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How to Design a 2 Layer Flexible PCB

Ultrasound probe 2 layer soft board

Name:Ultrasonic probe soft board

Number of layers: 2L Flexible PCB

Material: PI

Board thickness: 0.09mm

Copper thickness: 6um±1um

Minimum aperture: 50um

Size: 84mm*168mm

Surface treatment: electroplating soft gold

Application: Medical

Features: ultra-thin copper, micro-holes, appearance of zero defects FPC


2 Lay Flexible PCB

Flexible printed circuit boards (PCBs) allow circuitry to bend and flex to accommodate different shapes and motions. A 2 layer flex PCB contains a single flexible dielectric layer sandwiched between 2 conductive copper layers.

Compared to rigid boards, flexible PCBs require some unique design considerations regarding the materials used, layout techniques, manufacturability and reliability. This article provides a step-by-step guide on how to design a 2 layer flexible PCB covering the key aspects to consider.

Design Process Overview

The typical workflow for designing a 2 layer flex PCB is:

  • Define circuit requirements and interfaces
  • Select flexible material stackup
  • Create circuit schematic
  • Plan layout and component placement
  • Route traces and vias
  • Specify flexible outline and bend areas
  • Add text callouts and markings
  • Review design rules and margins
  • Verify electrical constraints
  • Export Gerber and CAD files

Flexible Materials

The main materials used in a 2 layer flex PCB include:

Flexible Dielectric

The most common flexible dielectric material is polyimide (PI). It provides excellent flexibility along with thermal and chemical resistance. Typical thickness is 25μm to 100μm. Other polymers like PEEK, PET, PC etc. are also used.

Copper Foil

Rolled annealed copper with 18μm to 35μm thickness allows flexibility while carrying required current.

Bonding Adhesive

Acrylic or epoxy adhesive bonds the copper foil to the dielectric. Should withstand flexing.


A polyimide or acrylic coverlay coats the circuits for protection and insulation. Cutouts are provided at bonding pads.

Solder Mask

LPI solder mask additionally insulates exposed copper traces from environmental factors and accidental contacts.

Stackup Selection

For a 2 layer flex PCB:

  • Core: 25μm-100μm flex dielectric film
  • Layer 1: 18μm-35μm rolled copper foil
  • Layer 2: 18μm-35μm rolled copper foil
  • Coverlay: 25μm-50μm polyimide or acrylic

Using thinner cores allows smaller bend radii but requires narrower trace/space. Thicker copper improves current capacity but reduces flexibility.

Schematic Design

flexible pcb circuit board

A schematic defines the circuit connectivity. Considerations for flex:

  • Show all interfaces like connectors
  • Define component placements
  • Call out any rigid sections
  • Note high speed signals
  • Mark all testpoints

Layout Considerations

Key aspects for 2 layer flex PCB layout:

Component Placement

Only low profile surface mount devices should be used. Place components in the rigid areas only. Avoid placing parts across bends. Distribute components evenly to balance stresses.

Trace Routing

Use 45° angle traces instead of 90° turns. Avoid traces crossing bend perforations. Leave extra trace length for flexing movement. Run critical traces on inner layers.


Allow cutouts for connectors, buttons and other mechanical parts to be mounted.

Bend Areas

Define by outlines/perforations where the board will bend. Allow sufficient bend radius. Consider stresses when routing across bends.


Add stiffeners made of rigid sections or covers near narrow flexible segments for support.


Include testpoints to access key nets for debugging.

Critical Design Rules

Key design criteria for 2 layer flex PCBs:

  • Minimum Bend Radius: Inside bend radius should exceed minimum specified for materials to prevent cracking
  • Minimum Trace Width: Thinner traces allow tighter bends but handle less current
  • Minimum Spacing: Adequate spacing prevents shorts and crosstalk
  • Annular Rings: Sufficient pad clearance from thru-hole walls to avoid opens
  • Testpoint Size: Must allow safe probing access
  • Via Tenting: All vias should be fully capped/tented for protection

Manufacturing and Assembly


Fabricating and assembling flex PCBs involves:

  • Imaging: Photolithography transfers the layout onto copper layers.
  • Etching: Removes unwanted copper to form traces.
  • Die Cutting: Cuts board outline and bend perforations.
  • Coverlay Lamination: Bonds protective coverlay onto circuits.
  • Solder Mask: LPI solder mask applied for additional insulation.
  • Route/Fold: Boards are routed apart and folded along bends if needed.
  • Component Mounting: Parts assembled only on rigid sections using adhesive or fixing points.
  • Debugging: Provides testpoints for easy probing access.
  • Conformal Coating: Optional coating increases reliability.

Design Verification

Prior to release for fabrication, the flex PCB layout must be thoroughly verified:

  • Confirm minimum electrical clearances
  • Check trace widths can carry required current
  • Verify impedance of high speed traces
  • Validate bend radii exceed minimum material and manufacturing capabilities
  • Review component placement near bends and edges
  • Confirm all design rules and margins
  • Test board flexibility and dynamic durability through prototyping


Designing reliable 2 layer flexible PCBs requires understanding the materials, layout techniques, manufacturability limitations and reliability factors involved. With the growing use of flex circuits in consumer and industrial products, rigid-flex PCBs and 3D packaging, understanding robust flexible PCB design principles is becoming imperative for engineers. The guidelines provided in this article can serve as a starting framework when developing a 2 layer flexible PCB design to achieve a board that successfully meets the functional, mechanical and quality needs of the application.


What are some typical applications of 2 layer flex PCBs?

Wearable devices, medical sensors, robotic arms, electronic display interconnections, wire harness replacement in equipment.

What design software is used for flex PCBs?

Cadence Allegro PCB Editor, Mentor Xpedition, Altium, Zuken CR-8000 offer flex and rigid-flex design capabilities.

Can surface mount components be assembled on the flex section?

It is not recommended since soldering stresses can crack the thin flex laminate and traces. Components should only be assembled on rigid sections.

What thickness of polyimide is typically used as the flexible dielectric?

12 micron to 50 micron polyimide films are commonly used. Thinner material allows smaller bend radius but requires narrower trace/space.

What are some key tests performed to qualify flex PCB durability?

Flex PCBs are dynamically tested for bend cycles, torsion, flexion, twist, vibration, shock, temperature cycling and bend radius verification.




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