Rigid-Flex PCB Design Guide

There are specifications and manufacturing process guidelines that must be met before you can successfully design a rigid-flex PCB.

rigid-flex pcb design

Understanding these rules make it easier for the fabricator cum PCB manufacturer to have a swell time when designing and manufacturing the board.

Having that in mind, we will like to reemphasize the importance of the rigid-flex PCB. That way, you are better informed on what you stand to gain from using this type of Printed Circuit Board (PCB).

What Is a Rigid-Flex PCB?

Worthy of mention is that the rigid-flex circuit board is a combination of the Flexible Printed Circuit Board and the hardboard.

Hence, it is a hybrid circuit board that combines the elements of the aforementioned PCB types to enhance the structure and design of the circuit board you are trying to make.

With that in mind, we will take you to the next part of this content, which is where we will be explaining all you need to know about designing the rigid-flex PCB. To simplify the process further, we will hint at the different rules and specifications by categorizing them.

Pre-Design Considerations

The first thing you must have in mind is that there are some considerations you must have in mind even before you start looking for the best rigid-flex PCB manufacturer.

More often than not, consumers make the mistake of hiring a PCB fabricator without actually knowing what is expected of the job. In turn, the PCB manufacturer tends to find it a bit confusing to understand the specifications or the expectations you expect of the circuit board.

That informs the reason why the first step you must take is to take into consideration the pre-design requirements, as they will help you to make an informed decision on the rigid-flex PCB design.

Here are some of the pre-design considerations you must have in mind at all times:

1. Work Closely with the PCB Fabricator

It is time you stopped being distant with the PCB manufacturer because that doesn’t create a good opening for collaboration.

Rather, teamwork is the main goal, and you must strive to enable that at the onset. The golden rule is to work and communicate closely with the rigid-flex PCB fabricator.

With a closer and working relationship, you rest assured that the manufacturer/fabricator will let you into the steps taken to design the circuit board. And if there is a need for an urgent respin, you are sure that you will be the first to know.

2. Quit Delivering Gerber Files

As technology continues to evolve, more inputs will be made to make life easier for everyone. The same applies to the rigid-flex PCB design niche where Gerber Files that once called the shots are set to be relegated to the background.

So, if you have been submitting Gerber Files or you intend to do for your next rigid-flex PCB job, it might not give off the excellent documentation expected for the success of the design.

Instead, consider delivering and submitting files that are compliant with the IPC-2581 standards. You may also want to deliver the files in Version 7 of ODB++. The reason is that such files help to provide clear documentation that highlights the different components and specific layer types that are expected to make the rigid-flex PCB design worthwhile.

3. Confirm the Designing Process

It is pertinent that you involve the rigid-flex PCB manufacturer at the early stages of the design. The reason is to enable the fabricator to be well informed with the history of the circuit board design so that any challenges can be fixed in real-time.

Also, consider confirming the designing process of the rigid-flex PCB manufacturer. This should be paramount because you want to be sure that the fabrication process designated by the manufacturer tallies with the layer stack design of the rigid-flex circuit board you want to be designed.

4. Fabricator’s Capability

You also need to confirm if the rigid-flex PCB manufacturer you intend to use can design the type of circuit board you want.

The earlier the confirmation, the better because it helps to save you time and money if the design doesn’t come along as expected.

5. Prioritize IPC-2223

The last but not the least is that you must do all you can to ensure that the communication between you and the rigid-flex PCB fabricator is optimum.

To do that, consider prioritizing the IPC-2223 as the reference point and standard method of communicating with the designer. With this in place, you are confident of zero breaches and errors in the course of the rigid-flex PCB design.

Best Practices in Rigid-Flex PCB Design

rigid-flex pcb applications

This is the important aspect of fabricating your rigid-flex PCB. Mistakes made here are either hard to correct or will take more time to be corrected.

That is not what you want considering that you need some time to finish the design in the shortest time possible and set the circuit board to use.

Having said that, here are some of the best practices that guide the design of the rigid-flex circuit board:

1. Board Flexibility

The flexibility of the rigid-flex PCB matters a lot. Most times, designing the trace routing can trigger chains of wrinkles and stress that might cause the circuits to frail.

For that reason, you want to follow the best practices that will help you retain the flexibility of the circuit board. The easiest way to go about that is by substituting the traditional solid copper pours with hatched polygons. The reason is that the polygons help to strengthen the flexibility of the rigid-flex PCB contrary to the buckling of the copper pours under the tight radius bends.

2. Define the Manufacturing Process

Many consumers make the mistake of leaving everything about the rigid-flex PCB in the hands of the fabricator. While that isn’t an entirely bad idea, it makes more sense if you already have something figured out ahead of time.

