Wearables
Thin adhesiveless polyimide that fits inside a wristband, copper from 0.5 oz and stiffened pads for the sensor and antenna connections.
Flex & rigid-flex
Polyimide circuits that have to bend and still connect
Single-sided and multilayer FPC plus rigid-flex built on polyimide film, with coverlay, adhesive or adhesiveless laminates and stiffeners in FR-4, polyimide or aluminium. Copper starts at 0.5 oz, finishes run from ENIG to OSP, and the same plant laminates rigid boards to 58 layers.
Adhesive or adhesiveless
The first decision is what holds the copper on
A flex laminate is a copper foil, a polyimide film and something in between. That something decides the achievable bend behaviour, the finished thickness and how the circuit survives reflow, so it is settled before the stackup is drawn.
| Dimension | Adhesive construction | Adhesiveless construction |
|---|---|---|
| Dielectric structure | Polyimide film bonded to copper foil with an acrylic or epoxy adhesive layer on each side | Copper cast or sputtered directly onto the polyimide film, with no separate adhesive layer |
| Cover layer | Coverlay, a polyimide film pre-coated with adhesive, laminated over the finished traces | Coverlay or photo-imageable solder mask, depending on the pad density and the bend radius required |
| Bend behaviour | Suits static bends and assemblies that are folded once and fixed in place | Preferred where the circuit flexes repeatedly in service; the requirement is confirmed per project against the stackup and stiffener plan |
| Finished thickness | Thicker for the same copper weight, because the adhesive adds to the build | Thinner for the same copper weight, which matters inside a wearable or a camera module |
| Copper weight | From 0.5 oz, which remains the practical starting point on both constructions | From 0.5 oz, with fine-line etching on thin foils for dense pad fields |
| Typical use | Ribbon connections, static folds, cost-sensitive consumer assemblies | Wearables, medical catheters and probes, camera modules, high-cycle hinge joints |
How a flex circuit is built
Eight stages that a rigid board does not have
Two stages here have no equivalent on a rigid line: the coverlay press, which replaces solder mask as the mechanical cover, and stiffener bonding, which is what makes a flexible circuit solderable on an SMT line.
1 · Material preparation
Polyimide laminate cut to panel size, with the adhesive or adhesiveless construction fixed on the released stackup.
2 · Drilling
Punched or drilled vias and registration holes; flexible cores are handled on carriers to stop the thin film stretching.
3 · Patterning
Dry film, exposure and etching of the copper traces, starting from 0.5 oz foil.
4 · Coverlay lamination
Coverlay pressed over the traces with openings for the pads, giving the circuit its mechanical and environmental cover.
5 · Surface finish
ENIG, immersion tin or OSP, chosen against the assembly process and the connector mating requirement.
6 · Stiffener bonding
FR-4, polyimide or aluminium stiffeners bonded where connectors, fine-pitch parts and mounting holes need a rigid base.
7 · Profiling
Laser cutting or die cutting of the outline, with the breakaway tabs and carriers that hold the part through SMT.
8 · Electrical test
Continuity and isolation test on the flexible panel, with impedance coupons where a controlled-impedance flex is specified.
What holds the bend
Coverlay, stiffeners and the transition zone
A flexible circuit only behaves predictably where the neutral axis is controlled. These four elements are how that is done, and each one is specified on the drawing rather than left to the shop floor.
- Coverlay, a polyimide film with its own adhesive, pressed over the etched traces. It carries the copper through the bend and protects the traces where a solder mask would crack, with cut-outs left at every pad.
- FR-4 stiffener, the default choice under a connector, a BGA or a screw-down area. It gives the SMT line a flat, rigid surface to place and reflow onto, and it stops the flex tail flexing at the solder joint.
- Polyimide or aluminium stiffener. Polyimide where the stiffened area must stay thin and survive high temperature, aluminium where heat has to be pulled away from the parts above it.
