Electronics Guide

Flexible Substrate Materials

The choice of substrate material fundamentally affects flexible circuit performance and processing requirements:

• Polyimide (Kapton): The most common flex substrate, offering excellent thermal stability (withstands soldering temperatures), chemical resistance, and mechanical durability; available in thicknesses from 12.5 to 125 micrometers; amber color is characteristic
• Polyester (PET): Lower-cost alternative to polyimide with reduced thermal stability; suitable for applications not requiring soldering or high-temperature exposure; clear or tinted versions available
• Polyethylene naphthalate (PEN): Improved thermal and dimensional stability compared to PET while maintaining lower cost than polyimide; emerging as middle-ground option
• Liquid crystal polymer (LCP): Low moisture absorption and excellent high-frequency performance; used in RF and microwave flexible circuits
• Thermoplastic polyurethane (TPU): Highly stretchable substrate for applications requiring elasticity beyond simple bending

Substrate thickness affects both flexibility and durability. Thinner substrates bend more easily but are more susceptible to handling damage. The trade-off between flexibility and robustness must be evaluated for each application.

Copper-Clad Laminate Options

Flexible copper-clad laminates (FCCLs) form the starting material for most flexible PCB fabrication:

• Adhesive-based laminates: Copper foil bonded to polyimide using acrylic or epoxy adhesive; lower cost but adhesive layer limits flexibility and high-temperature performance
• Adhesiveless laminates: Copper directly deposited or bonded to polyimide without separate adhesive layer; improved flexibility, reliability, and fine-pitch capability at higher cost
• Copper weight options: Thinner copper (typically 9 to 18 micrometers, corresponding to quarter-ounce to half-ounce) improves flexibility; thicker copper provides better current capacity and easier processing
• Single-sided and double-sided: Single-sided laminates simplify construction; double-sided requires through-hole or via connections between layers

Prototyping Services and Equipment

Multiple pathways exist for flexible PCB prototyping:

• Quick-turn fabrication services: Companies like OSH Park, PCBWay, JLCPCB, and Flex PCB Inc. offer prototype flexible PCB fabrication with turnaround times of days to weeks; minimum order quantities as low as one piece enable economical experimentation
• Desktop PCB mills: Machines like Bantam Tools and LPKF systems can machine flexible substrates with appropriate fixturing and cutting parameters; requires careful workholding to prevent substrate movement
• Laser cutting systems: CO2 and UV lasers cut polyimide effectively; useful for outline cutting and creating flex-to-rigid transitions
• Photolithography: Standard PCB photolithography works on flexible substrates with appropriate handling; vacuum or electrostatic fixturing prevents substrate movement during processing

For many development projects, quick-turn fabrication services offer the best balance of capability, cost, and convenience. In-house processing may be justified for projects requiring rapid iteration or unusual specifications not available from standard services.

Design Considerations for Flex Circuits

Designing for flexible substrates requires attention to factors beyond conventional PCB design:

• Bend radius specification: Define minimum bend radius based on substrate thickness, copper weight, and whether bending is static (one-time installation) or dynamic (repeated flexing during operation)
• Trace routing in bend zones: Orient traces perpendicular to bend axes where possible; avoid routing along the neutral bend axis where cyclic strain is maximum
• Via and pad placement: Keep vias away from bend regions where they create stress concentrations; use tear-drop pads for improved reliability
• Stiffener design: Define rigid areas for component mounting using stiffeners of polyimide, FR-4, or metal; stiffener edges should be designed to minimize stress concentration at flex-to-rigid transitions
• Coverlay design: Flexible coverlay (solder mask equivalent) openings must be carefully designed; incorrect openings can initiate delamination during flexing
• Strain relief: Design strain relief features at cable exits and flex-to-rigid transitions to prevent conductor fracture from concentrated stress

Conductive Ink Materials

Multiple conductive ink formulations serve different applications and printing methods:

