3.4 Flexible printed circuits

Flexible printed circuits are a game-changer in soft robotics. They allow electronics to be integrated into bendy, stretchy structures. These circuits use conductive traces on flexible substrates, enabling them to flex without breaking electrical connections.

Materials like polyimide and conductive inks are key to making these circuits work. Manufacturing techniques include printing, etching, and lamination. Designers must consider electrical and mechanical properties to create reliable, functional circuits for soft robots.

Flexible printed circuit basics

• Flexible printed circuits (FPCs) are a key component in soft robotics, enabling the integration of electronics into flexible and stretchable structures
• FPCs consist of conductive traces printed or deposited onto a flexible substrate material, allowing the circuit to bend and flex without damaging the electrical connections

Materials for flexible substrates

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• Polymeric films are commonly used as flexible substrates due to their mechanical properties and dielectric characteristics
• Polyimide (PI) is a popular choice for its high thermal stability, chemical resistance, and good mechanical strength
• Polyethylene terephthalate (PET) offers transparency and low cost, making it suitable for disposable or single-use applications
• Thermoplastic polyurethane (TPU) provides excellent stretchability and can be used for circuits that require high elongation

Conductive inks and adhesives

• Conductive inks are used to print the electrical traces on the flexible substrate
• Silver-based inks are widely used for their high conductivity and compatibility with various printing methods
• Carbon-based inks offer lower cost and can be used for resistive elements or electrodes
• Conductive adhesives are employed for bonding components or creating interconnects between layers

Single vs multilayer circuits

• Single layer circuits have all the conductive traces on one side of the substrate
• Multilayer circuits consist of multiple layers of conductive traces separated by dielectric layers
• Multilayer designs allow for higher circuit density and more complex routing
• Vias and through-holes are used to connect traces between layers in multilayer circuits

Flexible circuit manufacturing

Printing methods for circuits

• Screen printing is a common method for depositing conductive inks onto flexible substrates
• Inkjet printing offers high resolution and can be used for precise patterning of traces
• Flexography is a high-speed printing process suitable for large-scale production
• Gravure printing uses an engraved cylinder to transfer ink onto the substrate

Etching and plating techniques

• Etching involves selectively removing unwanted conductive material to create the desired circuit pattern
• Photolithography is used to transfer the circuit design onto the substrate before etching
• Copper is a common choice for the conductive layer due to its high conductivity and ease of etching
• Electroplating can be used to increase the thickness of the conductive traces for improved current carrying capacity

Lamination and bonding processes

• Lamination is used to bond multiple layers of flexible substrates together
• Adhesive lamination involves using a bonding agent between the layers
• Thermal lamination uses heat and pressure to fuse the layers together
• Bonding techniques are also used to attach components or connectors to the flexible circuit

Electrical properties of flexible circuits

Conductor resistance and current capacity

• The resistance of the conductive traces depends on the material, thickness, and width of the traces
• Current carrying capacity is determined by the cross-sectional area of the conductors and the maximum allowable temperature rise
• Increasing the thickness or width of the traces can reduce resistance and improve current capacity

Dielectric constant and loss tangent

• The dielectric constant (relative permittivity) of the substrate material affects the capacitance and impedance of the circuit
• Loss tangent is a measure of the dielectric losses in the substrate, which can impact signal integrity and power dissipation
• Low dielectric constant materials are preferred for high-frequency applications to minimize signal delay and crosstalk

Impedance control in flexible PCBs

• Controlling the impedance of the traces is crucial for high-speed signals and to prevent reflections
• Impedance is determined by the geometry of the traces, the dielectric constant of the substrate, and the spacing between traces
• Techniques such as adjusting trace width, using ground planes, and employing differential signaling can help maintain consistent impedance

Mechanical properties of flexible circuits

Bending and flexing characteristics

• The ability of a flexible circuit to bend and flex without damage depends on the materials used and the design of the circuit
• The minimum bend radius is a key parameter that specifies the tightest bend the circuit can withstand without failure
• Factors such as substrate thickness, copper thickness, and adhesive properties influence the bending performance

Stress and strain analysis

• Stress and strain analysis is used to evaluate the mechanical behavior of flexible circuits under loading conditions
• Finite element analysis (FEA) can be employed to simulate the stress distribution and identify potential failure points
• Understanding the stress and strain limits of the materials is essential for designing reliable flexible circuits

Fatigue life and reliability

• Fatigue life refers to the number of bending cycles a flexible circuit can endure before failure
• Factors such as the bend radius, copper thickness, and substrate material affect the fatigue life
• Accelerated life testing can be conducted to assess the long-term reliability of flexible circuits under repeated bending and environmental stresses

Designing flexible circuits for soft robotics

Circuit layout and routing considerations

• The layout and routing of flexible circuits should consider the mechanical requirements and motion of the soft robotic system
• Traces should be routed to minimize stress concentrations and avoid areas of high bending or stretching
• Using curved traces and avoiding sharp corners can help improve the flexibility and reliability of the circuit

Interconnects and terminations

• Designing robust interconnects and terminations is crucial for reliable connections between the flexible circuit and other components
• Zero insertion force (ZIF) connectors and flexible printed circuit (FPC) connectors are commonly used for connecting flexible circuits
• Soldering, conductive adhesives, and anisotropic conductive film (ACF) bonding are techniques used for attaching components to flexible circuits

Electromagnetic interference (EMI) shielding

• EMI shielding is important to protect the flexible circuit and sensitive components from electromagnetic interference
• Conductive materials such as copper or silver ink can be used to create shielding layers on the flexible circuit
• Proper grounding and the use of shielded connectors can help mitigate EMI issues

Integrating flexible circuits in soft robots

Embedding circuits in elastomeric structures

• Flexible circuits can be embedded within soft elastomeric materials to create integrated and compact soft robotic systems
• Molding processes such as injection molding or casting can be used to encapsulate the flexible circuit within the elastomer
• Considerations such as material compatibility, adhesion, and strain distribution should be taken into account when embedding circuits

Connecting to rigid components and sensors

• Soft robots often require interfaces between flexible circuits and rigid components such as microcontrollers or sensors
• Flexible-to-rigid connectors or board-to-board connectors can be used to establish reliable connections
• Techniques such as soldering, conductive epoxies, or crimping can be employed for attaching wires or connectors

Strain relief and mechanical protection

• Providing strain relief and mechanical protection is essential to ensure the longevity and reliability of flexible circuits in soft robots
• Strain relief can be achieved through the use of flexible adhesives, encapsulation, or mechanical clamping
• Protective layers such as additional substrate materials or elastomeric coatings can be applied to shield the circuit from abrasion or environmental factors

Applications of flexible circuits in soft robotics

Wearable and epidermal electronics

• Flexible circuits enable the development of wearable and epidermal electronic devices that can conform to the human body
• Examples include flexible sensors for monitoring physiological signals (ECG, EMG), and stretchable displays for visual feedback
• Flexible circuits allow for comfortable and unobtrusive integration of electronics into wearable soft robotic systems

Soft sensors and actuators

• Flexible circuits can be used to fabricate soft sensors and actuators for soft robotic applications
• Capacitive and resistive sensing principles can be employed to create flexible touch, pressure, and strain sensors
• Flexible heating elements can be integrated into soft actuators to enable shape memory alloy (SMA) or thermally responsive actuation

Soft controller boards and interfaces

• Flexible circuits can be designed as soft controller boards to process signals and control the behavior of soft robots
• Flexible microcontroller boards can be developed by integrating rigid components onto flexible substrates
• Soft-rigid hybrid designs can be used to combine the benefits of flexible circuits with the functionality of rigid electronic modules