Avoiding Common Flex PCB Errors and Designing for Success

A successful flexible PCB often depends on two things: the manufacturability of your design, and your relationship (e.g., level of collaboration) with your supplier. Say you've decided that flex is the right fit for your application. Already, you have a bevy of additional choices in front of you. Will you go with a single-sided (with or without a stiffener) or multilayer flex PCB, or might a rigid-flex PCB work in this situation? Can your PCB supplier make a board that also meets all of these requirements?

Answering these questions and avoiding costly errors and delays is much easier when you sit down with your supplier and assess the options available to you. Relatively simple designs such as single-layer flex are the most economical, but cost isn't everything. Make sure the flex PCB you choose is the best technical fit for your use case. Depending on application, a higher manufacturing cost will cut on overall cost in the long run. Pay close attention to the the complexity of the flex PCB application as well as the capabilities and recommendations of your supplier.

"Give [your supplier] an idea of what you want to do, and make sure they can manufacture your design," explains flexible circuits expert Vern Solberg. "I've always advocated knowing your supplier's capabilities before you dive into something. You can design a lot of things that can't be built. A lot depends on line spacing, circuit density, and the like."

Understanding some common mistakes with flex PCB design

Solberg also highlighted a few places where flex PCB design issues often occur. Common issues include:

  • Circuit traces that are too close to openings—the overall design might still be feasible, but its traces are close to the edge, and shortages may occur. For coverlay, the trace to mask opening can be as close as 3 mils. For soldermask, 3 mils could lead to misregistration or undercut issues (relating to the imaging of the soldermask). Sierra suggests staying a distance of at least 4 mils for soldermask.
  • Confusion about the coverlay. Keep in mind that coverlay is not the same as soldermask. One of the largest differences between coverlay and soldermask is their state: coverlay is a solid material made of plastic polyimides, or plastic with adhesive, while soldermask is a liquid. Keep coverlay dam sizes a minimum of 10 mils; soldermask dam sizes should be a minimum of 4 mils.
  • Incorrect hole sizes—they might be too big for component mounting, or there might be excessive soldering.

In order to optimize performance, Solberg recommends keeping the interfaces between components as short as possible. At the same time, designers should consider potential inspections, testing, and re-workability up front as they plan things out.

Keeping things simple in terms of the design and the materials can help you steer clear of some of the flex PCB problems we outlined here. Of course, simplicity isn't always possible, since some applications and designs will require more intricate PCBs.

Solberg recommends standard materials when possible because they have proven track records when it comes to their throughputs and yields. However, he also acknowledged that really thin alternatives (i.e., thin dielectrics and copper) may be necessary for certain applications.

"There is a learning curve [with flex] and you keep it somewhat simple at the beginning and you can go into more complex applications as you grow," says Solberg. "Designers have to be aware that if they need that [complexity], they need it. But if they don't, reconsider some simplification of the product."

Getting a great flex PCB starts with finding a top-notch PCB supplier like Sierra Circuits. Check out our main flex PCB page to find out more, and also be sure to take a look at our flex PCB design guide.

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