• Silver nanoparticle inks: Most common high-performance printed electronics ink; silver particles (typically 20 to 100 nanometers) in solvent carrier; requires sintering (thermal or photonic) to achieve low resistance; typical sheet resistance 10 to 100 milliohms per square
• Silver flake inks: Larger silver particles (1 to 10 micrometers) in polymer binder; lower cost than nanoparticle inks; air-dry or low-temperature cure; higher resistance but adequate for many applications
• Copper inks: Lower material cost than silver but require inert atmosphere sintering to prevent oxidation; emerging formulations with improved oxidation resistance
• Carbon-based inks: Carbon black or graphite in polymer binder; highest resistance but lowest cost; suitable for resistors, electrodes, and EMI shielding
• Conductive polymers: PEDOT:PSS and similar materials offer transparency and flexibility; lower conductivity than metal inks but unique properties for specific applications
• Particle-free inks: Metal-organic decomposition inks and reactive silver inks that form conductive traces without pre-formed particles; emerging technology with potential for improved resolution and substrate compatibility

Inkjet Printing Systems

Inkjet printing deposits precise droplets of conductive ink to form circuit patterns:

• Fujifilm Dimatix DMP-2850: Industry-standard materials development printer with piezoelectric printheads; accommodates custom ink formulations; 10-picoliter drop volume typical; suitable for research and low-volume prototyping
• Voltera V-One: Desktop conductive inkjet system designed for PCB prototyping; integrated dispensing and drilling capabilities; uses proprietary silver ink with room-temperature cure
• Nano Dimension DragonFly: Professional system for multilayer printed electronics including vias; combines conductive and dielectric printing for complete circuits
• Modified consumer printers: Desktop inkjet printers modified for conductive ink use; limited capability but lowest cost entry point for experimentation

Inkjet printing resolution depends on drop size, ink spreading characteristics, and substrate surface energy. Typical line widths range from 50 to 200 micrometers with careful optimization, though finer features are possible with specialized equipment and processes.

Screen Printing Platforms

Screen printing pushes conductive ink through a patterned mesh onto the substrate:

• Manual screen printing: Low-cost entry point using standard screen printing equipment; suitable for simple patterns with line widths above 200 micrometers
• Semi-automatic printers: Improved registration and consistency compared to manual printing; systems from companies like Systematic Automation and ATMA provide production capability
• Fully automatic printers: High-throughput systems for production volumes; precise squeegee pressure control and environmental management
• Stencil printing: Uses metal stencils rather than mesh screens for improved edge definition; common for solder paste also applicable to conductive inks

Screen printing achieves higher deposition thickness than inkjet in a single pass, providing lower resistance traces. However, resolution is typically limited to 75 to 100 micrometers minimum line width, coarser than inkjet capability.

Aerosol Jet and Direct-Write Systems

Aerosol jet printing creates fine aerosol of ink droplets focused by gas flow for high-resolution deposition:

• Optomec Aerosol Jet: Leading aerosol jet printing platform achieving line widths below 10 micrometers; excellent for fine-pitch interconnects, antenna patterns, and sensor elements
• nScrypt systems: Direct-write dispensing systems combining multiple deposition methods including precision syringe dispensing, jetting, and aerosol; suitable for thick-film deposition
• Integrated Deposition Solutions (IDS): Aerosol jet systems for electronics manufacturing applications

Aerosol jet systems achieve the finest resolution among printed electronics methods but at higher equipment cost. They excel for applications requiring precise trace geometry, conformal printing on 3D surfaces, or printing on temperature-sensitive substrates.

