Can PCB be flexible?

What are flexible PCBs?

Flexible PCBs, also known as flex circuits or flexible printed circuit boards, are a type of printed circuit board that can bend and flex without breaking. Unlike traditional rigid PCBs, flexible PCBs are made from thin, flexible materials such as polyimide or PEEK that allow the circuit board to conform to different shapes and fit into tight spaces.

Flexible PCBs offer several advantages over rigid PCBs:

  • Flexibility and durability
  • Lightweight and thin
  • Space-saving
  • Vibration and shock resistant
  • Dynamic flexing
  • Cost-effective for high volume

Flexible PCBs are used in a wide range of applications that require circuits to bend or fit into small spaces, such as:

  • Wearable electronics
  • Medical devices
  • Aerospace
  • Automotive electronics
  • Consumer electronics
  • Industrial equipment

How are flexible PCBs made?

The manufacturing process for flexible PCBs is similar to rigid PCBs but with some key differences to enable flexibility:

  1. Material selection – Flexible substrates like polyimide film are used instead of rigid FR-4
  2. Metallization – Copper is deposited on the flexible substrate through sputtering, plating, or lamination
  3. Patterning – The circuit pattern is printed and etched on the metallized substrate using photolithography
  4. Coverlay – A protective layer of coverlay film is laminated over the circuits
  5. Cutting & drilling – The flex PCB is cut out and any necessary holes or slots are drilled
  6. Surface finish – The exposed copper is coated to prevent oxidation, e.g. with ENIG or HASL
  7. Components – Any electronic components are soldered on using SMT or through-hole assembly

Some key considerations in flex PCB manufacturing are:

  • Using thinner copper to enable flexibility, typically 1/2 oz to 1 oz copper thickness
  • Ensuring copper traces are oriented parallel to the bend to prevent breaks
  • Using soft, flexible soldermask and coverlay materials
  • Avoiding large components that could impede flexibility
  • Defining keep-out areas to prevent components in flex zones
  • Specifying bend radius and number of flex cycles required

Types of flexible PCBs

There are several different types of flexible PCBs with varying degrees of flexibility:

Type Description Flexibility
Single-sided flex Circuits on one side of a flexible substrate Moderate
Double-sided flex Circuits on both sides of a flexible substrate Moderate
Multi-layer flex Multiple circuit layers bonded with flexible adhesive Low
Rigid-flex Rigid PCBs and flexible PCBs combined in one assembly High in flex areas
Sculpted flex Varies substrate thickness within flex circuit Very high

Other variations of flexible PCBs include:

  • ZIF (zero insertion force) flex – Flex PCBs designed for easy insertion and removal
  • HDI (high density interconnect) flex – High wiring density enabled by microvias and fine lines/spaces
  • Shielded flex – Flex circuits with EMI shielding to block interference
  • Extreme temperature flex – Materials rated for very high or low operating temperatures

Flex PCB design guidelines

Designing a flexible PCB requires some special considerations compared to rigid PCB design:

Mechanical design

  • Define the static and dynamic portions of the flex circuit
  • Specify the bend radius and number of flex cycles required
  • Avoid creases or folds that could damage the circuits
  • Define stiffener areas to support connectors or large components
  • Use curved traces and tear-drops to reduce stress concentrations
  • Avoid plated through-holes in the flex area

Layer stack-up

  • Use symmetric stack-ups to prevent curling
  • Define copper weights and polyimide thicknesses for each layer
  • Use hatched polygons instead of solid copper floods
  • Specify copper-polyimide adhesion method (adhesiveless, adhesive-based, or cast)

Trace routing

  • Route traces perpendicular to the bend direction
  • Avoid traces in the bend area if possible
  • Use curved traces and tear-dropping to reduce stress
  • Increase trace width for high-current or long traces
  • Avoid acute angles and T-junctions
  • Leave space between traces and board edges

Footprints & pads

  • Use surface mount components whenever possible
  • Place components on rigid areas or stiffeners, not in flex zones
  • Avoid large or heavy components that could impede flexing
  • Increase pad size and use anchoring holes for connectors
  • Define solder mask openings larger than the pads

By following these flex PCB design guidelines, you can ensure a robust and manufacturable flexible circuit that will perform reliably in your application.

Assembly of flexible PCBs

Assembling components on a flexible PCB requires some special handling procedures to prevent damaging the flex circuit:

  • Handle flex PCBs by the edges only, avoid touching exposed circuits
  • Support the flex PCB to prevent bending during assembly
  • Use low-temperature solder and a fine-tip soldering iron
  • Avoid excessive heat exposure which could delaminate the flex layers
  • Secure wires and cables to strain relief points, not directly to solder joints
  • Use slow actuation when mating flex connectors to prevent tearing
  • Avoid bending assembled flex PCBs beyond their rated bend radius

For high-volume assembly of flexible PCBs, specialized equipment may be used such as:

  • Vacuum or magnetic pcb pallets to hold the flex circuit flat
  • Selective soldering machines with soft-touch tooling
  • Conductive epoxy or anisotropic conductive film for ZIF connectors
  • Continuous ovens with low-temperature profiles for solder reflow

Applications of flexible PCBs

Flexible PCBs are used in a wide range of industries and applications, wherever circuits need to fit into tight spaces or flex during use. Some common applications include:

Medical devices

  • Hearing aids
  • Pacemakers
  • Insulin pumps
  • Catheters
  • Endoscopes

Wearable electronics

  • Smartwatches
  • Fitness trackers
  • VR headsets
  • Smart clothing
  • Wearable medical monitors

Aerospace & defense

  • Avionics
  • Satellites
  • Missiles
  • Radar systems
  • Night vision goggles

Automotive electronics

  • Engine control units
  • Sensors
  • Displays
  • Cabin lighting
  • Wire harnesses

Consumer electronics

  • Smartphones
  • Laptops
  • Cameras
  • Gaming systems
  • LED lighting

Industrial equipment

  • Robotics
  • Motors
  • Sensors
  • Control panels
  • Portable instruments

As electronics continue to shrink in size and increase in complexity, the use of flexible PCBs will likely continue to grow across all of these applications.

