The Learning Curve: Your First Flex Circuit

As seen in the July issue of The Flex007 Magazine: http://iconnect007.uberflip.com/i/1007258-flex-july2018/58

Can you relate to this? You are tackling your first flexible circuit design. It is a simple circuit, with just two layers. Lines and space are generous, the hole size isn’t pushing any limits, and this seems like a perfect design to cut your teeth on.

You do your research, complete the layout, send the design package in for a quotation and then place the order, confident in the process. There are a few engineering questions related to materials, and you make a note for future applications: Be more specific about the coverlay requirements and whether flexible solder mask or film-based coverlay is needed. Things are going smoothly. The circuits are delivered, assembled, installed right on schedule. But something isn’t working. Now the fun begins—troubleshooting. Where do you start? After the painful process is complete, you discover that one of the components was too heavy and bulky for the flexible circuit to support without reinforcement and traces had been broken during installation. A quick redesign adds a rigidized stiffener, the circuits are ordered again, and the project moves forward.
In my experience through the years, when first working with flexible circuits or rigid-flex circuits, this is a learning curve that everyone goes through.

With this learning curve in mind, I reached out to customers and industry friends to ask
them to share some of their first experiences with flex, “gotcha” moments and advice for
those new to flexible circuit design. A few common themes stood out.

Materials
Material selection is critical in some designs, especially dynamically flexing applications,
and anecdotal information tells the story that the options available are more complicated
than one would anticipate when first working with flex. Several decisions must be made:
RA or ED copper, adhesive-based materials, or adhesiveless materials, copper thickness,
dielectric thickness, coverlay or flexible solder mask, what type of stiffener, polyimide or
FR-4? Skill and knowledge is required to balance those decisions with the end use of the
circuit, available materials, and cost.

There were a few stories about “the flex that didn’t flex” when a multilayer stack-up
became so thick there was no way to bend the circuit without cracking the copper. This
seems to be a common occurrence—it has happened to me in the past! As a side note,
most were resolved using unbonded layers in the stack-up. Another common message was that material lead time seems to be longer than expected, with more questions about the stack-up than anticipated. It is true there are a lot of variables in inventory, preferences, and capabilities between fabricators. The piece of advice given most often was, “Work with your fabricator during the design and to understand their capabilities.” Great advice.

Conductor Routing
Conductor routing practices was another category that stood out in the conversations
about lessons learned. Nearly everyone has a story about cracked traces and the learning
curve they went through to be confident in the flex design and performance. A flexible circuit is a hybrid of mechanical and electrical design. This introduces a lot of variables. I’ll share one story that stood out. The application required a double-sided circuit that was expected to be flexed during installation and test, but not over the life of the product. The first design used solid copper for shielding and was manufactured with adhesive-based materials. It cracked in the bend area during installation.

Several new ideas were implemented for the second revision. The traces were rerouted
perpendicular to the bend area, materials were changed to adhesiveless, and crosshatch shielding was added. These are all great options for improving flexibility. The second
revision cracked in the same location.

For the third revision, traces were routed to just one side of the bend area, and all
copper was removed in that area. In addition, polymide stiffeners were added to help
more specifically direct where the bend was occurring. Even though all the best practices
were employed in this design, the third revision cracked also. The problem was resolved
when they realized that the circuit was not just absorbing the stress from the known bend area, but as the unit was working stress was being applied in another axis. A slight redesign of the unit eliminated the cracking. This had to be a painful and frustrating experience for all involved, but it also was a good lesson in ways to improve the flexibility in any design.

I received a lot of real-world advice for conductor routing. A few of the key items
included: avoid abrupt changes in conductor size and direction, route conductors uniformly and perpendicular to the bend area, add radius to all inside corners, make pad patterns bigger to add stress relief, and add anchoring tie points to the solder pads to reduce the opportunity for pad lifting during assembly.

Improving Flexibility
Another common topic of discussion was the learning curve for options to improve flexibility. The previous example provides many tips and tricks pertaining to conductor routing. Wisdom was shared for additional options to consider, especially relevant for dynamically flexing and applications and when tighter than recommended bend radiuses are required. To share a few key pieces of advice, consider removing material in the bend area; this could be cut-outs in the circuit or removing coverlay and adhesive to provide a thinner package. Eliminate the ED copper in a design by requesting button plating for your design and adding copper only to the plated through-holes, not the rest of the panel. Add stiffeners to move stress points to other areas in the package that may be better able to withstand the stress.

This process was certainly interesting. Everyone seems to have a favorite story of lessons
learned when starting to work with flexible circuits. Most are told with a slightly humorous spin after the fact, but I am certain it felt anything but funny at the time. Flexible circuits are a growing portion of the PCB market and more and more applications are expected to require flexible circuits.

For those new to flex, or anyone considering using a flexible circuit in their next design,
there was one piece of advice that was repeated by nearly everyone I spoke to: Work with your fabricator early in the design. I couldn’t agree more with that advice. Not only will this help avoid material availability issues, your fabricators work with flexible circuit designs day after day and are happy to share their experience to help ensure the product works as you intend it. Take advantage of the expertise!

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