Flex PCB thickness: typical stacks, bend radius, and tolerances

A flex PCB typically finishes at 0.05–0.20 mm for 1‑layer and 0.12–0.30 mm for 2‑layer, depending on copper, polyimide, and coverlay.

What defines flex PCB thickness

“Thickness” for a bare flex tail means the sum of copper foil(s), base polyimide (PI), and coverlay (PI + adhesive). Stiffeners (FR‑4, PI, steel) and PSA are add‑ons and are usually called out separately. Surface finishes (e.g., ENIG) add only a few microns at pads and don’t materially affect overall thickness.

Typical material thicknesses

• Copper: 9 µm (¼ oz), 12 µm (⅓ oz), 18 µm (½ oz), 35 µm (1 oz)

• Base polyimide: 12.5 µm (0.5 mil), 25 µm (1 mil), 50 µm (2 mil)

• Coverlay: PI 12.5–25 µm + adhesive 12.5–38 µm per side (adhesiveless coverlay removes the adhesive term)

• Stiffeners (not part of flex thickness): FR‑4 0.20–1.00 mm typical; PI 50–200 µm; stainless 50–200 µm

Example finished thickness calculations

These examples assume adhesiveless base for the core (lower thickness, better bending) and a common coverlay of PI 12.5 µm + adhesive 12.5 µm (≈25 µm per side):

• 1‑layer, thin: 12 µm Cu + 12.5 µm PI + 25 µm coverlay ≈ 0.050 mm

• 1‑layer, standard: 18 µm Cu + 25 µm PI + 25 µm coverlay ≈ 0.068 mm

• 2‑layer, thin: 18 µm Cu ×2 + 25 µm PI + 25 µm coverlay ×2 ≈ 0.111 mm

• 2‑layer, robust: 35 µm Cu ×2 + 50 µm PI + 25 µm coverlay ×2 ≈ 0.210 mm

If an adhesive‑based core is used (not adhesiveless), add ~12.5–25 µm per copper‑to‑PI interface (×2 for two‑layer cores).

How thin can a flex PCB be?

Ultra‑thin 1‑layer flex down to ~0.04–0.05 mm is practical with 9–12 µm Cu, 12.5 µm PI, and a thin coverlay. Below ~0.04 mm, handling risk rises sharply, pad lifting and coverlay registration become critical, and yield drops. Two‑layer flex below ~0.10 mm generally requires 12 µm Cu, 12.5–25 µm PI, and adhesiveless constructions.

What bend radius for my thickness?

Per common IPC‑2223 design guidance: single‑layer static bends work at ~6–10× thickness (t); dynamic bends at ~100×t. Double‑sided static bends: ~12×t; multilayer static bends: ~20×t. Example: a 0.11 mm 2‑layer tail should keep static radius ≥1.3 mm; for dynamic flexing, plan much larger radii. Use rolled‑annealed (RA) copper, keep traces perpendicular to the bend, and avoid through‑holes in bend zones to protect the copper grain.

Does a stiffener change “thickness”?

Yes at the connector, no in the bend. Stiffeners set the local stack to a connector spec (e.g., ZIF fingers often target 0.30 ± 0.03 mm with PI stiffener and PSA), but they are not counted as the flexible section thickness and should stop short of the bend line.

Impedance vs thickness for 50 Ω and 90 Ω

For controlled impedance you need a reference plane (usually a 2‑layer flex). With PI εr ≈ 3.2–3.4 and 18 µm Cu:

• 50 Ω microstrip over 25 µm PI typically lands near 60–90 µm trace width, depending on coverlay/adhesive stack.

• 90 Ω differential (USB) on the same stack often uses 60–90 µm lines with 80–130 µm spacing.

Thicker dielectrics (e.g., 50 µm PI) relax trace widths but increase overall thickness. Request a field‑solver stackup; specify target Ω, copper, PI thickness, and whether coverlay adhesive sits over the routing.

Tolerances and measurement callouts

IPC‑6013 focuses on flexible printed boards; in practice, finished flex section thickness is usually controlled to about ±10% or ±0.03 mm, whichever is greater, with local variation near coverlay openings and etched areas. Call out thickness separately for: flexible area, stiffened connector area, and any PSA‑laminated zones. Note that copper plating at pads and wraps can shift local thickness by several microns.

DFM choices that drive thickness

• Go adhesiveless for the core when you must minimize thickness and improve bending life.

• Use 12.5–25 µm PI for tight bend zones; keep heavier PI only under stiffeners.

• Prefer 9–18 µm copper in dynamic bends; reserve 35 µm copper for stiffened, low‑flex regions.

• Minimize coverlay adhesive in bend areas (thinner adhesive, larger openings over tight bends).

• Align trace direction with the bend (perpendicular to the bend axis) and spread copper to avoid local stiffness peaks.

Compliance and build criteria at HyperPCB

We support builds and inspection to IPC Class 2 or IPC Class 3 acceptance criteria. Our quality management system is ISO 9001:2015 certified (SGS CN-2023-Q1156, valid to June 2027). RoHS and REACH compliance is supported for hazardous substance control. Need to converge quickly? We return quotes within 24 hours and DFM feedback within 1 business day.

Summary targets you can use

• 1‑layer flex: 0.05–0.20 mm typical; static bend radius ~6–10×t; dynamic ~100×t

• 2‑layer flex: 0.12–0.30 mm typical; static bend radius ~12×t

• Call out connector‑area thickness separately with stiffener and PSA stack