Flexible pcb materials

The materials used in flexible PCBs must be able to withstand repeated bending and flexing without breaking or delaminating. Some common flexible PCB materials include:

Material Description Advantages
Polyimide Heat-resistant polymer film Excellent thermal stability, high strength-to-weight ratio
Polyester PET film Low cost, good electrical properties
PEEK Polyetheretherketone film Excellent mechanical and thermal properties, chemical resistance
LCP Liquid crystal polymer Low moisture absorption, high frequency performance
PEN Polyethylene naphthalate Transparent, high tensile strength

The choice of material depends on the specific requirements of the application, such as:

  • Operating temperature range
  • Chemical exposure
  • Mechanical stress and strain
  • Electrical performance (Dk/Df)
  • UL flammability rating
  • Flexibility (bend radius and cycles)
  • Cost

In addition to the base substrate material, flexible PCBs also use specialized materials for the copper traces, cover layers, and adhesives:

  • Rolled annealed copper for high ductility
  • Low-flow prepregs and coverlays for tight bend radii
  • Adhesiveless substrates for improved flexibility and reliability
  • Flexible soldermask inks for component protection

By selecting the appropriate combination of materials, flexible pcb designers can create circuits that meet the unique mechanical, environmental, and electrical requirements of each application.

Testing and qualification of flexible PCBs

Flexible PCBs must be rigorously tested to ensure they can withstand the stresses of repeated flexing and environmental exposure. Some common tests for flexible PCBs include:

  • Bend testing – Repeated bending to a specified radius for a certain number of cycles
  • Flexibility testing – Measuring the stiffness and flexibility of the PCB
  • Tensile testing – Pulling the PCB to measure its tensile strength and elongation
  • Fatigue testing – Cycling the PCB through a range of mechanical stresses
  • Moisture resistance testing – Exposing the PCB to high humidity and measuring its performance
  • Thermal cycling – Subjecting the PCB to alternating high and low temperatures
  • Shock and vibration testing – Simulating the mechanical shocks and vibrations the PCB may experience in use
  • Microsectioning – Cross-sectioning the PCB to inspect for delamination or other internal defects

In addition to these mechanical and environmental tests, flexible PCBs must also pass standard electrical tests such as:

  • Continuity testing – Verifying that all connections are intact
  • Insulation resistance testing – Measuring the resistance between conductors
  • Dielectric withstanding voltage – Applying a high voltage to check for leakage or breakdown
  • Impedance testing – Measuring the impedance of controlled-impedance traces

Flexible PCBs used in regulated industries such as medical devices or aerospace may also need to be qualified to industry-specific standards such as:

  • IPC-6013 – Qualification and performance specification for flexible printed boards
  • ASTM F2752 – Standard specification for medical flexible printed circuit materials
  • MIL-PRF-31032 – Performance specification for flexible circuit boards

By thoroughly testing and qualifying flexible PCBs, manufacturers can ensure that they will perform reliably in the field and meet all applicable quality and safety standards.

FAQ

What is the difference between a flexible PCB and a rigid PCB?

The main difference between flexible and rigid PCBs is the substrate material. Flexible PCBs use thin, bendable plastic films like polyimide, while rigid PCBs use stiff fiberglass-epoxy laminates like FR-4. This allows flexible PCBs to bend and fit into tight spaces, while rigid PCBs are limited to flat or gently curved surfaces.

How long do flexible PCBs last?

The lifespan of a flexible PCB depends on factors like the substrate material, copper thickness, bend radius, and number of flex cycles. A well-designed polyimide flex circuit can withstand tens of thousands of bend cycles at a moderate radius. However, repeated sharp bends or exposure to high temperatures or chemicals can degrade the circuit over time. Proper design and material selection are key to maximizing the lifespan of a flexible PCB.

Can you solder wires to a flexible PCB?

Yes, you can solder wires directly to the pads on a flexible PCB, just as you would with a rigid PCB. However, the soldering process requires more care to avoid damaging the thin, flexible substrate. Use a low-temperature solder and a fine-tipped iron, and avoid applying too much heat or pressure. It’s also a good idea to use strain relief on the wires to prevent them from pulling on the solder joints.

How small can a flexible PCB be?

Flexible PCBs can be made very small and thin, with minimum feature sizes down to a few microns. The limit is usually set by the capabilities of the manufacturing process and the size of the components being used. For example, a single-sided polyimide flex circuit can be as thin as 0.05mm, while a multilayer flex-rigid PCB might be a few millimeters thick. The minimum bend radius also limits how small a flex PCB can be folded or rolled up.

Are flexible PCBs more expensive than rigid PCBs?

In general, flexible PCBs are more expensive than rigid PCBs of the same size and complexity. The higher cost is due to the specialized materials, processes, and tooling required for flex circuit manufacturing. However, the total system cost may be lower with a flex PCB due to reduced weight, space, and assembly time. The cost of a flex PCB also depends on factors like the number of layers, overall size, and production volume. At high volumes, flex PCBs can be very cost-effective compared to rigid PCBs or wiring harnesses.