Top Tips from Experienced Flex Designers
In general, advice given about flexible circuit fabrication is split between knowledge of the fabricator and their process, and communication. Knowledge of the printed circuit board fabrication process is not as common as it once was, and this is even more apparent with the subtleties of flex manufacturing. Understanding the manufacturing process helps to illuminate areas that are more challenging for fabrication, as well as areas that are less challenging, with limits that could perhaps be pushed in the give-and-take of design trade-offs. Are you able to visit a fabricator for a day or two to see the process in person? If not, ask a fabricator to come to you, most are more than happy to help educate customers. Take advantage of their knowledge!
Another key piece of PCB fabrication knowledge is understanding your fabricator’s capability matrix. A facility specializing in two- or four-layer flex is likely not the best choice for a complex rigid flex. Designing with a 3-mil line/space or micro vias when your fabricator does not have those capabilities, will require the qualification of a new supplier. This is something that needs to be considered as part of the project timeline. Understanding your fabricator’s capabilities and advanced planning when you are going to push those limits or need to qualify a new source is key.
Communication with your fabricator was another often-repeated recommendation. Topping the list of suggestions was planning to have a design review with your fabricator early in the design process to ensure it is buildable with in-stock materials and processes. This was followed closely by being extremely diligent about reviewing the documentation to be sure it matches the design. Following, is one of my favorite quotes from these conversations:
“For rigid boards, I really don’t need a drawing and can get the fabrication vendor where he’s going based on material and a starting point. But flex is different, often called the black magic of the business.” As part of the documentation, it is also helpful to clearly communicate the flex requirements. It is difficult for a fabricator to know how your design will be flexed in end-use but taking the time to communicate this clearly and asking for their review of the flex zones, has, and will certainly help avoid missteps and costly revisions that could be avoided.
There were several flex-specific things that were mentioned repeatedly:
- Anchors; be sure to add anchor tie points from a solder pad to reduce the opportunity for pad lifting.
- Cut-outs; use cut-outs to keep the flex more pliable (picture a net)
- Mechanically reduce all stress and tear points
- Add layer-to-layer crosstalk by offsetting traces on adjacent layers
- To avoid cracks or stress points, add stiffeners when required
- Route with smooth or curved lines to avoid sharp bends
- Route uniformly and perpendicularly to fold lines
- Avoid abrupt changes in conductor size and direction
- Via locations are critical—avoid placing them close to a fold or stress or tear point
- Know how it will bend and if a fold could become a crease which can break the trace
- Make pad patterns bigger to add stress relief
- Radius all cut lines, never have a sharp inside corner
There are three key questions that stood out when collecting this feedback. First, is the flex going to be flex-to-install or dynamic flex? Second, how flexible does the flex need to be? And third, is the material selected common material and in stock or does it require a special order? Understanding bend radius calculations and how material selection can impact thickness and flexibility is critical. I have done previous blog posts that provide some (almost) comical references to the “flex that didn’t” flex. It happens to most of us at some point! With multi-layer construction, flexibility can diminish quickly depending on materials selected. There are also past blog posts that provide more detail about material selection. From this discussion a few top tips to take away are:
- Be careful not to over-spec copper weight
- Work with your fabricator early in the process to use common, in-stock materials when possible
- Reduce adhesive and coverlay in bend radius areas if possible, to keep that area thinner and more flexible
- Use “unbonded” layers with multilayer construction to improve flexibility
- The material callouts need to be perfectly clear
- Model or use “paper dolls” to test proper alignment, orientation and bend areas
- Interestingly, when talking with eight highly experienced flex designers, their messages were nearly identical. If there are areas that you would like to dig into more deeply, please comment or reach out to me.