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Problems like designing the layers, placing parts, and cutting the circuit are usually easy to spot. However, flexible circuits also face several common material weaknesses. These can include adhesives that expand too much, thin copper layers peeling off flimsy plastic bases, or copper hardening or cracking.
Keeping Flex Circuits Flexible
You must decide how bendable the flexible circuit needs to be before designing it. This seems obvious, but it’s worth repeating.
If the flexible part only needs to bend once during assembly (like in a handheld ultrasound probe) and then stays fixed, you have more choices. You can use more layers and different types of copper (RA or ED).
However, if the flexible part will be constantly bent, moved, or twisted during use, you should reduce the number of layers and choose materials without adhesive.
Do not bend sharply at corners
Ideally, bend flexible circuit copper traces vertically (up/down). When that’s not possible:
1. Keep bends as small and infrequent as you can.
2. Use smooth, tapered curves instead of sharp bends, following the mechanical design.
Use Smooth Curves for Routing
Route traces using smooth curves instead of sharp corners or straight 45° angles. This reduces stress on the copper when bending.
Avoid Sudden Trace Width Changes
Never change a trace’s width abruptly where it connects to a pad – especially at flexible circuit terminals. This creates weak spots where copper can crack over time.
Solution: Use a gradual taper design, unless you add reinforcement, or the circuit won’t bend during use.
Use polygon shapes
It is essential to incorporate a power supply or ground plane on a flexible board. If the significant reduction in flexibility and the potential for copper skin wrinkling are acceptable, solid copper can be utilized. However, to maintain a high degree of flexibility, it is generally recommended to employ shadow polygon copper plating. Additionally, traditional shadow polygons may introduce excessive copper reinforcement along the 0°, 90°, and 45° directions. A more optimized approach involves utilizing a hexagonal pattern. This issue can be addressed by implementing negative layers and arrayed hexagonal pads, allowing for the rapid establishment of shadow polygons through copy-and-paste methods.
Add support to the pads
Plated holes (vias) work well for strengthening. They act like anchors, holding two flexible layers together firmly. This anchoring effect also allows some movement up and down. That’s why many manufacturers suggest adding plated holes, up to 1.5 mils deep, to both rigid-flex boards and flexible circuits.
Pads used for surface-mount parts (SMT pads) and pads for holes without plating don’t have any built-in strengthening. So, you need to add extra reinforcement to these pads to stop the copper from peeling off.
