Last month, my team was producing 500 rigid-flex PCBs for a industrial sensor. We needed precise 0.3mm holes to fit tiny connectors—even a 0.05mm deviation would make the connectors too loose (or too tight). But when we unpacked the first batch, we found a disaster: 80% of the holes were 0.35mm (too big), and 10% were 0.28mm (too small). “These are useless,” our production manager, Jake, said, holding up a PCB. “The connectors won’t fit, and we can’t rework them.”
We traced the problem to our drilling process: the rigid FR4 section and flexible PI section have different hardnesses, so the drill bit was slipping in the PI layer. “Rigid PCBs drill easily—they’re uniform,” our drill technician, Mei, explained. “But rigid-flex has two materials. The bit speeds up in PI (softer) and slows down in FR4 (harder)—that’s why the holes are off.”
Over the next two weeks, we tested 10+ drilling setups, adjusted tools, and finally got the holes to stay within 0.3mm ±0.02mm. This experience taught us: rigid-flex PCB drilling isn’t just “drilling holes”—it’s balancing two materials with different properties. The tips we learned saved us from another batch failure—and they’ll help you avoid the same mistake.
Rigid-flex PCBs mix hard FR4 (rigid sections) and soft PI (flexible sections). This mix causes three big issues during drilling—issues that don’t happen with rigid PCBs:
FR4 is hard (Rockwell hardness ~M100), while PI is soft (~M80). When the drill bit hits FR4, it slows down; when it hits PI, it speeds up. This speed change makes the hole bigger in PI (bit spins faster, removes more material) and smaller in FR4 (bit slows, removes less).
“In our failed batch, the drill bit sped up in the PI layer, widening the hole to 0.35mm,” Mei said. “Then it hit FR4 and slowed—leaving the bottom of the hole at 0.29mm. The hole was uneven, not just deviated.”
PI film is flexible—when the drill bit presses down, the PI bends instead of staying rigid. This makes the bit “wander” slightly, creating holes that are off-center or oval (not round).
“We found oval holes in 15% of the failed PCBs,” Jake said. “The PI flexed when drilling, so the bit didn’t stay straight. The holes looked like eggs, not circles.”
PI film is abrasive—it wears down drill bits faster than FR4. After drilling 50+ PCBs, the bit becomes dull, so it cuts smaller holes (dull bits can’t remove material as well). In our batch, the first 20 PCBs had 0.3mm holes, but by the 50th, holes were down to 0.28mm.
“Dull bits are a silent killer,” Mei said. “With rigid PCBs, a bit lasts 200+ holes. With rigid-flex, it only lasts 50. We didn’t change the bit often enough—that’s why later holes were too small.”
Fixing deviation isn’t about “drilling harder”—it’s about adjusting for FR4 and PI’s differences. Below are the tips that got our holes to 0.3mm ±0.02mm:
A step drill bit has different diameter sections—we used one with a 0.25mm “pilot” section (to guide the bit) and a 0.3mm “final” section (to cut the full hole). This stops the bit from speeding up in PI.
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The pilot section (0.25mm) drills through PI first—since it’s smaller, the bit doesn’t spin too fast.
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The final section (0.3mm) then cuts through FR4—by this point, the bit is guided straight, so it doesn’t slow down unevenly.
Hole deviation dropped from ±0.05mm to ±0.02mm. “The pilot section keeps the bit on track,” Mei said. “No more speeding up or slowing down.”
Choose a step drill bit with a titanium coating—it resists wear from PI, so it lasts 100+ holes instead of 50.
PI flexes because the PCB isn’t clamped hard enough. We used a vacuum clamp (sucks the PCB flat) plus rubber pads on the PI section to keep it rigid.
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The vacuum clamp holds the FR4 section flat (no movement).
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We added 0.5mm-thick rubber pads under the PI section—they press the PI down, so it doesn’t bend when the bit hits it.
Oval holes disappeared. “The PI stays flat now,” Jake said. “The bit goes straight through—no wandering.”
Don’t clamp too hard—too much pressure can crack the rigid-flex transition zone. Test clamp pressure on a scrap PCB first.
We set two different speeds: slower for FR4, faster (but controlled) for PI. This matches the bit speed to the material’s hardness.
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PI section: 15,000 RPM (faster, but not so fast the bit speeds out of control).
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FR4 section: 10,000 RPM (slower, so the bit cuts evenly without slowing down).
Holes were uniform through both layers—no more “big top, small bottom” issues. “Matching speed to material fixes the core problem,” Mei said.
Use a drill press with “variable speed” settings—cheaper presses with fixed speeds can’t adjust for rigid-flex.
Since PI wears bits fast, we now change bits every 60 holes (instead of 200). We also mark each bit with a “use count” to avoid forgetting.
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We keep a log: each bit is numbered, and we write down how many holes it’s drilled.
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When a bit hits 60 holes, we replace it with a new one—even if it looks “fine.”
Hole size stayed consistent from the first PCB to the 500th. “Dull bits were our biggest mistake before,” Jake said. “Now we never skip bit changes.”
Save old bits for testing—don’t use them for production. A “slightly dull” bit still causes 0.02mm deviation.
We added a metal drill guide (with 0.3mm holes) on top of the PCB. The guide keeps the bit straight, even if the PI flexes a little.
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The guide is a thin metal sheet with pre-drilled 0.3mm holes—we align it with the PCB’s hole marks.
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The drill bit goes through the guide first, then the PCB—so it can’t wander.
Off-center holes dropped to 1% (from 10% before). “The guide is a safety net,” Mei said. “Even if something goes wrong, the hole stays in place.”
Use a guide made of stainless steel—aluminum guides wear down too fast from drill bits.
After using these tips, we re-produced the 500 PCBs. Here’s how they turned out:
The industrial sensor client was thrilled: “We’ve had rigid-flex PCBs fail because of bad holes before,” their engineer said. “Yours are perfect—we can assemble the sensors without any delays.”
Our 500 failed PCBs taught us that rigid-flex drilling isn’t just a “production step”—it’s about respecting the differences between FR4 and PI. A step bit guides the cut, tight clamping stops flex, adjusted speed matches material hardness, frequent bit changes fight wear, and a guide adds precision.
For any rigid-flex project, ask: How will FR4 and PI act differently during drilling? Then adjust your process to match. A 0.05mm deviation might seem small, but it can ruin an entire batch. With the right tips, you’ll get holes that are precise, consistent, and ready for use.
Next time you hold a rigid-flex PCB, look at the holes. If they’re round, even, and fit the connectors perfectly, chances are someone used these tips. Drilling isn’t glamorous—but it’s the difference between a working PCB and a useless one.
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