The best way to go about that is to define the manufacturing or fabricating process ahead of time. The reason for this is that depending on how the rigid-flex PCB is designed, it might end up not taking the pattern you hoped it will take.

Some instances will suffice to drive home this point:

· Frequent Movement

The first point to consider is the frequency of the movement on the circuit board. If the use cases/applications demand frequent vibrations, movements, and shocks, it makes sense to concentrate the more on the number of layers, copper coils, and adhesives.

· Fixated Position

The second consideration is whether the rigid-flex PCB wouldn’t move. If no or non-frequent movements are expected, then you have to re-strategize.

The areas of the manufacturing process worth considering here are the number of layers, the grade of copper, copper type, and method of designing the circuit board.

3. Copper Trace Placement

The placement or positioning of the copper of the rigid-flex PCB is another important consideration to have in mind before contacting the PCB fabricator.

Worthy of mention is that the copper traces will play more significant roles in the rigid-flex circuit boards if they were to be placed or positioned to the right angle of the circuit board.

You don’t have to wonder why that is so because the reason is simply that such a placement helps to prevent bending at the corners of the circuit board.

Yet, you must have in mind that some rigid-flex PCB manufacturing processes might demand that the corners of the circuit board will be bendable. If that is the case, you might consider using the conical radius bends that bolster such flexibility.

4. Vias Placement

Also, the vias used for the fabrication of the rigid-flex PCB must be placed appropriately.

Have in mind that fatigue tends to take prominence in the circuit board if there is frequent bending.

Resolving and preventing that can be facilitated by placing the vias in the stationary areas of the board, which are not susceptible to continuous movements that might bend the circuit board.

Besides, consider leaving half-millimeter spacing between the adjacent vias and the copper annulus.

5. Pad Reliability

The reliability of the pads on the rigid-flex PCB might be threatened when the bending of the circuit board becomes more frequent. In such cases, it wouldn’t be long before the circuitry copper would give in to the pressure and detach itself from the polyimide substrate.

The best way to avoid this from happening is by providing enough pad support for the exposed copper traces. The ideal way to provide that support is by using overlay anchoring stubs and openings.

6. Consider Using Curved Traces

Stresses on the circuit board can wear it out. And since you don’t want that to happen (especially in the early stages of the design), you must consider other options.

Have in mind that the major cause of stresses and twists in the circuit board are because of the continuous bending. The antidote to that is to use a special design that will inhibit the stresses from continuing.

Consider using curved traces instead of 45-degree hard corners and right angle traces. By using curved traces, you will discover that the stresses wouldn’t be much because these types of traces are already designed by default to withstand pressure because of their curves.

7. Reduce Tension with Staggered Traces

Tensions aren’t out of place on rigid-flex PCBs. The reason for the dominance of tensions is because of the continuous running of traces over each other and in the same direction.

Rather, you can put a stop to those tensions by staggering the double-sided flex traces. That will help reduce the tension in the short-term and the long run, the tensions would be eliminated.

8. Avoid Abrupt Change of Trace Widths

Understandably, you want to make some changes that you didn’t figure out at the onset. While that isn’t a bad idea, it can mar the success of the rigid-flex PCB stackup because an abrupt change in the widths of the traces can create a weak spot on the circuit board.

Instead of doing that, consider using the Teardrop Pattern that aids in the swift alignment of the width traces until they connect to the vias and pads in the rigid-flex circuit board.

Rigid-Flex PCB Stackup Requires Teamwork

rigid-flex pcb cost

You got it all wrong if you have been working on the assumption that you and the rigid-flex PCB fabricator can work solo. Teamwork is the best way to get the best designs for the circuit board while ensuring that you get value for your money.

Several reasons suggest why you must liaise and be in a good relationship with your rigid-flex PCB fabricator. Some of the areas you will need the help of the fabricator include:

Unique Design

The rule of thumb is to partner with your rigid-flex PCB manufacturer at different points of the design. Whether it is before the start of the design, during the designing processes, or after the design; you have to be in constant communication with the fabricator.

The reason is not far-fetched from the fact that the fabricator comes across thousands of PCB designs every year. That places the designer in a better position to advise you on the best design that will make your rigid-flex PCB look nice.

Change of Materials

There are some rare cases when you might want to change the materials intended for use in fabricating the rigid-flex PCB.

This is not something that must be rushed because the rigid-flex PCB stackup fabricator must be informed before anything else. The reason for involving the fabricator is to draw from the wealth of experience on the best way to use the adhesive laminates and non-standard base dielectric materials.

Final Words

You and your rigid-flex PCB manufacturer can do an excellent job on the circuit board when you must have mastered these helpful rigid-flex PCB design rules.

Have you designed a rigid-flex circuit board before? Do you mind sharing with us some of the best practices and guidelines that helped you get the best design? Share your thoughts via the comment section!

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