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6 Flex PCB Design Guidelines and Rules

Author Profile img: Amit Bahl

By Amit Bahl

February 3, 2020 | 0 Comments


When designing a printed circuit board for devices such as fitness trackers and hearing aids, reliability is going to be at the top of your list of requirements. Flex PCBs are the go-to options for these types of applications because they’re rugged and reliable, especially compared with conventional ribbon cables. But here are some flex PCB design guidelines you need to follow.

What is a flexible PCB?

In addition to its reliability, a flex PCB also has the ability to fit into tighter spaces and provide higher packaging density, all without the bulky connectors traditionally needed for interfacing with other boards.

“We use flex because we need a good, rugged, dependable interface,” explained flexible circuits expert Vern Solberg in a recent interview. “Wire cables are not always dependable, but flex is. At other times, it is because of the shape or the need to fit the product into a small space, and their flex allows us to connect two rigid boards in a very small space.”

Solberg outlined a few tips for capitalizing on the inherent advantages of flex PCB in the use cases we mentioned earlier:

1. Consider the flexible electronics applications

Before the design stage begins, make sure you know the answers to the following questions: How many times is your PCB going to flex—will it be a dynamic or static board? What is the bend radius? How thin or thick can the overall PCB be? Stack-ups for dynamic and static PCBs vary, so the application and type of board must be known ahead of time.

2. Design with your PCB stack-up in mind

Request a stack-up from your manufacturer before designing begins. It is crucial that you know what stack-up you are designing. Rigid-flex is the simplest configuration that will allow you to reduce the number of connectors, which will also increase wiring density and reliability. Take a look at our Flex Design Guide, for example, stack-ups.

Flex PCB Design Guide - Cover Image

Flex PCB Design Guide

10 Chapters - 39 Pages - 45 Minute Read
What's Inside:
  • Calculating the bend radius
  • Annular ring and via specifications
  • Build your flex stack-up
  • Controlled impedance for flex
  • The fab and drawing requirements

One way to get off to a good start here and avoid rework (like dimensional modifications) down the road is to build models early on, using paper or mylar. Primary components can be arranged to ensure proper fit, while rigid PCB sections can be glued on as well.

Having a face-to-face meeting with the supplier is the best way to ensure that you’re on the same page in terms of where the overall PCB process is headed. This meeting can also help ensure that flex PCB design guidelines and capabilities are well-understood.

3. Work with your PCB Supplier

Understand that flex and rigid-flex design rules are different. Flex designs require button plating. For flex, annular rings need to be larger than in rigid designs. Each supplier may have its own set of design rules and recommendations. PCB design and layout will also be affected by your planned circuit density and line spacing.

Another thing you should always work with your supplier on is material selection. The material should be suitable for the environment and the application in which the flex PCB will operate. Flex materials themselves are pretty durable, but flex laminates may be less suitable for certain applications.

We are just scraping the surface of what you can do to keep your flex PCB design on track. In the second part of this series, we will look at a few other things to keep in mind, including more on modeling and design rules.

During a recent interview, he offered several helpful pointers for ensuring maximum reliability, manufacturability, and economy when planning out your flex PCB, including:

4. Stick to standard flex PCB design guidelines

Stick to standard design rules, since it will enable you to avoid using advanced technologies and also keep your overall costs down. Putting your flex layer within your rigid layers is one of the most reliable ways to design your rigid-flex PCB. However, multilayer boards with greater complexity are increasingly in demand as pin counts rise and thousands of I/Os get incorporated into some devices.




5. What does your stack-up look like?

Optimize for the application in question. There is no “one size fits all” approach to flex PCB design. For example, you may find yourself needing something more sophisticated than single-layer flex, meaning you may need to consider rigid-flex and also make key decisions about options, such as whether to use stiffeners.

Know your PCB supplier and their capabilities—you don’t want to end up designing a board that can’t be manufactured. Suppliers have their own particular design rules and recommendations that should be taken into account from the get-go.




6. Testing and modeling

Prototyping allows you to find out what requires such attention—e.g., maybe you should look again at the reinforcement in the board or modify its dimensions. However, it is still a good idea to avoid this type of trial and error and the rework that comes with it, if possible. Solberg recommends making paper doll models to ensure that you get it right the first time.

“I have created a number of models simply with a piece of stiff paper, or mylar,” explains Solberg. “I will glue rigid sections to it and put on the primary components. I make sure the thing fits first, and then it is up to working with the supplier to choose materials that are going to withstand whatever environment you have to deal with.”

The polyimides used in coverlay are rugged enough to handle most environments. However, issues are more common with laminates, which are less robust under harsh conditions. Laminates are not suitable with certain chemicals or adhesives, and without reinforcement, the vias are liable to crack. The copper found in flex PCBs differs from the copper found in rigid PCBs. Flex materials also do not have glass weave, or suffer from caffing—i.e., material cracking once we drill into it.

Sierra Circuits is an experienced PCB provider, with years of expertise in flex manufacturing. We offer turnkey service for high-reliability flex PCB in two days or less.

For more information on how Sierra Circuits can assist you with your next design, visit our flex PCB capabilities.

Read flex PCB design guidelines for manufacturing.

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