The Top PCB Layout Design Rules for Rigid-Flex PCBs

November 14, 2019 , in Blog

Being a parent is one of the most natural, joyful, rewarding, thankless, annoying and complicated jobs anyone can have. Not many things can compare with seeing your child graduate after all the nights you spent reading to them and helping with their homework. And not many things can compare with having to explain to your boss that you have to leave work and pick up your child from school early because of their foolish actions. Raising a child breathes life into the words there is a time for everything.

Section view of rigid-flex PCB

Rigid-flex PCB

When it comes to printed circuit board assemblies (PCBAs), the same is true. Some applications require the board to be rigid, flexible, or both. There are PCB design essentials that should be applied for all circuit boards. However, there are also design considerations that depend on whether your board is rigid or flex. And when your board as both of these properties, the situation is more complicated. Let’s take a look at all of these board types and then how to apply the best PCB layout design rules for rigid-flex boards and simplify development.

Similarities and Differences between Rigid, Flex and Rigid-Flex PCBs

As the name implies, rigid boards are intended to maintain rigid structural integrity irrespective of the environmental conditions of the area in which they operate. Even minor deviations from this requirement or warping are problematic and can cause erratic operation and tombstoning, where a component becomes detached from its pad on one side.  Although rigid PCBAs are utilized the most, there are advantages to using flexible boards. For example, flex and rigid-flex boards can be mounted in geometric configurations that rigid PCBAs cannot.

Both flex (where the total board area is flexible) and rigid-flex (where the board contains both flexible and rigid areas) are classified according to their construction type under IPC-6013D Qualification and Performance Specification for Flexible/Rigid-Flexible Printed Boards. Classification criteria include single or multilayer stackup and via usage. Although similar in taking advantage of flexibility for trace routing and board mounting, there are differences between flex and rigid-flex PCBs, as shown in the table below.

Comparison of Flexible PCB Types

Comparison of Flexible PCB Types

As shown in the table above, rigid-flex boards are more widely used than flex-only boards. This is because many rigid board applications have or can be replaced with a comparable rigid-flex one. Unlike flex-only boards that are usually limited to six or fewer layers, rigid-flex PCB stackups may be comprised of 20 or more layers. Another advantage is that rigid areas may be located at any convenient location on the board to provide good solder joint quality for SMT components and allow for the use of blind and buried vias, which is not available with flex-only. And all indications are that rigid-flex PCBAs usage will continue to grow.

Flex PCBA Manufacturing

The comparison above also indicates that manufacturing flex boards can be complex. For PCB fabrication, the process of constructing flex boards is similar to the method for rigid boards. Notable exceptions are thinner materials and coverlay instead of solder mask, for flex. Rigid-flex fabrication is more involved as it includes both the rigid stackup, which may include multiple cycles for blind and buried vias, and flex stackup processes. Assembly is the opposite, as mounting components on flex boards typically requires stiffeners or fixtures to keep the dielectric material from moving during the reflow step of component soldering. Rigid-flex, on the other hand, is similar to rigid board assembly, if component placement is limited to the rigid areas.

Rigid-flex manufacturing technology is advancing as the board’s utilization grows. And it is likely that you have or will have to design boards for this process. To do so effectively requires the implementation of good PCB layout design rules for rigid-flex PCBs.

PCB Layout Design Rules for Rigid-Flex Boards

Rigid-flex technology provides the most versatility for PCB development, as it provides the solid foundation preferable for component mounting and the flexibility to accommodate limited or geometrically challenging PCBA trace routing and board deployment requirements. However, there are design rules that should be followed to fully realize these advantages. These are listed below:

Rule 1: Place components in the rigid area(s)

By placing components in rigid areas, high-quality soldering can reduce or eliminate the possibility of problems such as tombstoning, and the full range of via options can be used.

Rule 2: Make sure component footprints are on solid rigid planes

Regardless of where components are mounted, the entire pad area should be coplanar. Placing components across flex areas can place unreasonable stress on solder joints that can result in components becoming detached.

Rule 3: Distribute traces evenly in the flex area(s)

Adjacent traces run along or through flex areas should be distributed with consistent spacing. Failing to do so can negatively impact signal integrity, depending on signal types and trace lengths.

Rule 4: Keep trace geometries constant in flex area(s)

Trace dimensions and spacing along the flex material may be altered during bending or twisting of the flex material. This can alter electrical and physical properties that can impact board operation; therefore, it is better to maintain a constant trace width in these areas.

Rule 5: Use rounded traces in the flex area(s)

It is advisable to only use rounded curves for your traces in flex areas if curves are used at all. Acute angles in traces may be a problem during fabrication on any board, although much less so than in the past. For flex traces, bending can result in wrinkles in the coverlay and excessive stretching may tear the material.

Rule 6: Only use through-hole vias in the rigid area(s)

Currently, it is advisable to avoid designing boards with blind and buried vias in the flex area. Doing so can present several complications for manufacturing, as well as lead to component detachment and signal flow inconsistency once deployed. Advances in flex PCBA technology, such as embedded component-in-flex (CIF) and component-on-flex (COF), are emerging solutions that are easier to manufacture.

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Rigid-flex PCBAs are increasingly being utilized where small size, reliability, and versatility in trace routing and board mounting are desired. This usage extends across virtually all industries, but especially in medical devices, aerospace, and industrial applications. At Tempo Automation, we are experienced in building high-quality, reliable PCBAs for these industries and commercial products. Our industry-leading fast turnkey prototyping and low-volume production process will help you meet your rigid-flex development objectives.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on PCB layout design rules for rigid-flex boards, contact us.

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