Flexible PCB (FPC) — Single & Double Layer

Bend Radius Design Rule

The minimum bend radius is the single most critical FPC design parameter. Violating it causes conductor fatigue fractures — often intermittent failures that are difficult to diagnose in field returns.

The IPC-2223 design guideline for static flex (bent once during assembly, not repeatedly) specifies a minimum bend radius of 6× the total FPC thickness. For dynamic flex (repeatedly bent during operation, e.g., a flip-phone hinge or wearable band), minimum bend radius is 10–20× the total FPC thickness.

Practical calculation: a single-layer FPC with 25µm PI base + 35µm copper + 25µm coverlay = 85µm total. Minimum static bend radius: 6 × 0.085mm = 0.51mm. Minimum dynamic bend radius: 10 × 0.085mm = 0.85mm.

Using 50µm PI instead of 25µm approximately doubles the minimum bend radius requirement. For tight-radius applications (wearable devices, compact camera hinges), specify the thinnest adequate PI thickness. Over-specifying material thickness to “be safe” often creates a design that physically cannot be folded to the required radius.

ZIF vs. FFC Connector Pitch Compatibility

ZIF (Zero Insertion Force) connectors are the most common FPC termination method. Key parameters:

Pitch. 0.5mm and 1.0mm are standard. 0.3mm is possible but significantly increases FPC manufacturing difficulty and connector cost. Confirm the connector pitch matches both the FPC contact pitch and the PCB footprint on the host board.

Contact thickness. ZIF connectors specify the FPC contact thickness they accept: either 0.2mm or 0.3mm total FPC thickness at the contact area. A mismatch causes unreliable connection or connector damage. The FPC coverlay and stiffener thickness at the contact area must match the connector specification.

FFC vs. FPC. FFC (Flat Flexible Cable) is a parallel conductor ribbon cable — conductors embedded in a PET film, no PCB. FPC (Flexible Printed Circuit) is a true circuit board on PI film with etched conductors and optional components. They use the same connector footprints at 0.5mm and 1.0mm pitch, but FPC allows arbitrary trace routing and component mounting. Confirm which type your application requires.

Static Flex vs. Dynamic Flex Material Selection

Static flex. Bent once or a few times during product assembly (e.g., connecting a display to a PCB inside an enclosure). Standard PI base material (25–50µm) is adequate. Standard rolled-annealed copper (RA copper) at 35µm (1oz) is the default.

Dynamic flex. Bent continuously during operation (e.g., printer head cable, wearable hinge). Requires:

• RA (Rolled Annealed) copper — not ED (Electro-Deposited) copper. RA copper has a grain structure aligned with the rolling direction that gives 3–5× better fatigue life than ED copper.
• Thinner PI base (12.5–25µm) to reduce bending stiffness
• Possible use of adhesiveless copper laminate to eliminate the adhesive layer, which is a common fatigue initiation site
• Finite bend radius compliance — confirm with your mechanical design team that the actual installed bend radius exceeds the minimum dynamic flex design rule

Coverlay vs. Solder Mask Trade-off

Polyimide coverlay. Laminated PI film with adhesive, punched or laser-cut to expose pads. Better flexibility and adhesion to PI base material. Standard for FPC with dynamic flex requirements. Minimum hole-to-hole spacing: 0.5mm (for punching) or 0.1mm (for laser cutting).

Liquid Photoimageable (LPI) solder mask. Screened or spray-applied, photo-developed. Finer feature resolution than punched coverlay. Lower cost for complex pad geometries. Reduced flexibility compared to PI coverlay — not recommended for tight dynamic flex applications.

For wearables and consumer FPC with bend radius below 5mm and any dynamic flex cycling, PI coverlay is the correct choice. LPI is adequate for rigid-flex PCBs where the flex section has a large bend radius.