electromechanical project requiring close collaboration between the mechanical and electrical domains. Here, it is possible to save time provided there is ability to import mechanical CAD data and create multiple and complex board outlines automatically, thereby reducing the potential for errors.

Interaction with Fabricator — this is a very crucial aspect of the flexible board design. The fabricator offers helpful feedback to the designer regarding stiffness, bend requirements, keepout regions, material, stackup, etc. All this helps ensure the design can undergo fabrication with high reliability.

Analysis of Power and Signal Integrity — It is necessary to make sure the analysis tools recognize flexible layers and stackups and relate the analysis results to them, rather than assuming a single layer PCB with a uniform stackup.

Verification of 3-D Design — it is important the design team is capable of taking advantage of the available space in the product in all its three dimensions, while being able to identify potential design issues with the flexible circuit. They must be able to define the bend parameters such as bend origin, angle, and radius, while editing the design in 3-D. They must also be capable of viewing the design in relation to the enclosure, while performing 3-D flex-aware design rule checks (DRC).

As most circuit designers will likely be new to flexible design requirements, Rush PCB Inc. offer some guidelines as follows:

• Keep both trace spacing and widths as large as possible.
• Route traces using round corners, rather than ninety-degrees.
• Round the corners of the traces using true arcs, as segmented arcs create stress fractures.
• Allow the trace contour to mimic the outline contour of the board. Use an ECAD tool that does this automatically, as this will save time.
• When routing more than one layer, make sure adjacent conductors have their traces staggered.
• Follow electrical requirements while cross-hatching ground/power planes. Note that the cross-hatching significantly influences the impedance of any conductor using the plane as a return path.
• Ensure the use of stiffeners where the flex circuitry demands placing parts such as a jack, plug, or an SMD connector.

Apart from the above, the area of the flex circuit, also called the flex bend region, where there will be a bend or twist, must follow a set of fundamental guidelines and best practices for maximizing its reliability:

• Distribute the traces evenly through the region.
• Make sure the traces are routed perpendicular to the direction of the bend.
• Make sure the width of traces does not change within the region, as lack of symmetry may increase the chance of a stress buildup.
• Do not place vias in the bend region.
• Keep the crosshatched ground/power planes parallel to the region, with the crosshatch pattern at a 45-degree angle to the bend line. The ECAD tool should be able to calculate the angle of the crosshatch in relation to the bend line—this will save time when creating planes using bend lines with odd angles.
• The bend radius is crucial to the flex circuitry, especially as the flex may have a static or a dynamic bend. While a static bend can stand a tighter bend radius, as it will bend only once during installation, the dynamic bend requires a greater radius, since this area of the circuit will face numerous bending throughout the lifespan of the product. Although the bend radius requirements vary based on the application, defining the bend radius properly is critical to avoid reliability issues from compression or tension in the area inside the bend.

Summary

Of course, the above guidelines and best practices are not all-inclusive, and are only introductory. However, these are the prime areas where a first-time flex designer needs to focus and familiarize him/her quickly, as these areas are important even for the simplest of flexible designs.

Along with the above fundamental areas of concern, designers will need to review additional items that include laminates and bonding materials, construction options, hole to bend distance, impedance control, coverlay design, and copper surface finish. The designer needs to confer with the fabricator for the review.

Finally, designers should take advantage of the latest ECAD tools available. This will not only improve their productivity but also reduce the development costs. For a highly probable first-pass success, it is necessary to combine proper tool deployment along with adequate education.