Sintering and Curing Methods

Most conductive inks require post-deposition treatment to achieve optimal conductivity:

• Thermal sintering: Heating printed traces to 150 to 300 degrees Celsius removes organic binders and sinters metal particles into continuous conductors; requires heat-tolerant substrates
• Photonic sintering: Intense pulsed light flash-heats the printed layer without bulk substrate heating; enables sintering on low-temperature substrates like paper and PET
• Plasma sintering: Low-temperature plasma treatment removes organic material and improves particle contact; suitable for temperature-sensitive substrates
• Chemical sintering: Chemical agents trigger particle coalescence at room temperature; emerging approach for extremely temperature-sensitive applications
• Laser sintering: Focused laser beam selectively sinters printed material; enables patterning without masking and sintering on heat-sensitive substrates

Stretchable Substrate Materials

Elastomeric substrates provide the mechanical foundation for stretchable circuits:

• Polydimethylsiloxane (PDMS): Most common stretchable electronics substrate; biocompatible, transparent, and stretchable to over 100% strain; Sylgard 184 is the standard formulation for research
• Thermoplastic polyurethane (TPU): Stretchable thermoplastic enabling heat-activated bonding and standard film processing; available in various hardness grades
• Ecoflex: Ultra-soft silicone elastomer with Shore hardness below 00-10; extremely stretchable for skin-contact applications
• Dragon Skin: Silicone with excellent mechanical properties and tear resistance; available in multiple hardness grades
• Hydrogels: Water-containing polymer networks providing ionic conductivity and tissue-like mechanical properties for biomedical applications

Stretchable Conductor Approaches

Creating conductors that maintain connectivity under stretch requires specialized approaches:

• Serpentine metal traces: Conventional metal traces patterned in serpentine shapes that unfold during stretching; enables use of standard metals like copper and gold; geometric design determines stretchability
• Metal mesh structures: Thin metal films patterned into mesh or fractal geometries that accommodate strain through local bending and rotation
• Liquid metal: Gallium-based liquid metals (galinstan, EGaIn) remain liquid at room temperature and maintain conductivity at any strain; encapsulated in elastomer channels
• Conductive composites: Conductive particles (silver, carbon) dispersed in elastomer matrix; stretchable but typically lower conductivity than metal approaches
• Carbon nanotube networks: Randomly oriented CNT networks maintain percolation under strain; can achieve transparency for display applications
• Conductive polymers: Intrinsically stretchable conducting polymers like PEDOT:PSS with appropriate additives

Stretchable Electronics Development Kits

Several platforms support stretchable electronics development:

• Academic reference designs: Published research from groups at UIUC, Stanford, and other universities provides detailed fabrication protocols for stretchable circuits
• Liquid metal starter kits: Suppliers like Indium Corporation provide liquid metal materials with application guides for stretchable interconnects
• Stretchable PCB services: Emerging services offer fabrication of serpentine-patterned stretchable circuits on flexible-rigid substrates
• Soft robotics platforms: Companies like Soft Robotics Inc. provide stretchable sensor and actuator development platforms

Design Strategies for Stretchability

Designing stretchable circuits requires consideration of mechanical deformation:

• Island-bridge architecture: Rigid islands containing conventional components connected by stretchable interconnects; concentrates strain in interconnect regions designed to handle it
• Strain engineering: Pattern conductors to localize strain in conductor segments while protecting component connections
• Prestrain strategies: Fabricating on prestrained substrates creates buckled structures that accommodate subsequent stretch by flattening
• Kirigami patterns: Strategic cuts transform materials to enable stretching through rotation and out-of-plane deformation
• Encapsulation design: Encapsulation layers must accommodate strain without delaminating or cracking; neutral plane placement minimizes conductor strain

Paper Substrate Options

Different paper types serve various paper electronics applications:

• Office paper: Standard copy paper provides an accessible starting point; high surface roughness and porosity limit fine-feature resolution
• Photo paper: Coated papers with smoother surfaces enable finer printed features; plastic coating may improve moisture resistance
• Cardboard and cardstock: Thicker substrates provide mechanical stability for larger devices and packaging integration
• Specialty papers: Papers engineered for printed electronics with controlled porosity, surface energy, and dimensional stability
• Nanopaper: Cellulose nanofiber papers with exceptional smoothness and mechanical properties; emerging high-performance paper substrate

Printing on Paper

Paper substrate characteristics influence printing process selection:

