Best practices and guidelines for PCB Flex and rigid flex design

14 January 2020
New

Currently, training on terminology, requirements, processes, and best practices is critical to mitigating the challenges of flexible pcb and rigid-flex design. For PCB designers who need to respond to rigid-flex design, the following is a list of guidelines and best practices for successful design:

1. Make sure that the cabling width and spacing are as large as possible;
2, the wiring should use round corners, avoid the use of 90° corner, the designer should ensure that the round corner is a true arc. The segmented arc will be cracked due to stress. In most cases, the contour of the wiring should mimic the contour of the flexible plate. If the ECAD tool can automatically route according to the circuit board contour, it will save the designer a lot of time;
3, if the design requirements in multi-layer wiring, confirm the wiring of adjacent conductors is staggered;
4, the electrical requirements allow a cross-open power/ground plane. The use of the cross-open plane as the return path has a significant effect on the impedance of any conductor;
5, if any part of the flexible circuit is required as a flexible connector, plug or socket, you need to use a reinforcing plate.

Best practices and rigid flex pcb guidelines

There is a set of best practices and guidelines for flex areas in flexible circuits that designers can consider to maximize reliability.

1) never change the cabling width of the area;
2) ensure uniform cabling;
3) make the wiring perpendicular to the bending direction, because lack of symmetry will increase the chance of stress accumulation;
No conduction holes are allowed;
4) grid-like power/ground should be parallel to the area. It is best to cross-open the power/ground, but the cross-open pattern should be 45° to the curved line. (When creating a planar fill) you can save time by using an ECAD tool that calculates the angle of intersection openings and curved lines, especially for designs with curved lines with irregular angles;
5) The bending radius is likely to be the biggest challenge associated with the bending zone, and therefore requires close cooperation with the PCB manufacturer. Bend radius requirements vary depending on the application. A flexible circuit can be static (bending only once during assembly), or it can be dynamic (bending countless times during the product life);
6) The bending radius of a static flex circuit is more demanding than the bending radius of a dynamic flex circuit. The bending radius plays an important role in avoiding compression (inner bending area) or tension reliability problems.

While these guidelines and best practices are not comprehensive, they are very helpful for new designers who have just entered the field of flexible and rigid-flex design. These methods can help PCB designers to better understand the characteristics of flexible and rigid-flex technology. PCB designers should also consider several aspects of flexible technology, such as impedance control, the distance between the hole and the interface (flexible interface and rigid interface distance), structural parts, laminate and bonding materials, surface coatings and cladding design. These are not discussed in this article, but further research should be conducted to address future needs.

ECAD design tools for flexible pcb and rigid-flex pcb design

The product development team should adopt more advanced ECAD design tools than the traditional “individually designed post-assembly” approach to design new-generation rigid-flex pcb products. Today’s powerful electronic design tools, such as the Mentor Xpedition and PADS professional processes, make it easy to set up and implement changes in the stack and board contours to increase productivity.

By supporting true 3D design and validation, rather than just a 3-dimensional view, the ECAD tool helps users generate stable and clear manufacturing drawings. To ensure a successful trial run, a rigid-flex-specific structural verification is performed, i.e. a structure that affects signal integrity/power integrity/Thermal integrity and manufacturability. This will increase PCB productivity, reduce development costs, and ensure design intent is achieved.