Flexible PCB

01

Choose the flex laminate and copper foil

DuPont describes Pyralux AP as a double-sided copper-clad laminate with an adhesiveless, all-polyimide dielectric for multilayer flex and rigid-flex, certified to IPC-4204/11 and UL 94 V-0. Its datasheet lists typical values for AP9121: glass transition 220 °C, dimensional stability −0.04 to −0.08% after etching and after thermal exposure, XY expansion 25 ppm/°C below Tg and 30 above, moisture absorption 0.8%, flexural endurance 6,000 cycles (IPC-TM-650 2.4.3), peel strength 1.4 N/mm as received and after solder, and a pass at 288 °C solder float for 10 s. [1]

The same datasheet offers copper from 6 to 140 µm and dielectric from 12 to 150 µm, with the product code suffix marking the foil: R for rolled-annealed, E for electro-deposited and D for double-treated rolled-annealed copper. Dk is 3.4 at 1 MHz and 3.2 at 10 GHz, Df 0.002 and 0.003, and the laminate should be stored at 4–29 °C below 70% relative humidity, not refrigerated. [1] DuPont notes that these are typical values and performance varies with construction and processing, so ask the fabricator which laminate grade and foil it will actually use. [1]

Editorial calculation: AP9121R is listed as 35 µm copper on a 50 µm dielectric; as a double-sided clad that is about 35 + 50 + 35 = 120 µm before coverlay is added. [1]

Give the fabricator the layer count and IPC-6013 type, laminate (adhesive or adhesiveless polyimide), copper weight and foil type, coverlay or LPI, stiffener locations and materials, bend locations and radii, whether each bend is install-only or dynamic with required cycles, surface finish, impedance targets, operating temperature, IPC-6013 class, and quantity.

02

Pick the flex type and bend radius

Minco's flex circuit overview lists IPC-6013 Type 1 (one conductive layer), Type 2 (two conductive layers with plated through-holes) and Type 3 (three or more flexible conductive layers, with blind or buried vias possible); stiffeners, pins, connectors and components are optional on each. It gives minimum bend radii of 12 times circuit thickness for double-layer and 24 times for multilayer, with circuit thickness about 0.006 in (0.15 mm) per layer, and notes that static bend-to-install single-layer circuits can take sharper permanent bends, while dynamic applications require special consideration. [2]

Editorial calculation: a double-layer flex of about 2 × 0.15 = 0.30 mm needs at least 12 × 0.30 = 3.6 mm bend radius by Minco's rule, while a three-layer multilayer of about 0.45 mm needs at least 24 × 0.45 = 10.8 mm. [2]

Minco lists standard materials for −65 to 150 °C, withstanding a 5-second solder immersion at 260 °C; covers and substrates of polyimide film from 12 to 125 µm or liquid photoimageable coverlay (LPI); copper from 5 to 107 µm; acrylic, flame-retardant acrylic, epoxy and polyimide prepreg adhesives; finishes including ENIG, ENEPIG and OSP; and typical insulation resistance of 100 MΩ minimum and dielectric withstand of 1,000 VRMS at 60 Hz for 30 seconds. It notes extended exposure above 105 °C darkens the adhesive. [2]

03

Check a fabricator's published flex capability

JLCPCB's flexible PCB capability page lists 1-, 2- and 4-layer FPCs (rigid-flex not yet supported) on 25 µm or 50 µm polyimide dielectric, or 36 µm PET for transparent boards, with outer copper of 12, 18 or 35 µm, ENIG finish, LDI exposure and a regular maximum size of 234 × 490 mm. Finished thickness without stiffener is 0.07 or 0.11 mm for a 1-layer and 0.11, 0.12 or 0.2 mm for a 2-layer 25 µm board, and 0.2–0.45 mm for 4-layer. [3]

The page also requires coverlay on inner layers with 1 oz copper in 4-layer builds to prevent delamination and blistering, and gives hole diameters of 0.1–6.5 mm with ±0.08 mm tolerance. [3] Compare these build values with the laminate data and the IPC-6013 types and bend-radius rules above. [1] [2]

Editorial calculation: applying Minco's 12× rule to JLCPCB's 2-layer finished thicknesses of 0.11–0.2 mm gives a minimum bend radius of about 1.3–2.4 mm; the 24× multilayer rule on a 0.2–0.45 mm 4-layer gives about 4.8–10.8 mm. 4/4 mil is about 0.10/0.10 mm.

Send the fabricator your stack-up, copper weight and minimum trace/space, and confirm the finished thickness in the bend area so the bend radius can be checked.

04

Plan the pre-bake and stiffeners for assembly

Flex circuit maker Epec Engineered Technologies states that the polyimide materials in flex and rigid-flex circuits can absorb 2 to 3 percent moisture by weight, and that this moisture must be baked out before any soldering, because entrapped water can turn to steam during reflow and delaminate the circuit. Its pre-bake guidance starts at about 120–121 °C (250 °F): simple single- or double-layer flex may need as little as 2 hours, while multilayer rigid-flex with stiffeners and thicker builds can need 6 to 10 hours or more. It asks for soldering or reflow within 30 minutes of the bake, because polyimide re-absorbs moisture quickly, and lists vacuum ovens at 65–80 °C as a lower-temperature option that also limits pad oxidation. [4]

Epec's stiffener guidance adds that stiffener and coverlay terminations on both sides should overlap by at least 0.030 in to reduce stress concentrations in dynamic flex areas, and that choosing standard laminate thicknesses and one stiffener thickness across the design helps material availability and cost. [5] Editorial calculation: 0.030 in is about 0.76 mm, and the stock thicknesses are about 0.25, 0.79, 1.19 and 1.57 mm. The 2–3% figure is higher than the 0.8% moisture absorption DuPont lists for its adhesiveless AP9121 laminate, so ask the fabricator for the bake time it uses for your specific stack-up and adhesive system. [5] [4] [1]

05

Compare what suppliers offer

Suppliers linked to this page list flexible PCBs; each card links to the page reviewed.

Listings include flexible PCBs alongside rigid-flex, HDI and rigid boards, and some publish flex capability pages; ask each for laminate brand and grade, foil type, minimum bend radius for your stack-up and flex-cycle test data.

06

Trace the flex circuit construction

This is a functional reference built from the sources cited; it is not one maker's specification.

07

Fill the specification fields with values for your circuit

A flexible PCB quotation needs values for your circuit. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

08

Send the stack-up, bends and materials together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Construction: [IPC-6013 type; layers; laminate; adhesive or adhesiveless]
Copper: [weight; RA or ED]
Mechanics: [bend locations; radii; static or dynamic; cycles]
Cover and support: [coverlay or LPI; stiffeners]
Electrical: [trace/space; impedance; current]
Finish and environment: [finish; temperature; humidity]
Quality: [IPC-6013 class; tests; quantity]

See manufacturer records Send an enquiry through CNFX