A finished rigid-flex PCB is one board, but it is built from two material systems that meet in a single lamination: polyimide (PI) and coverlay in the flex zone, FR-4 and prepreg in the rigid zone. It goes through every step of a normal multilayer board and adds several steps that exist only for the flex zone.
Whether you need rigid-flex at all is covered in flex vs rigid-flex construction boundaries; stackup material choice is covered in rigid-flex stackup and material selection. This article covers process order only.
Process overview
| # | Step | Purpose | Design input | Where problems surface |
|---|---|---|---|---|
| 1 | Flex core inner-layer imaging | Pattern the flex layers on PI copper-clad laminate | Flex-layer Gerbers, bend-zone marking | Inner-layer AOI, final electrical test |
| 2 | Coverlay opening and lamination | Protect flex-zone circuitry | Coverlay opening layer (separate data) | Post-lamination visual, exposed copper on pads |
| 3 | Rigid-layer imaging and pre-routing | Mill or depth-route rigid cores over the flex zone | Rigid/flex boundary, flex-zone outline | Clean cap removal |
| 4 | No-flow prepreg windowing | Bond rigid layers to the flex core with no adhesive in the flex zone | Same boundary | Resin squeeze-out, stiffened flex zone |
| 5 | Hybrid lamination | Press flex core and rigid layers into one board | Full stackup table | Delamination, misregistration |
| 6 | Drilling and desmear | Hole walls pass through PI, adhesive and FR-4 | Drill file, hole attributes | Hole-wall quality, plating adhesion |
| 7 | Plating and outer-layer imaging | Hole metallization and outer circuitry | Outer Gerbers, copper weight | Hole copper, line width |
| 8 | Solder mask, finish, legend | Applied to rigid zones only | Mask layer, finish requirement | Mask edge at the rigid/flex boundary |
| 9 | Cap removal, routing, electrical test | Remove rigid material over the flex zone, route outline, 100% test | Outline layer, test netlist | Final test, IPC-6013 acceptance |
Steps 2, 3, 4 and 9 do not exist on a standard multilayer board. Step 6 exists, but the hole-wall materials differ.
Flex core: built first, frozen first
Flex-layer circuitry is imaged on the flex core and inspected before lamination. Once laminated, the flex layers are buried and a pattern defect can only be found by electrical test, not repaired. Inner-layer AOI is therefore the release gate; see inner-layer AOI one-way release.
The designer supplies separate flex-layer Gerbers and a clear bend-zone marking. Routing rules inside the bend zone are in the rigid-flex PCB design guide.
Coverlay: its own data layer
The flex zone is protected by coverlay (PI film plus adhesive) instead of liquid solder mask. Coverlay is opened at pad locations first, then registered and laminated onto the flex core.
- Coverlay openings need their own data layer. If only a solder mask layer is supplied, the factory has to derive the coverlay openings and confirm them in an EQ.
- Coverlay material choice is covered in flex PCB coverlay and polyimide material selection.
Rigid-layer windows and no-flow prepreg: the boundary line drives both
The flex zone starts out covered by rigid material. To remove it at the end, two things happen before lamination:
- Rigid core pre-routing: the rigid core is milled open or depth-routed over the flex zone, then removed along the boundary after lamination.
- No-flow (low-flow) prepreg windowing: the bonding layer is cut back over the flex zone so resin cannot flow into it. Standard prepreg flows too freely; resin that reaches the flex zone stiffens it and can crack it in bending.
Both steps use the same data: the rigid/flex boundary and the flex-zone outline. If the boundary is unclear, neither step can be done accurately. Draw the boundary on a mechanical layer and state in the stackup table, layer by layer, whether material stays or is removed in the flex zone. Transition-zone design is covered in rigid-flex transition zone manufacturing.
Hybrid lamination: two materials, one press cycle
The flex core, windowed bonding layers and rigid layers are stacked and pressed once. This step locks in every earlier data error:
- Registration: flex-core and rigid-layer patterns must align. PI and FR-4 differ in dimensional stability, and the more layers there are, the more alignment depends on the factory compensating for measured movement. General methods are in multilayer lamination registration control.