- Rigid-flex transition zone, the bonded band where the rigid FR-4 islands meet the polyimide tail. Copper weight starts at 0.5 oz, and the layer count on both sides is agreed per stackup so the through-vias land correctly.
Rigid-flex in detail
The transition is where the design is won or lost
Everything either side of the transition zone is ordinary rigid and ordinary flexible work. The bonded band between them is not, and it is the first thing an engineer looks at when a rigid-flex drawing arrives.
- Rigid islands and flex tails pressed together with the correct bond ply, so the acrylic or epoxy flow does not squeeze into the bend area
- Stiffener and coverlay openings placed together, so the connector and the mounting holes land on a rigid base rather than a flexible one
- Gold fingers and connector areas finished in ENIG, immersion tin or OSP according to the mating cycles and the assembly process
- Dynamic bend requirement, bend radius and stiffener positions confirmed per project against the released stackup, no figure is assumed from a catalogue
Where flex pays for itself
Four applications, four different constraints
The same polyimide process serves all four sectors, but the constraint that decides the build is different in each one.
Medical devices
Medical-grade process capability with serialisation and full traceability, ionic cleanliness to 1.56 µg/cm² and parylene or polyurethane coating above 500 MΩ for probe and catheter assemblies.
Industrial control
Automotive-grade process capability with PPAP and IMDS documentation where the harness runs into a moving axis: multilayer rigid-flex, Tg ≥170 °C laminate and −55/+125 °C for 1,000 cycles.
RF & antenna
Rogers and Taconic laminates with impedance held to ±10% standard and ±5% on premium builds, verified by TDR coupons on the panel border.
Flex capability window
What we need before we can say yes
Flexible work is quoted from the drawing rather than from a table, because the bend and the stiffener plan change the process. The rows below are the inputs the review works from.
| Item | Window | How it is confirmed |
|---|---|---|
| Layer count | Reviewed per stackup | The rigid and flexible sections are counted separately on the drawing; the rigid side draws on the same 1–58 layer lamination process |
| Copper weight | from 0.5 oz | Chosen with the line width and the bend requirement, since heavier copper stiffens the flex tail |
| Stiffener material | FR-4, polyimide or aluminium | Selected from the connector type, the heat load and the space inside the enclosure |
| Panelisation & carriers | With breakaway tabs and tooling strips | Array layout and carrier design agreed so the flexible panel can run across a 510 × 460 mm conveyor envelope |
| Surface finish | ENIG, immersion tin or OSP | Matched to the mating cycles on gold fingers or the solderability window on the assembly side |
| Electrical test | Continuity and isolation on every panel | Flying probe or fixture test, with an impedance coupon read where controlled impedance is specified |
| Bend requirement | Confirmed per project | Static or dynamic, the intended bend radius and the stiffener positions are fixed at DFM review against the released stackup |
Flex parts in production
Ribbons, shaped circuits, bonded panels, modules
Four representative parts, all built on the eight-stage flow above: a straight ribbon, an irregular profiled circuit, a rigid-flex array and a stiffened edge-connector module.
40-way flex ribbon
Parallel traces on polyimide with gold contact fingers at one end and a stiffener patch behind them for the connector.
Shaped FPC
Routed to the enclosure rather than to a rectangle, so the circuit follows the available space instead of consuming it.
Rigid-flex array
Five rigid islands joined by polyimide bridges on one panel, with routed tabs and tooling strips for the SMT line.
Stiffened edge-connector module
Hard-gold fingers at 150–200 HK25 on a stiffened edge, rated above 10,000 mating cycles; the bevel angle follows the connector's insertion count and is confirmed at engineering review.
Quotation in 24 hours
Send the stackup, the bend and the stiffener plan.
Mark where the circuit bends, whether it bends once or repeatedly, and where the stiffeners have to sit. An engineer confirms the construction and returns both lead-time brackets within one working day. NDA on request.
