Last year, my team was building a commercial drone for agricultural mapping. The drone used a rigid-flex PCB to connect the GPS module (on the rigid FR4 section) to the flight controller (on the flexible PI section). During field tests, disaster struck: every time the drone’s arm bent slightly (from wind), the GPS signal would cut out—causing the drone to drift off course.
“We checked the wiring a dozen times,” our RF engineer, Leo, said, holding the PCB under a microscope. “The traces look fine, but the signal dies at the rigid-flex transition zone.” We ran signal tests and found the problem: the transition zone (where FR4 meets PI) was causing signal reflection—like a speed bump for electrical signals. The rigid section transmitted signals smoothly, but the flexible section distorted them, and the gap between them made it worse.
Over the next two weeks, we redesigned the PCB’s transition zone, adjusted trace routing, and finally got the GPS signal to stay stable—even in 20mph winds. That experience taught us: rigid-flex PCB signal transmission isn’t just about “connecting two zones”—it’s about making the signal forget there’s a gap between rigid and flexible. The fixes we found saved the drone project—and they’ll help anyone struggling with signal issues in rigid-flex designs.
Rigid and flexible zones are made of different materials, and that difference creates three signal-killing issues you don’t see in all-rigid or all-flex PCBs:
FR4 has a high dielectric constant (εr ≈ 4.5), while PI film has a lower one (εr ≈ 3.5). When a signal moves from FR4 to PI (or vice versa), it hits a “dielectric boundary”—some of the signal bounces back (reflection) instead of moving forward. This weakens the signal and causes delays.
“In our drone’s PCB, 30% of the GPS signal reflected at the transition zone,” Leo said. “By the time it reached the flight controller, it was too weak to use—hence the drift.”
Impedance (resistance to electrical signals) needs to stay consistent across the PCB. But FR4’s higher dielectric constant makes impedance lower in the rigid zone, while PI’s lower constant makes impedance higher in the flexible zone. This “impedance mismatch” twists the signal’s shape (distortion), turning clear data into garbled noise.
“We tested the signal with an oscilloscope,” said our PCB designer, Priya. “In the rigid zone, the signal was a clean square wave. In the flexible zone, it was a messy, rounded wave—unrecognizable to the flight controller.”
The transition zone bends more than other parts of the PCB (especially in moving devices like drones). Over time, the copper traces here stretch or crack—creating “intermittent breaks” in the signal. One minute the signal works; the next, it cuts out.
“After 500 bends of the drone’s arm, the transition zone trace had a tiny crack,” Leo said. “It was invisible to the eye, but it killed the GPS signal every time the arm moved a certain way.”
Fixing rigid-flex signal issues isn’t about “making the signal stronger”—it’s about smoothing the gap between rigid and flexible. Here are the changes that fixed our drone’s GPS:
Impedance mismatch is the #1 cause of signal problems. To fix it, adjust the trace width and dielectric thickness in each zone to keep impedance consistent (usually 50Ω or 75Ω for most signals).
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Rigid Zone (FR4): We narrowed the trace width from 0.2mm to 0.15mm. FR4’s high dielectric constant needs a narrower trace to keep impedance high.
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Flexible Zone (PI): We widened the trace width from 0.15mm to 0.2mm. PI’s low dielectric constant needs a wider trace to keep impedance low.
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Result: Impedance stayed at 50Ω across both zones—no more distortion.
Use an impedance calculator (free online tools work!) to design trace widths. Input the dielectric constant (FR4 = 4.5, PI = 3.5) and desired impedance— the tool will tell you the right trace width.
Instead of letting the signal jump directly from FR4 to PI, create a 5–10mm “gradient zone” where the trace width changes gradually. This eases the signal across the dielectric boundary, reducing reflection.
We made a 8mm gradient zone: the trace started at 0.15mm (rigid FR4) and slowly widened to 0.2mm (flexible PI) over 8mm. No sudden jumps—just a smooth transition.
Signal reflection dropped from 30% to 5%. “The oscilloscope showed almost no bounce-back,” Leo said. “The signal moved from rigid to flexible like there was no gap.”
Avoid sharp corners in the gradient zone. Use gentle curves (radius ≥1mm) to keep the signal flowing smoothly.
To prevent trace cracking (and intermittent signals), reinforce the transition zone with extra material:
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Double-Layer Trace: We added a second layer of copper to the transition zone trace—making it 0.03mm thick instead of 0.015mm. Thicker traces resist stretching.
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FR4 Stiffener: We glued a small 3mm × 8mm FR4 stiffener over the transition zone. It reduced bending stress on the trace.
After 1,000 bends of the drone’s arm, the trace had no cracks. “The signal stayed stable even when the wind was blowing hard,” Priya said.
Don’t cover the entire transition zone with stiffener—leave 2mm of flexible PI exposed. Too much stiffener will make the zone rigid, defeating the purpose of the flexible section.
Ground planes act like “signal guides”—they keep the signal focused and reduce noise. But they only work if they’re present on both zones and connected properly.
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We added a copper ground plane to the bottom of both the rigid and flexible zones.
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We used two “vias” (holes through the PCB) in the transition zone to connect the rigid ground plane to the flexible one. This created a continuous ground path.
Signal noise dropped by 40%. “The GPS signal was no longer drowned out by electrical noise from the drone’s motor,” Leo said.
Place the ground plane as close to the signal trace as possible (≤0.5mm). The closer they are, the better the signal guidance.
Long, winding traces weaken signals—especially in rigid-flex PCBs, where each bend adds tiny losses. Keep traces between rigid and flexible zones as short (≤50mm) and straight as possible.
We rerouted the GPS trace from a winding 70mm path to a straight 35mm path. We removed two unnecessary bends that were adding signal loss.
Signal strength increased by 25%. “The flight controller received a strong, clear GPS signal—even at the drone’s maximum altitude,” Priya said.
Avoid “stubs” (short, unused trace segments) in the transition zone. Stubs act like antennas, picking up noise and reflecting signals.
After applying these 5 fixes, we tested the drone in the field. Here’s how the rigid-flex PCB’s signal performed:
The agricultural client was thrilled: “We used to lose 10% of our mapping data due to GPS drift,” their field manager said. “Now we get 100% accurate maps—your rigid-flex design fixed the problem.”
Our drone’s GPS failure taught us that rigid-flex PCB signal transmission is all about harmony between two different zones. FR4 and PI will never be the same, but you can design the PCB to make the signal treat them as one. By matching impedance, adding a gradient transition, reinforcing traces, using ground planes, and keeping traces short, you eliminate the “gap” that kills signals.
For any rigid-flex project, ask: How will the signal move from rigid to flexible? Then design every part of that path to ease the transition. A signal that doesn’t notice the gap is a signal that works—whether it’s powering a drone’s GPS, a medical device’s sensor, or an industrial machine’s controller.
Next time you use a device with a rigid-flex PCB, remember: the clear, stable signal you’re relying on isn’t an accident. It’s the result of careful design that bridges the gap between rigid and flexible. And that’s the key to great rigid-flex signal transmission.
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