Rigid-Flex Stackup Design Optimization

December 20, 2019 Cadence PCB Solutions

Earthquake-resistant building

Rigid-flex structure

 

I have never been afraid of heights. I am just terrified of falling from a great height! Nevertheless, I am captivated by skyscrapers, such as; the One World Trade Center (New York), Shanghai Tower (Shanghai), Lotte World Tower (Seoul), Lodha The Park 1 (Mumbai), Tour First (Paris) and Azrieli Sarona Tower (Tel Aviv) and the structural rigidity necessary to support them. Even more amazing is that the world’s tallest building, the Burj Khalifa in Dubai, is flexible enough to withstand up to a 7.0 magnitude Richter scale rated earthquake. The consolidation of rigid-flex construction to satisfy the height and safety objectives is amazing.

When designing circuit boards, there are also times that rigidity and flexibility must be merged to meet the functionality objectives. For example, the board may have a complex mounting requirement; such as the bending needed to minimize the space needed for pacemakers. Although these boards have the benefit of increased flexibility over the more common rigid PCB structure, there are also drawbacks. A good understanding of both advantages and disadvantages will help us determine when to use this technology. Then we can look at how to optimize our rigid-flex stackup design. 

Rigid-Flex Stackup PCBs: Advantages and Disadvantages

Rigid-flex is one of the three types of common classification for PCBs. The other two are rigid, which most boards are, and flex, sometimes referred to as full flex. 

 

Types of PCB:

  1. Rigid - standard PCB stackup construction type.

  2. Flex - fully bendable board, typically used in vibratory environments or when board installation requires flexibility.

  3. Rigid-Flex - partially rigid and partially flexible construction. 

 

All of the board types listed above may be single or multilayer, where signal traces exist on more than one layer. And for all multilayer boards, you should apply good PCB layer stackup tips. However, in order to choose the best stackup type for your board, you need to know its primary advantages and disadvantages. For rigid-flex stackups, these are listed below.  

 

RIGID-FLEX STACKUP

Advantages*

Disadvantages*

Mounting flexibility

Complex production process

Better thermal resistance due to polyimide usage

Lower production yield

Flex part has no solder joints, connectors

Long manufacturing cycle

Can withstand vibrations and shocks better

Higher fabrication cost

* As compared to a rigid board with the same number of layers.

 

As shown above, there are significant advantages and disadvantages when opting for a rigid-flex stackup design. However, for certain applications, as discussed below, the benefits make this technology well worth the costs.

Applications of Rigid-Flex Circuit Boards

Rigid PCBs dominate the PCB market; however, the use of rigid-flex technology is growing based upon its many benefits. This growth is expected to continue as more industries adopt rigid-flex boards for their applications. Some of the most notable of these are shown below. 

 

Rigid-Flex Applications by Industry

 

  • Consumer Appliances

    • Washing systems

    • Ovens

    • Solar energy systems

  • Telecommunications

    • Wireless communications

    • Routers and servers

    • Communications satellites

  • Medical Equipment

    • Pacemakers

    • Imaging equipment

    • Drug delivery systems

 

Pacemaker and ECG printout

Pacemaker

 

  • Automotives

    • Control systems

    • Air conditioners

    • Navigation systems

  • Industrial

    • Automation systems

    • Test equipment

    • Surveillance systems

  • Aerospace

    • Sensors

    • Communications

    • Control systems

  • Military

    • Communications

    • Guidance systems

    • Tracking systems

 

Rigid-flex boards are utilized in a wide range of applications across many industries, as shown above. Even with this breadth of usage, there are commonalities that can help you optimize your rigid-flex stackup design.

How to Optimize Your Rigid-Flex Stackup Design

Although becoming more common, rigid-flex stackups are special designs. Creating the best boards requires that you incorporate the principles of manufacturing into your design. In order to efficiently and effectively incorporate your contract manufacturer’s (CM’s) DFM and DFA you need the best PCB design tools. 

 

Rigid-flex PCB design

3D view of Rigid-flex design

 

One advanced tool that is particularly helpful in designing rigid-flex stackups is 3D viewability and analysis, as shown in the figure above. This capability allows for evaluation of the board from various angular perspectives and in multiple states of flex. Integration with other capabilities; such as trace layout and signal analysis, allow for accurate design decisions that impact impedance calculations and signal integrity. 

The best PCB design and analysis software for 3D integrated real-time design is Cadence’s OrCAD. Additionally, analog/mixed signal analysis capabilities promote shorter design turnarounds and optimization. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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