Rigid-board habits do not transfer to flex. Thin stacks deflect, adhesives smear, and the failure that matters most is a particle pressed into the surface rather than a burr left on it — the parameter side sits in the drilling parameter ranges, and the consumable side in the backing grades used on thin panels.
Flex and polyimide drilling is usually treated as rigid drilling with a thinner panel. It is not. The stack deflects under the pressure foot, the adhesive layers soften at a lower temperature than the dielectric, and the defect that ruins the part is often a particle embedded in the coverlay rather than a burr on the exit. This page covers what changes.
| Property | Rigid board | Flex / polyimide | Consequence for the drill stack |
|---|---|---|---|
| Stiffness | Self-supporting | Deflects under the foot and between supports | The backup is doing structural work, not only exit support |
| Thickness | 0.8–3.2 mm typical | Often 0.05–0.3 mm | Entry and backup thickness become a large fraction of the stack |
| Adhesive layers | None or thin | Acrylic or epoxy adhesive between films | Smear and adhesive pick-up at the hole wall |
| Dimensional stability | Good | Moves with humidity and handling | Position error can come from the material, not the machine |
| Surface | Solder mask | Coverlay, often softer | Particles embed instead of sitting on top |
The entry sheet on flex has two jobs that pull in opposite directions. It has to guide the bit at the surface, which argues for stiffness, and it has to sit flat on a panel that will not hold itself flat, which argues for conformity.
In practice a thin aluminium entry in the 0.10–0.15 mm range is the usual starting point, with three caveats:
The thickness arithmetic is in the entry board thickness calculator, and the material data is in the AA1100 H18 profile.
The backup has to flatten a panel that wants to curl, and then let the drill exit cleanly. Those two requirements do not always point at the same grade. The rigid-line techniques these defaults come from are in the PCB drilling techniques guide.
| Condition | Backup property that matters | What to avoid |
|---|---|---|
| Thin panel, high curl | Flatness and sufficient hardness to press the panel flat without marking it | A soft board that conforms to the curl instead of correcting it |
| Adhesive-heavy construction | Clean exit so adhesive does not string | A rough or fibrous surface that catches softened adhesive |
| Small holes, high hit count | Density stability through the run | A grade that compresses early and then changes |
| Coverlay on the exit side | A surface that will not emboss the coverlay | Coarse texture or embedded debris |
Reuse is the trap. A backup board that has been drilled once has a hole pattern in it, and on a thin panel that pattern prints through. On flex, single use is the safer default.
On a rigid board a stray particle usually shows up as a burr or a scratch and gets caught at inspection. On flex it gets pressed into a soft coverlay and becomes part of the part. It is then found at assembly, at bend test, or in the field.
The sources, in the order they usually turn out to be:
Control is a sequence rather than a single measure: clean the stack before loading, remove shavings by vacuum rather than brushing, change the bit before adhesive build-up starts, and keep the panel in its carrier between steps. Brushing is the step that feels effective and moves the problem instead of removing it.
Two panels per stack is common on flex; more than that usually trades position for throughput. Pinning or taping is not optional on thin material, because the panel will move relative to the backup under lateral load even when it looks flat.
If position is the problem rather than hole quality, the causes are separated in hole position accuracy, and the fixturing discussion there applies to flex with more force.
| Metric | Why it matters here |
|---|---|
| Hole position against datum | Distinguishes material movement from machine error |
| Hole wall roughness at the adhesive layer | Smear shows here before it shows electrically |
| Particle count on the exit surface | The defect that rigid-board inspection practice misses |
| Coverlay embossing depth | Reveals backup texture and embedded debris |
| Bend-test result on the drilled zone | The functional consequence, not a proxy |
For the rigid-board version of the same decisions, the backup board selection route covers grade choice by failure mode. If the flex part also goes through back-drilling, the depth conventions are in the back-drill design rules. For a supplier-side view of documents and qualification, see how a drilling material supplier is qualified.
Usually a thinner one. A stiff entry sheet bridges over a flexible panel instead of conforming to it, which leaves the middle of the span unsupported. A lubricated coating also earns more on flex, because adhesive smear is a friction problem.
It is not advisable. A used board carries a hole pattern, and on a thin panel that pattern prints through to the exit surface. Single use is the safer default on flex.
The coverlay is softer than solder mask, so a particle is pressed into the surface rather than sitting on it. It survives cleaning and is then found at assembly or at bend test.
It can be, on the rigid sections of a rigid-flex construction. The depth reference and stub budget follow the same rules as a rigid board, but thickness variation across the rigid-flex transition has to be accounted for.