Flexible printed circuit boards.

A flexible PCB (flex circuit) is a printed circuit board built on flexible substrate materials, allowing the circuit to bend, fold or twist during use or installation. Unlike rigid PCBs, flex circuits are designed to fit compact, dynamic or irregular spaces — making them ideal for modern products where size, weight and reliability are critical. They commonly replace traditional wiring harnesses and connectors, improving electrical performance and mechanical reliability at the same time.

Dynamic flex vs static flex

There are two major design categories for flex PCBs, and each requires different materials and process discipline.

Dynamic flex

Designed to be bent or flexed many, many times — laptop hinges are the classic example. The board is part of the hinge so the screen and base can stay electrically connected through repeated open/close cycles. Material and process choices directly limit the achievable cycle count. Two important examples:

• Use rolled-annealed (RA) copper — its grain structure handles repeated bending far better than electrodeposited copper.
• Avoid copper plating in the flexible section of the PCB whenever possible.

Static flex

Common in tightly packed products like cell phones where real estate is at a premium. A static flex design eliminates cables between PCBs — the flex is bent once during manufacturing and then stays in that position for the life of the product. There are many ways to fabricate static flex; the key is good communication between designer and PCB manufacturer about the actual installation and expected flexibility.

IPC-6013 flex types

• Type 1 — single-sided flex circuits.
• Type 2 — double-sided flex circuits with plated through-holes.
• Type 3 — multilayer flex circuits with PTHs.
• Type 4 — multilayer rigid-flex circuits with PTHs.

Key features

Flexible substrate materials

• Typically polyimide film for thermal stability and flexural endurance.
• Optional adhesive or adhesiveless bonding systems.
• Polyimide coverlay for protection of conductors.

Reduced size and weight

• Replaces bulky cables, harnesses and connectors.
• Enables compact, lightweight, often 3-D form factors.

Improved reliability

• Fewer interconnects and solder joints.
• Better resistance to vibration and mechanical stress.

Enhanced electrical performance

• Shorter signal paths than wired connections.
• Reduced impedance discontinuities; better signal integrity.

Construction

Typical flex PCB construction includes:

• Polyimide base film.
• Rolled-annealed (preferred for dynamic flex) or electrodeposited copper.
• Adhesive or adhesiveless bonding systems.
• Polyimide coverlay with adhesive.
• Optional stiffeners (FR-4, polyimide or metal) where rigid mounting is required.

Design considerations

Bend radius control

Proper bend radius prevents copper fatigue and cracking. Rule of thumb: minimum bend radius scales with copper weight and number of layers — your fabricator can give you exact targets.

Copper type and thickness

• Rolled-annealed copper for dynamic flex.
• Thinner copper improves flex life.

Layer transitions

• Avoid vias and pads in bend areas.
• Smooth transitions reduce stress concentration at the rigid-to-flex interface.

Applications

• Consumer electronics — smartphones, wearables, cameras.
• Medical devices — diagnostic equipment, implants, portable devices.
• Automotive — instrument clusters, sensors, ADAS modules.
• Aerospace and defense — avionics, robotics, missile guidance.
• Industrial and robotics — articulated and moving subsystems.