A flex PCB fabrication drawing is where a design either becomes buildable or becomes a week of back-and-forth. A rigid-board drawing can lean on defaults; a flex or rigid-flex drawing cannot, because the fab house has to know where the board bends, what the coverlay does, where the stiffeners sit, and which acceptance class the tolerances are held to — none of which is obvious from the Gerbers alone. Leave those off and the order stalls at pre-production engineering review with a list of questions; put them on, and the board runs.
This guide is the callout-by-callout list of what a flex fabrication drawing must state, written for engineers and buyers who want a package a supplier can quote and build without a round of clarifications. It complements the flex PCB DFM checklist, which covers the design itself; this is about documenting it correctly.
TL;DR
- A flex fab drawing must add, on top of the usual outline and hole table: bend lines and bend zones, coverlay extent and openings, stiffener locations and materials, the layer stackup, controlled-impedance callouts, and the IPC acceptance class — the things Gerbers do not carry.
- Bend zones need an explicit bend radius and a "no plated features in the bend" note; call out the bend line so the fab and the assembler read it the same way. See the bend radius design guide.
- Coverlay is not solder mask — specify coverlay extent, opening sizes, and registration tolerance separately; the coverlay vs solder mask and coverlay opening registration guides cover the detail.
- Name the IPC-6013 class (2 or 3) and the design standard on the drawing so tolerances and acceptance are defined, not assumed — see IPC-6013 Class 2 vs Class 3.
- The most common cause of a stalled flex order is a drawing that leaves stackup, hole attributes, board-edge treatment, or surface finish unstated — the fab has to ask before it can start.
What a flex fab drawing adds over a rigid one
A complete rigid-board drawing already carries the board outline, a hole/drill table, layer count, material, surface finish, and general notes. A flex or rigid-flex drawing keeps all of that and adds the features that make flex flex:
- Bend lines and bend zones — where the board flexes, at what radius, and how many cycles if it is dynamic.
- Coverlay — the flexible insulation over the copper, its extent, and its openings (distinct from any solder mask on the rigid sections).
- Stiffeners — added rigidity under connectors or components, with material and thickness.
- Layer stackup — the exact build, because flex thickness and symmetry drive both impedance and flex life.
- Controlled-impedance callouts — target impedance and reference layers, verified on a coupon.
State each one on the drawing. The Gerbers show geometry; the drawing states intent, tolerance, and acceptance.
Bend lines and bend zones
Draw the bend line and label the bend radius and, for a moving application, the flex cycle target. The single most important note in the bend zone: no plated through-holes, vias, or abrupt copper transitions inside the bend — plated features crack under repeated flexing. Keep traces perpendicular to the bend, route them staggered on double-sided flex, and state the minimum bend radius as a multiple of thickness. The mechanics behind the number are in the bend radius design guide; the drawing just has to make the zone and its rules unambiguous.
Coverlay callouts (not solder mask)
On flex, the outer insulation is usually coverlay — a laminated polyimide film — not the liquid photoimageable solder mask used on rigid boards. They are documented differently, and treating them the same is a frequent drawing error. Call out the coverlay extent, each opening's size, and the registration tolerance (coverlay openings register less tightly than solder mask, so pads must be sized for it). Where the rigid sections of a rigid-flex board use solder mask, note that separately. The coverlay vs solder mask and coverlay opening registration guides give the tolerances to specify.
Stiffeners, stackup, and impedance
Call out stiffeners by location, material (FR-4, polyimide, or steel), and thickness, and say which side they mount on — the fab cannot infer them. Put the layer stackup on the drawing as a table: layers, copper weights, dielectric, adhesive vs adhesiveless, and total finished thickness, because flex thickness and symmetry drive flex life as much as impedance. For controlled impedance, state the target, tolerance, and reference layers, and require verification on a test coupon rather than assuming the stackup delivers it. Stiffener detail is in the stiffener guide.
IPC callouts that fix the acceptance class
A flex drawing should name the standards explicitly so tolerances and acceptance are defined rather than left to the fab's default:
- IPC-6013 — the performance/qualification spec for flex and rigid-flex. State the class (2 or 3); Class 3 tightens acceptance for high-reliability work. See IPC-6013 Class 2 vs Class 3.
- IPC-2223 — the sectional design standard for flex; reference it so the design rules are shared.
- IPC-A-600 for acceptability of the finished board, and a coupon/microsection requirement where reliability matters.
Naming the class on the drawing is what turns "build it well" into a measurable acceptance bar.
Representative project (anonymized)
An electronics manufacturer in the Asia-Pacific region paid for a rigid-flex batch and asked for a completion date so it could schedule downstream work. Before production could start, the pre-production engineering review surfaced two groups totalling 14 items that needed customer confirmation — stackup and impedance files, board-edge treatment, the outline layer, solder-mask openings, character over pads, microvia plugging, copper-to-board-edge clearance, hole attributes, and surface finish (the team also recommended switching HASL to ENIG at the same cost). Production was placed on hold while those 14 questions went back to the customer. Every one of them is a callout that belongs on the fabrication drawing. The lesson is direct: a drawing that states stackup, coverlay/mask openings, hole attributes, edge treatment, and surface finish up front is the difference between a board that runs on receipt and one that waits on a 14-item engineering query.
FAQ
What is a flex PCB fabrication drawing?
It is the controlling document that tells the fab house how to build the board to your intent — outline, hole table, stackup, and, for flex, the bend lines and zones, coverlay extent and openings, stiffeners, controlled-impedance targets, and the IPC acceptance class. The Gerbers carry the geometry; the drawing states tolerance, intent, and acceptance.
How do I show bend lines on a flex drawing?
Draw the bend line, label the bend radius (as a multiple of thickness), and, for a dynamic application, the flex-cycle target. Add a note that no plated through-holes, vias, or abrupt copper transitions sit inside the bend zone, and keep traces perpendicular to the bend. The zone and its rules must be unambiguous to both the fab and the assembler.
How do I call out coverlay versus solder mask?
Coverlay is a laminated polyimide film used as the flexible insulation; solder mask is the liquid film used on rigid sections. Document them separately: state coverlay extent, opening sizes, and registration tolerance (which is looser than solder mask, so size pads for it). On rigid-flex, note where solder mask applies on the rigid areas.
Which IPC standards should the drawing reference?
Reference IPC-6013 with an explicit class (2 or 3) for performance and acceptance, IPC-2223 for the flex design rules, and IPC-A-600 for board acceptability, plus a coupon/microsection requirement where reliability matters. Naming the class fixes the acceptance bar instead of leaving it to a default.
Why do flex orders stall at pre-production review?
Almost always because the drawing left something unstated — stackup, impedance files, hole attributes, board-edge treatment, coverlay/mask openings, or surface finish — so the fab has to ask before it can start. A complete drawing that specifies these up front prevents the round of engineering questions that pauses production.
Do I need a fabrication drawing if I send Gerbers and an ODB++ file?
Yes. Gerbers and ODB++ carry geometry and some attributes, but the drawing is where you state acceptance class, stackup intent, bend-zone rules, coverlay/stiffener callouts, controlled-impedance targets, and general notes. The data files and the drawing together are the buildable package; either one alone leaves gaps.
A fabrication drawing is where flex PCB quality is defined before a single board is built. If you want a package reviewed for exactly these callouts before you release it, our flex PCB service runs a pre-production DFM and documentation review, and the DFM checklist is the companion list for the design itself.