• Inkjet printing: Works on paper with attention to ink formulation and paper absorbency; coated papers generally provide better results than uncoated
• Screen printing: Effective for thicker deposits that provide continuity over paper surface texture
• Pencil drawing: Graphite from standard pencils provides resistive traces; simple and accessible for educational and prototyping purposes
• Pen-based conductive inks: Conductive ink pens (Circuit Scribe, AgIC) enable hand-drawn circuits on paper for education and rapid prototyping
• Stamping: Conductive stamps transfer patterns to paper; suitable for simple repeated patterns

Paper Electronics Applications

Paper electronics excel in applications leveraging their unique characteristics:

• Smart packaging: Integrated sensors and indicators for package condition monitoring; disposable with the packaging
• Diagnostic devices: Paper-based biosensors and diagnostic strips combining microfluidics with electronic readout
• RFID tags: Low-cost paper-based RFID antennas for item-level tracking
• Interactive media: Paper with embedded touch sensors, lights, or sound for interactive books, games, and marketing materials
• Environmental sensors: Disposable sensors for distributed environmental monitoring
• Educational platforms: Low-cost circuits for STEM education without specialized equipment

Paper Electronics Development Resources

Resources for paper electronics development include:

• Circuit Scribe: Conductive silver ink pen with educational kits demonstrating paper electronics principles
• Bare Conductive: Conductive paint and materials for paper and other porous substrates
• Chibitronics: Peel-and-stick LED circuits for paper crafts and education; demonstrates component integration with paper
• Academic resources: Extensive published research on paper electronics provides fabrication protocols and application examples

Conductive Yarns and Threads

Conductive yarns form the interconnects and sometimes the active elements in e-textiles:

• Metal fiber yarns: Stainless steel or other metal fibers spun into yarn; durable and highly conductive but may affect fabric hand feel
• Plated yarns: Textile yarns with metallic coating (silver, nickel, copper); balance of conductivity and textile properties depends on coating thickness and base fiber
• Wrapped yarns: Conductive wire wrapped around textile yarn core; provides conductivity while maintaining yarn flexibility
• Carbon fiber yarns: Carbon fibers provide moderate conductivity with excellent mechanical properties
• Conductive polymer fibers: Fibers made from or coated with conductive polymers; typically lower conductivity but fully polymeric composition

Fabric-Based Sensors

Textiles can incorporate various sensing modalities:

• Resistive strain sensors: Conductive materials that change resistance when stretched; knitted sensor structures provide engineered strain response
• Capacitive sensors: Conductive fabric layers separated by dielectric form pressure-sensitive capacitors
• Piezoresistive sensors: Pressure-sensitive conductive materials (Velostat, Eeonyx) change resistance under compression
• Temperature sensors: Resistive elements with temperature-dependent resistance integrated into fabric
• Humidity sensors: Conductive materials with moisture-dependent resistance for sweat and environmental monitoring
• Electrochemical sensors: Textile electrodes for biosignal acquisition (ECG, EMG) or chemical sensing

E-Textile Development Platforms

Platforms supporting e-textile development include:

• LilyPad Arduino: Pioneering sewable microcontroller platform with ecosystem of matching sensors, actuators, and power components
• Adafruit Flora: Circular sewable controller with Bluetooth option; extensive tutorial and project library
• SparkFun e-textile components: Range of sewable sensors, LEDs, and connectors compatible with LilyPad ecosystem
• Loomia: Industrial e-textile solutions including flexible circuit strips designed for textile integration
• Textile research looms: Computer-controlled jacquard looms enable weaving of custom electronic textile structures

Textile Integration Challenges

Successful e-textile development must address unique challenges:

• Washability: Electronics must survive water, detergent, and mechanical agitation; solutions include encapsulation, removable modules, and inherently water-resistant designs
• Mechanical durability: Connections and components must survive repeated bending, stretching, and abrasion during wear
• Comfort: Electronics should not create pressure points, stiffness, or skin irritation
• Manufacturing integration: Electronic components must integrate with textile manufacturing processes (sewing, weaving, knitting)
• Power and data connections: Interfaces between textile electronics and external systems require appropriate connector design

