Last year, my team was testing a prototype for a foldable phone with a rigid-flex PCB. We’d spent months perfecting the hinge and screen, but on the 200th fold test, we heard a faint “crack.” The phone’s inner screen went black—when we pried it open, the rigid-flex PCB’s transition zone (where the rigid section meets the flexible one) had split cleanly.
“We designed it to fold 10,000 times,” our engineer, Lila, said, holding the broken PCB. “But we forgot to reinforce the part that bends the most. This isn’t just a manufacturing flaw—it’s a design mistake.”
That moment was a wake-up call. Rigid-flex PCBs are supposed to handle bending, but they break when you ignore the “stress points” where folds and twists happen most. Over the next two months, we overhauled our design, tested 20+ prototypes, and finally created a PCB that survived 15,000 folds without breaking.
In this article, I’ll share the design mistakes that broke our foldable phone PCB, the practical tips that fixed it, and how to apply these lessons to any rigid-flex project—from wearables to industrial sensors.
Before we fixed our design, we first identified the three main reasons rigid-flex PCBs fail during bending. These “stress points” are where 90% of breakages happen:
The area where rigid FR4 meets flexible PI (the transition zone) is the biggest weak spot. When you bend the PCB, all the stress 集中 s here—if there’s no reinforcement, the PI film tears or the adhesive between layers splits.
Our foldable phone PCB had a transition zone with no extra support. “Every fold pulled the rigid and flexible sections apart,” Lila said. “After 200 folds, the adhesive gave out, and the PCB split.”
Flexible sections made of thin PI film (0.075mm or thinner) or narrow widths (less than 3mm) can’t handle repeated bending. The copper traces inside stretch and break, or the PI film itself cracks.
“We tested a prototype with a 2mm-wide flexible zone,” said our lab technician, Raj. “It broke after 500 folds— the PI film was too narrow to distribute stress evenly.”
If copper traces run parallel to the direction of the bend (the “bend axis”), they stretch every time the PCB folds. Over time, the copper fatigues and snaps—even if the PI film is intact.
“Our first PCB had traces running along the fold,” Lila explained. “After 100 folds, the traces looked fine, but under a microscope, we saw tiny cracks. By 200 folds, they’d completely broken.”
Fixing our foldable phone PCB wasn’t about making it “stronger”—it was about distributing stress and protecting the weak points. Below are the tips that took our PCB from 200 folds to 15,000 folds:
The transition zone needs extra support to handle bending stress. Add a small FR4 stiffener (0.1–0.2mm thick) that covers part of the rigid section and part of the flexible section—this spreads stress across a larger area.
-
We cut a 6mm × 3mm FR4 stiffener and glued it over the transition zone: 4mm on the rigid FR4 section, 2mm on the flexible PI section.
-
We used a flexible acrylic adhesive (not rigid epoxy) so the stiffener could bend slightly with the PCB—no cracking.
The transition zone stopped splitting. Our next prototype survived 5,000 folds without any damage to the transition area.
Don’t cover the entire flexible section with the stiffener—leave enough PI film exposed to keep the PCB bendable. A 2–3mm overlap on the flexible side is enough for reinforcement.
Thicker PI film and wider flexible sections distribute bending stress better. For PCBs that fold often (like foldable phones or wearables), use:
-
PI Film Thickness: 0.125mm (not 0.075mm)—it’s 50% more durable and resists tearing.
-
Flexible Zone Width: At least 4mm (not 2–3mm)—wider zones spread stress so no single point takes too much force.
-
We switched from 0.075mm PI film to 0.125mm and widened the flexible zone from 3mm to 5mm.
-
We also rounded the edges of the flexible zone (radius 1mm) to avoid sharp corners that catch and tear.
The flexible zone stopped cracking. Our prototype went from 500 folds to 8,000 folds before showing any wear.
“Test different widths and thicknesses in the lab. We found that 0.125mm PI film with a 5mm width was the sweet spot—strong enough to resist bending, but flexible enough for the phone’s hinge.”
Traces that run perpendicular to the bend axis (across the fold, not along it) don’t stretch when the PCB bends—they just move slightly. This prevents copper fatigue and breakage.
-
We redrew the copper traces so they ran across the flexible zone (perpendicular to the fold) instead of along it.
-
For traces that had to run along the bend (like power traces), we added small “zigzag” sections—these act like springs, absorbing stretch without breaking.
Copper trace breakage dropped to zero. Our prototype’s traces looked intact even after 15,000 folds.
“Use a 45° angle if you can’t route traces perfectly perpendicular. Avoid straight lines along the bend axis—even a small zigzag helps reduce stress.”
For extra durability, add a thin PI reinforcement film (0.025–0.05mm thick) to both sides of the flexible zone. This doubles the PI film’s strength without making it too stiff.
-
We bonded a 0.05mm PI film to both sides of the flexible zone, extending it 2mm onto the rigid section to reinforce the transition area too.
-
We used a heat press (120°C for 5 minutes) to ensure the reinforcement film stuck firmly—no peeling.
The flexible zone became resistant to punctures and tears. We accidentally dropped a small tool on the flexible zone, and it didn’t break—something the old design would have failed.
Use the same brand of PI film for the reinforcement as the base flexible zone—this ensures they bond well and bend together.
Heavy components (like connectors, capacitors, or chips) add weight and stress to the flexible zone. Mount them on the rigid section instead—this keeps the flexible zone light and free to bend.
-
We moved a small connector from the flexible zone to the rigid section, using a short FPC “branch” to connect it (the branch was part of the flexible zone but only held the connector’s pins).
-
We also replaced two 0402 capacitors on the flexible zone with smaller 0201 capacitors (lighter and thinner) to reduce stress.
The flexible zone no longer had extra weight pulling on it. Our prototype folded more smoothly and lasted longer in tests.
“If you must put a component on the flexible zone, use the smallest, lightest one possible. A 0201 capacitor weighs 0.002g—half the weight of a 0402—and puts half the stress on the PI film.”
After applying these five tips, we tested the final prototype in our lab:
A phone manufacturer tested it too, and they were impressed: “We’ve had rigid-flex PCBs break after 2,000 folds,” their engineer said. “Yours lasted 15,000—this is the durability we need for foldable phones.”
Our broken foldable phone PCB taught us that rigid-flex durability isn’t about using the thickest materials or the strongest adhesive—it’s about designing with the bend, not against it. The transition zone needs reinforcement, traces need to be routed correctly, and flexible zones need the right size and thickness.
For any rigid-flex project, ask yourself: Where will this PCB bend most? What parts will take the most stress? Then design to protect those areas. A small FR4 stiffener, a wider flexible zone, or a well-placed trace can turn a PCB that breaks after 200 folds into one that lasts 15,000.
Next time you use a foldable phone, a smartwatch, or any device with a bending PCB, remember: it’s not just the materials that make it durable—it’s the design choices that protect it from breaking. And that’s the key to great rigid-flex PCB design.
Founded in 2009, our company has deep roots in the production of various circuit boards. We are dedicated to laying a solid electronic foundation and providing key support for the development of diverse industries.
Whether you are engaged in electronic manufacturing, smart device R&D, or any other field with circuit board needs, feel free to reach out to us via email at sales06@kbefpc.com. We look forward to addressing your inquiries, customizing solutions, and sincerely invite partners from all sectors to consult and collaborate, exploring new possibilities in the industry together.
English
Wechat
+86 13670210335
+86 13670210335
E-mail