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Understanding Rigid-flex PCB: Benefits, Applications, & Manufacturing

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Different Types of Rigid-flex PCB Provided By GlobalWellPCBA

Simply select your Rigid-flex PCB below to access the guide.

Introduction

Rigid-flex PCBs are commonly used in consumer appliances and electronics. It is a combination of both rigid and flexible PCBs. It means we can derive intricate benefits of rigid and flex PCBs in one rigid-flex PCB. There are different reasons behind using rigid-flex PCBs. Mechanical support is a major advantage, but we can aim for enhanced component mounting as well. 

Since it is a duo of rigid and complex PCBs, it is clear that one part of this PCB is bendable while the other is not. These are not only lightweight but also gather less space, which is why we can use them in devices of automotive, aerospace, and other industries. We are going to study rigid-flex PCBs in detail through this post. 

Rigid-flex PCB

Before we start our discussion, let’s define rigid-flex PCBs. A rigid-flexible PCB contains both rigid and flexible PCBs. These are among multilayer PCBs. We can make these PCBs more intricate depending on the nature and function of the applications. There is no doubt that due to the complex nature of these PCBs, we have to be careful while designing them. The intricacy of rigid-flex PCBs shows that any negligence in the design development can ruin the performance later. The best part about these PCBs is the freedom of using 3D space while designing these PCBs. Therefore, twisting and turning is possible even with the presence of rigid parts in these boards. 

Working on Rigid-flex PCB

Rigid-flex PCBs are different in working as compared to the rest of PCBs. We use both substrates, including Polyimide and FR4, for these PCBs. So that these can perform dynamically and enhance the performance of electronics on all counts. Here, we are going to briefly learn about the basic functioning of rigid-flex PCBs. 

  • The rigid part of these PCBs is for adding mechanical strength to the boards.
  • We can also rely on rigid portions for better component mounting 
  • Heat dissipation through the boards is also possible through the rigid side.
  • The flex portion is for developing interconnections between the board, its components, layers, and the rigid segment.
  • Both rigid and flexible PCBs use conductive layers for etching.
  • We use plated through holes for vertically joining the layers, and this applies to both rigid and flex segments.
  • The flex part is not used for component mounting.
  • We can twist and turn the flex part as we like without compromising the surrounding elements.
  • Other than this, we can also develop 3D structures out of rigid-flex PCBs.

The purpose of developing 3D configurations is to enhance the support provided by rigid parts of these PCBs. So that better and stronger interconnects can be created within the setup that adds to its durability. We can introduce the materials for rigid and flex PCBs through a method called lamination. During this, we should make sure that the materials are going to stick properly so that there will be no thermal and mechanical complications at all. 

Since rigid-flex PCBs are useless without immaculate designs, below are some effective tips that you should keep in mind. 

  • Don't forget to traceroute the flexible layers so that you can achieve dynamic bending
  • There should be no excessive transition stress between rigid and flex segments
  • It is crucial to ensure optimum bend radius control 
  • The rigid and flex segments should be given enough room for thickness fluctuations

Designing Considerations for Rigid-flex PCB

After going through these recommendations, you will be able to develop flawless rigid-flex PCBs. Let’s see how: 

  • Layer Stackup

There should be no errors in layer stackup for the rigid-flex PCBs. The individual number of rigid and flex layers is important to determine, or else it can ruin the designs. Factors like routing, dynamic flexing, and component density are crucial to consider as well. The laminates for rigid layers should always be FR4 and glass laminates. Flexible dielectric and polyimide are advisable for flexible layers. Also, to combine these layers, additional adhesive layers are important to integrate into the setup. On testing, the stack-up should comply with thickness, impedance control, flexibility, and rigidity on all counts. 

  • Transitions (Rigid-to-Flex)

Since we are working with two segments here, there are higher chances of breakage and bumps in the transition segment. Therefore, it is important to carefully handle this transition, or else it can ruin both parts.

  • It will be better to use tapers and rounded corners for transition configuration
  • The annular rings should be broad, which can help reduce mechanical stress
  • If the transition is of copper, then it is important to reduce the uncovered portion for the desired performance 
  • Curving corners are recommended instead of acute angles. 
  • Bending Radius

Another important thing is the bend radius. The flexible portion cannot cross a certain length of bend radius, which is why we should consider it while developing rigid-flex PCBs. If it is too tight, it can break the copper traces and also ruin the lamination, which we don't want. Therefore, copper thickness, layer stack up, and dynamic flexing should be optimum to avoid complications related to bend radius. 