R2R Process Steps

Roll-to-roll processing can incorporate various unit operations:

• Web handling: Unwinding, tension control, and rewinding of flexible substrates at controlled speed
• Printing: Gravure, flexographic, screen, or inkjet printing of functional materials
• Coating: Slot-die, knife-over-roll, or spray coating for uniform material deposition
• Drying and curing: Hot air, infrared, or UV curing of deposited materials
• Lamination: Bonding of multiple web layers
• Patterning: Laser cutting, die cutting, or rotary screen patterning
• Inspection: Inline optical inspection for quality control

R2R Development Equipment

Equipment for R2R process development ranges from laboratory to pilot scale:

• Laboratory coaters: Benchtop systems from companies like MTI Corporation, Coatema, and RK PrintCoat enable coating process development on small web widths
• Pilot R2R systems: Larger systems with production-representative process conditions; facilities at national laboratories and universities provide access for development projects
• Modular R2R platforms: Configurable systems allowing different process combinations; Coatema and Yasui Seiki offer modular platforms
• R2R printing systems: Gravure and flexographic printers adapted for electronic materials

R2R Process Considerations

Developing for R2R manufacturing requires specific considerations:

• Registration: Aligning multiple printed layers requires accurate web tracking and registration systems
• Web tension: Consistent tension prevents wrinkling and stretch variation; tension requirements differ for different substrate materials
• Drying constraints: Inks and coatings must dry within available dwell time at acceptable temperatures
• Edge effects: Web edges often behave differently than center regions; designs should account for edge variation
• Material utilization: R2R processing typically leaves unused margins; designs should maximize material utilization

Rigid-Flex Architectures

Several rigid-flex architectures serve different applications:

• Flex-to-rigid transition: Flexible cable sections connect to rigid board areas; simplest architecture with established design rules
• Multilayer rigid-flex: Flexible layers integrated within multilayer rigid stackup; enables complex routing across flex zones
• Sculptured flex: Varying thickness within flex region optimizes bend characteristics
• Bookbinder construction: Rigid sections connected by flex hinges that fold like book pages
• Flying-lead flex: Flex conductors extending from rigid section for direct bonding to displays or other devices

Design Tools for Rigid-Flex

EDA tools supporting rigid-flex design include:

• Altium Designer: Comprehensive rigid-flex support with bend-line definition, 3D visualization of folded assemblies, and clearance checking across fold configurations
• Cadence Allegro: Professional rigid-flex capabilities with advanced constraint management
• Mentor PADS: Rigid-flex design support in mainstream PCB tool
• KiCad: Open-source option with emerging rigid-flex capabilities

3D visualization is particularly valuable for rigid-flex design, enabling verification that folded assemblies fit intended enclosures and that bend stresses remain within acceptable limits.

Rigid-Flex Prototyping Services

Prototype rigid-flex fabrication is available from specialized manufacturers:

• Advanced flexible circuits specialists: Companies like American Standard Circuits, Flexible Circuit Technologies, and Epec focus on rigid-flex and flex fabrication with appropriate process expertise
• Quick-turn services: Some quick-turn PCB services offer rigid-flex options, though with more limited complexity than specialists
• Full-service EMS providers: Contract manufacturers with rigid-flex capability handle both fabrication and assembly

Rigid-flex prototyping typically requires longer lead times and higher costs than standard rigid boards due to more complex fabrication processes. Early engagement with fabricators helps ensure manufacturability.

