Behind the Scenes with Flex PCBs, Part 1

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. In addition to its reliability, a flex PCB also has the ability to fit into tighter spaces and provide higher packaging density, and 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 there flex allows us to connect two rigid boards in a very small space.”

How to ensure maximum reliability and manufacturability in a flex PCB

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

Consider the application
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? Stackups for dynamic and static PCBs vary, so the application and type of board must be known ahead of time.

Design with your PCB stackup in mind
Request a stackup from your manufacturer before designing begins. It is crucial that you know what stackup you are designing to. 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 Flexible Design Guide for example stackups.

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 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 design rules and capabilities are well-understood.

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 for flex rather than rigid. 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.

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

To continue reading about flexible PCBs, check out part two.



Flex Design Guide

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