  • Placement of Components

You already know that we cannot surface mount components on the flex part of PCBs. Therefore, while we are doing this, we should not let the components slide in the flexible part, which can damage the structure of PCBs. Also, connectors are stress components, and we should maintain an accurate distance between them and the transition segment. The copper area is considerable for letting out excess heat as well. 

  • Routing 

You should keep the following tips for tracing in mind for better results: 

  • The traces should be wider in size, and there should be appropriate spacing for better dynamic bending
  • Try not to create acute angles, which is not recommendable for rigid-flex PCBs
  • The better will be to route where a neutral bend axis is present 
  • Make sure to use optimum copper density and maintain it throughout the setup

Manufacturing Procedure of Rigid-flex PCB

This is how we can develop industrial-grade rigid-flex PCBs:

  • Materials Preparation

The process begins with the procuring of materials for rigid and flex parts. These materials undergo proper cleaning and conditioning before further processing. 

  • Imaging

The next step is to start the imaging of the inner and outer layers with the help of a conductive pattern, and for this, we use lithography and etching. The purpose of imaging is to develop pads and traces on PCB layers. 

  • Hole Formation

Drilling is important for creating holes, and we can punch them as well. Different holes like component and tooling holes are possible to make with this process. We plate them along with copper so that vertical interconnects can be formed. 

  • Lamination

Lamination is important for the PCB layers so that they can be aligned. High temperature and controlled pressure make this possible. Also, we should not compromise the bonds between both segments at this point. To enhance them, we use highly strong adhesives. 

  • Imaging

It is time to image the outer layers, and the same process of lithography and etching is used for this step. 

  • Solder Mask and Finish

Solder mask is for protecting the copper layer from corrosion and possible damage. We don't laminate the pads and traces at this point. Only the PCB uses surface finishes based on its nature. Some PCBs use ENIG, while others use HASL, OSP, and tin. 

  • Singulation

We also have to route the large panel of PCBs. This step shows how rigid-flex PCBs have complex configurations that are easily identifiable from the rest. 

  • Testing

Testing and inspection are the most crucial steps of this process. We cannot only enhance our PCBs with testing but also point out harmful flaws in time and fix them as well. Different inspection methods are available to make this possible. However, AOI, aka automated optical inspection, is a common process to follow. 

  • Assemblage

We can acquire 3D configuration in case of rigid-flex PCBs when needed. Also, during assembly, the components and layers are mounted as per the designs developed in the pre-production phase. 

Cost Aspects of Rigid-flex PCB

  • Layer Count

The logic is simple. More layers mean more cost. Due to this reason, all the multilayer PCBs in which rigid-flex PCBs are included are not affordable and also take money for production. 

  • Panel 

If the panel area utilization is less, then it will increase the cost. Therefore, for not blowing up your budget, it is important to use the space efficiently. 

  • Finishing & Coatings

Different surface finishes are available at affordable prices but also come with downsides. Due to this reason, surface finishes can also influence the cost of these PCBs. 

  • Flexible Material Kind

PCB materials like polyimide are expensive, and if you are using rare and high-end materials, then be ready to face a big dent in your pocket. 

  • Registration Accuracy

When you are aiming for tight tracing and more accuracy, it will demand high-end equipment. Therefore, to access this gear, you would have to disturb your budget. 

  • Design Complexity

If you want to go for dense routing and HDI features, then it will also increase your production cost. 

  • Testing

It is obvious that different testing methods include many tools and equipment, and due to this reason, a producer has to go above and beyond their budget for testing their PCBs. 

Benefits of Using Rigid-flex PCB

Consolidated Packaging

It is obvious that when we are using rigid-flex PCBs, the size of the package reduces. Due to this reason, we can use a single rigid-flex PCB to not only save space but also aim to enhance the quality and performance at the same time. 

Higher Reliability

The best part about rigid-flex PCBs is that they offer more reliability, which comes in handy for high-frequency and powerful electronics. Using two PCBs in a single unit is no less than a miracle, and due to this reason, there are vast applications that rely on rigid-flex PCBs for their performance. 

Summary

Rigid-flex PCBs are a great innovation to sustain fields like electronics, automotive, and aerospace. The reason is quite simple. Quality and reliability. Other than this, space efficiency and strength are also common features of these PCBs. Hopefully, now you can give rigid-flex PCBs a shot for your future projects!

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