Last month, my team finished lamination for 300 rigid-flex PCBs destined for a industrial robot controller. We peeled back the protective film, excited to ship them—only to freeze. Half the PCBs had silvery bubbles between the rigid FR4 and flexible PI layers, and 10% were already delaminating (the layers were peeling apart). “These are worthless,” our lamination technician, Lee, said, picking up a PCB and squeezing it—bubbles popped like tiny balloons.
We’d followed the standard lamination process for rigid PCBs, but rigid-flex is a different beast. “FR4 and PI have different heat expansion rates,” our materials engineer, Priya, explained. “When we heated them too fast, the PI expanded quicker than the FR4—trapping air and creating bubbles.” Over the next 10 days, we tested 8 different lamination setups, adjusted temperatures, and swapped adhesives until we got bubble-free, tight bonds.
That disaster taught us: rigid-flex PCB lamination isn’t just pressing layers together—it’s balancing two materials that behave differently under heat and pressure. The solutions we found saved us from scrapping another batch—and they’ll help you avoid the same headache.
Rigid-flex PCBs mix rigid FR4 (glass-reinforced epoxy) and flexible PI (polyimide) layers, held together by adhesive. This mix causes two big problems during lamination that rigid PCBs rarely face:
FR4 expands slowly when heated (coefficient of thermal expansion, CTE: ~13 ppm/°C), while PI expands much faster (CTE: ~25 ppm/°C). If you heat the stack too quickly, the PI stretches more than the FR4, creating gaps where air gets trapped—those gaps become bubbles.
“In our failed batch, we heated from 25°C to 180°C in 10 minutes,” Lee said. “The PI expanded so fast it pulled away from the FR4, and air got stuck. The bubbles were everywhere—some as big as a pinhead, some as big as a dime.”
Standard rigid PCB adhesives (like FR4 prepreg) aren’t designed for PI. They either don’t stick well to PI’s smooth surface or cure too quickly, leaving unbonded spots that turn into delamination.
“We used FR4 prepreg for the first batch,” Priya said. “It bonded to the FR4 great, but it just peeled off the PI. After 3 days in storage, 10% of the PCBs had layers coming apart—you could lift the PI with your fingernail.”
Even tiny dust particles or oil from fingers can get between the FR4, PI, and adhesive. These contaminants act as barriers— the adhesive can’t stick to the layers, so bubbles form around the dirt, and delamination follows.
“We found dust specks in 80% of the bubbles we cut open,” Lee said. “Our cleanroom had a leak—dust was getting on the PI sheets before lamination. The adhesive couldn’t bond over the dust, so air filled the space.”
Fixing rigid-flex lamination isn’t about “pressing harder”—it’s about matching the process to FR4 and PI’s properties. Below are the steps that gave us bubble-free, delamination-resistant PCBs:
Instead of heating the stack quickly, use a 3-stage profile to let FR4 and PI expand gradually. This prevents air from getting trapped.
-
Preheat (25°C → 80°C, 20 minutes): Softens the adhesive without causing major expansion.
-
Ramp (80°C → 150°C, 30 minutes): Slow heating lets FR4 and PI expand at similar rates—no gaps form.
-
Cure (150°C → 180°C, 40 minutes): Final cure to set the adhesive, with pressure held steady.
Bubbles dropped from 50% to 2%. “The slow ramp is key,” Lee said. “We watched the stack with a thermal camera—no more PI pulling away from FR4.”
Don’t skip the preheat stage. Skipping it makes the adhesive cure too fast, trapping air before it can escape.
Use adhesives designed specifically for bonding FR4 to PI—these have a “tack” that sticks to PI’s smooth surface and cures at temperatures that work for both materials.
We switched from FR4 prepreg to a modified epoxy adhesive (rated for PI bonding) with a 120°C cure temperature. It has a rubbery texture after curing, which flexes with the PI instead of cracking.
Delamination dropped to 0%. “We tested the bond by pulling the PI and FR4 apart,” Priya said. “The adhesive tore before the layers separated— that’s how strong it is.”
Ask your adhesive supplier for “peel strength” data (how hard it is to pull layers apart). Aim for ≥5 N/cm—this ensures the bond survives flexing.
Even small contaminants ruin adhesion. We added 3 cleaning steps to our process to keep layers spotless:
-
Wipe PI/FR4 with isopropyl alcohol (IPA): Removes oil from fingers or machinery.
-
Use a lint-free cloth (not paper towels): Paper leaves tiny fibers that cause bubbles.
-
Vacuum the lamination press: Sucks up dust before closing the press—we added a HEPA filter to the vacuum.
Contamination-related bubbles dropped to 1%. “We haven’t found a single dust speck in the bubbles since,” Lee said.
Wear nitrile gloves (not latex) when handling layers—latex leaves powder that acts like dust.
Uneven pressure in the press creates thin spots in the adhesive—these turn into bubbles. We adjusted our press to apply pressure evenly across the entire PCB.
-
We used a “pressure pad” (a thick silicone sheet) between the press plate and the PCB stack. The pad flexes to fill gaps, so pressure is the same everywhere.
-
We checked pressure with a test film (changes color where pressure is low) before every batch.
Hot spots (areas with low pressure) disappeared. “The test film used to show light spots (low pressure) near the PCB edges,” Lee said. “Now it’s all dark—even pressure everywhere.”
Use 2–3 kg/cm² pressure for rigid-flex. Too much pressure crushes the PI; too little leaves gaps.
Adhesives often have tiny air bubbles trapped inside. If you don’t remove them, they expand during heating and become bubbles in the PCB. We added a degassing step to remove air from the adhesive.
We put the adhesive sheets in a vacuum chamber (0.1 bar pressure) for 15 minutes before lamination. The vacuum pulls air out of the adhesive, so there’s nothing to expand during heating.
Adhesive-related bubbles dropped to 0.5%. “We cut open a few PCBs to check—no air bubbles in the adhesive at all,” Priya said.
Degas at room temperature—heating the adhesive during degassing can make it cure early.
After applying these 5 solutions, we re-laminated 300 PCBs. Here’s how they performed:
The industrial robot client was relieved: “We’ve had rigid-flex PCBs fail in the field because of delamination,” their engineer said. “Yours hold up—our robots run 24/7, and the PCBs don’t crack or peel.”
Our failed batch taught us that rigid-flex lamination isn’t a copy-paste of rigid PCB lamination. FR4 and PI expand differently, need special adhesives, and hate contaminants—ignore these, and you’ll get bubbles and delamination.
The solutions are simple: heat slowly, use the right adhesive, clean thoroughly, press evenly, and degas the adhesive. These steps don’t add much time, but they save you from scrapping batches and losing clients.
Next time you hold a rigid-flex PCB, look at the FR4-PI edge. If there are no bubbles and the layers don’t peel, someone followed these steps. Lamination isn’t glamorous—but it’s the difference between a PCB that works for years and one that fails in weeks.
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