Rigid-Flex Design Guidelines

Successful rigid-flex design requires attention to several factors:

• Layer stackup: Plan layer transitions between rigid and flex regions carefully; not all layers need to extend through flex zones
• Bend zone routing: Follow flexible circuit routing guidelines in flex regions; perpendicular traces, no vias, appropriate copper weight
• Transition design: Rigid-to-flex transitions should be gradual with proper strain relief; avoid sharp corners that concentrate stress
• Material selection: Compatible materials for rigid and flex sections; consider coefficient of thermal expansion matching
• Assembly considerations: Plan component placement and assembly sequence considering fold requirements

Substrate Suppliers

Sources for flexible and specialty substrates include:

• DuPont: Kapton polyimide films in various thicknesses and formulations; industry standard for flexible electronics
• Panasonic: Flexible copper-clad laminates for flex PCB fabrication
• Sheldahl: Specialty flexible materials including flex circuits and laminates
• Dow: PDMS (Sylgard) and other elastomers for stretchable electronics
• Smooth-On: Dragon Skin and Ecoflex silicones for stretchable substrates

Conductive Ink Suppliers

Sources for conductive inks and pastes include:

• DuPont: Comprehensive line of screen-printable conductive inks
• Henkel: Conductive adhesives and inks for electronics assembly
• Sun Chemical: Printed electronics inks including silver and carbon formulations
• Novacentrix: Silver nanoparticle inks optimized for photonic sintering
• Voltera: Inks optimized for their desktop conductive inkjet system
• Electroninks: Particle-free reactive silver inks

E-Textile Materials

Sources for e-textile development materials include:

• Adafruit: Conductive thread, fabric, and sewable components
• SparkFun: LilyPad ecosystem components and conductive materials
• Less EMF: Shielding fabrics and conductive textiles
• Shieldex: Silver-plated conductive yarns and fabrics
• Eeonyx: Piezoresistive conductive materials

Wearable Devices

Wearable electronics benefit from flexible and conformable form factors. Fitness trackers, smartwatches, and health monitors use flex circuits for internal connections and increasingly incorporate stretchable sensors for body-contact measurements. Smart textiles integrate sensing and communication into garments for athletics, workplace safety, and medical monitoring.

Medical Devices

Medical applications leverage the biocompatibility and conformability of flexible electronics. Skin-mounted patches monitor vital signs continuously. Implantable devices benefit from flexible circuits that conform to tissue. Disposable diagnostic devices use printed electronics for cost-effective single-use sensors.

Consumer Electronics

Consumer products increasingly incorporate flexible electronics. Foldable displays require flexible driving circuits. Curved televisions and monitors use flex circuits to connect components across non-planar surfaces. Wearable audio devices fit flex circuits into compact, ergonomic enclosures.

Internet of Things

Distributed sensing applications benefit from low-cost printed sensors and flexible form factors. Smart packaging with printed RFID and sensors enables supply chain monitoring. Environmental sensing networks deploy large numbers of low-cost sensors. Building-integrated electronics conform to architectural surfaces.

Automotive and Aerospace

Transportation applications use flexible electronics for weight reduction and packaging efficiency. Interior lighting integrates into curved surfaces using flex circuits. Structural health monitoring embeds flexible sensors into composite materials. In-mold electronics combine printed circuits with injection-molded plastic parts.

Starting with the Application

Effective flexible and printed electronics development begins with clear understanding of application requirements. Not every application benefits from these technologies; conventional rigid electronics may be more appropriate where flexibility provides no advantage. Identify specific requirements that drive the choice of flexible or printed approaches, whether conformability, weight, cost, or unique form factor.

Iterative Prototyping

Flexible and printed electronics development typically requires more iteration than conventional PCB design. Material behavior, processing parameters, and reliability characteristics may differ from expectations based on rigid electronics experience. Plan for multiple prototype iterations and include characterization steps to build understanding of material and process behavior.

Reliability Testing

Flexible electronics face mechanical stresses that conventional electronics do not. Develop appropriate reliability test protocols including bend testing, flex cycling, stretch testing (for stretchable designs), and environmental exposure. Understand failure modes early in development to inform design improvements.

Manufacturing Considerations

Consider manufacturing requirements from the beginning of development. Processes that work well for prototyping may not scale to production. Engage with potential manufacturing partners early to ensure designs are manufacturable at intended volumes and costs.