- Delamination risk: moisture in the materials, incomplete adhesive coverage or squeeze-out can show up as delamination after lamination or during reflow.
The stackup table should list each layer's material type (PI laminate, coverlay, adhesive, FR-4 core, prepreg) and copper weight, not just overall thickness. SKPCB's published rigid-flex range is 2–20 rigid layers, 1–8 flex layers, 0.3–4.0 mm board thickness, FR-4+PI base; see the rigid-flex PCB product page.
Drilling and desmear: more than one hole-wall material
Vias in the rigid zone pass through FR-4, adhesive and PI. Each material responds differently to drilling heat, chip removal and desmear, and hole-wall cleanliness sets plating adhesion. Problems here usually surface as barrel cracks after thermal stress or reflow.
What the designer can do: keep vias away from the rigid/flex boundary and inside the rigid zone, and separate PTH and NPTH in the drill file.
Plating, outer layers and finish: rigid zones only
Hole metallization and outer-layer imaging are the same as on a standard multilayer board. Solder mask, surface finish and legend go on rigid zones only; the flex zone is protected by coverlay. Specify on the drawing how the mask edge overlaps the coverlay at the boundary, or you risk exposed copper or mask creeping onto the flex zone.
Cap removal, routing and test: the last place errors surface
During outline routing, the rigid material over the flex zone is removed along the boundary (cap removal), then the board outline is routed. Errors from steps 3 and 4 show up here as incomplete removal, surface damage on the flex zone or squeeze-out at the boundary.
Then comes electrical test. SKPCB tests 100% of orders by flying probe or fixture; see flying probe vs fixture test. Rigid-flex boards are accepted to IPC-6013 Class 2 / Class 3; full process parameters are in the rigid-flex capability sheet.
Data to freeze before release
- Separate flex-layer and rigid-layer Gerbers, with layer names matching the stackup table.
- Coverlay opening layer (separate).
- Rigid/flex boundary and flex-zone outline on a mechanical layer.
- Stackup table: material type and copper weight per layer, and whether each layer stays or is removed in the flex zone.
- Bend-zone location and bend type (static or dynamic).
- Drill file with PTH/NPTH separated and holes kept off the boundary.
- Mask-to-coverlay overlap requirement at the boundary.
- Acceptance class: IPC-6013 Class 2 or Class 3.
With this data complete you can request a quote; engineering returns stackup and window confirmations in the EQ.
Summary
Almost every extra step in rigid-flex manufacturing follows one line: the rigid/flex boundary. The flex core is built and frozen first; coverlay, rigid-layer windows and no-flow prepreg are all opened along that boundary; cap removal at the end follows it again. When the design data defines that line and each layer's keep-or-remove status, every later step has something to work from.
FAQ
Q: Which steps does rigid-flex add to a standard multilayer flow?
A: Mainly flex-core coverlay lamination, rigid-layer pre-routing, no-flow prepreg windowing and final cap removal. Drilling and desmear exist on standard boards too, but rigid-flex hole walls pass through PI, adhesive and FR-4.
Q: Why is no-flow prepreg required?
A: Standard prepreg flows too much during lamination and can run into the flex zone, stiffening it or causing cracks in bending. No-flow or low-flow bonding layers keep resin in the rigid zone.
Q: Can I send only a solder mask layer and no coverlay layer?
A: You can get a quote, but the factory has to derive the coverlay openings and ask you to confirm them in an EQ. A separate coverlay layer saves a round of questions.
Q: Can vias sit near the rigid/flex boundary?
A: Not recommended. Stress concentrates at the boundary and the hole wall crosses several materials, so vias belong inside the rigid zone.
Q: How many layers can SKPCB build in rigid-flex?
A: The published range is 2–20 rigid layers and 1–8 flex layers, at 0.3–4.0 mm board thickness.
Q: Which acceptance standard applies?
A: IPC-6013 Class 2 or Class 3, a different standard from IPC-6012 for rigid boards. Every order gets 100% electrical test.