Introduction
The raw materials that make up a printed circuit board dictate many aspects of its performance, manufacturability and reliability. PCBs integrate a wide array of specialized substrates, resins, reinforcers, foils and coatings together into a high performance electronic circuit platform.
This article provides an in-depth overview of key PCB raw materials including:
- The composition and properties of major PCB material types
- Characteristics of common PCB substrates and laminates
- Reinforcing fibers, foils and conductive layers used
- PCB coatings like solder masks and legends
- Specifications for critical material properties
- Leading global manufacturers of PCB raw materials
- Recent developments in materials for advanced PCBs
- How material selection affects PCB design and fabrication
- Examples of selecting optimal materials for applications
Understanding PCB material options allows electrical engineers to make informed selections to fulfill cost, performance and manufacturing goals.
Major Types of PCB Raw Materials
The core raw materials in a PCB include:
Substrate – The base dielectric material forms the core and prepreg layers. Common options:
- Woven fiberglass reinforced epoxy (FR-4)
- Ceramic-filled PTFE composites (Rogers RO3003)
- Cyanate ester epoxy blends (Panasonic Megtron 6)
- Polyimide films (Dupont Kapton)
Reinforcing Fabric – Woven fiberglass cloth adds mechanical strength. Options:
- Standard E-glass (~10um fiber diameter)
- Ultra-thin E-glass (0.5-3um diameters)
- Quartz glass (for low loss)
- Non-woven mats
Metal Foil – Thin copper foils form circuit traces. Two types:
- Rolled annealed copper
- Electrodeposited copper
Final Finishes – Coatings protect and facilitate soldering:
- Organic Solderability Preservatives (OSP)
- Immersion Silver (I-Ag)
- Immersion Tin (I-Sn)
- Electroless Nickel Immersion Gold (ENIG)
Solder Mask – Primary PCB coating:
- Liquid photoimageable solder mask (LPSM)
- Dry film solder mask
Bonding Films – Adhesives for laminating multilayers:
- B-stage epoxy
- Acrylic
- Phenolic butyral
Via Fill Material – Fills holes in HDI boards:
- Plating resins
- Conductive pastes
- Laser direct structuring (LDS) materials
These comprise the major raw materials integrated into PCBs during fabrication. Next we examine key properties and characteristics.
Properties of PCB Raw Materials
The properties of the raw materials profoundly impact PCB performance. Some key considerations include:
Electrical Properties
- Dielectric constant (Dk)
- Loss tangent (Df)
- Dielectric strength
- Volume resistivity
Physical Properties
- Coefficient of thermal expansion (CTE)
- Glass transition temperature (Tg)
- Decomposition temperature
- Moisture absorption
Mechanical Properties
- Flexural strength
- Compressive strength
- Tensile strength
- Fracture toughness
Chemical Properties
- Water absorption
- Solvent resistance
- Thermal oxidative stability
- Flame retardance
Manufacturing Properties
- Bonding compatibility
- Platability
- Solderability
- Drillability
- Coplanarity
Material selection requires optimizing this multidimensional property space for the application.
PCB Substrate Materials Types
One of the first material choices is the substrate laminate or dielectric. Common options include:
FR-4
- Glass fabric reinforced epoxy resin
- Flame retardant (FR) rating
- Low cost, widely available
- Good manufacturability
- Dk ~4.5, Loss tangent 0.02
- Tg 130-140°C
High Frequency Laminates
- PTFE or ceramic filled hydrocarbon resin systems
- Engineered for stable and low loss RF properties
- Tight dielectric tolerances
- Examples: Rogers RO3003 (PTFE), RO4350B (hydrocarbon)
High Thermal Conductivity Laminates
- Ceramic or carbon filled polymers
- Thermal conductivities up to 4X better than FR-4
- Manage high power densities
- Examples: Ventec VT-5A, Panasonic Megtron6-CA-HE
High Temperature Laminates
- Withstand over 280°C during assembly and operation
- Polyimide, cyanate ester, Quartet resins
- Demanding aerospace and automotive applications
Metal Core Boards
- Dielectrics bonded to aluminum or copper substrate
- Vias transfer heat to metal core
- Excellent thermal management
The substrate constitutes a major portion of the finished PCB material composition.
Reinforcing Fabrics for PCBs
Most PCB substrates incorporate reinforcing woven glass fabrics. Varieties include:
Standard Woven E-Glass
- Widely used with excellent strength and cost
- Yarn diameter around 10 um
- Square weave pattern common
Ultra-thin Glass Fabrics
- Yarn diameters from 0.5 to 3 um
- Enables finest PCB features <5um
- Handles lead-free assembly temperatures
Quartz Glass Fabrics
- Lower dielectric losses for high frequencies
- Moderate strength with limited drapability
- Costs more than standard E-glass
Non-woven Fabrics
- Mats with randomly distributed fibers
- Isotropic properties and reduced stresses
- Applications like antenna substrates
The fiber weave style, density, and electrical properties determine optimal applications.
Metal Foils for PCBs
Copper foils form the conductive traces that distribute signals across a PCB. Types include:
Rolled Annealed Copper Foil
- Produced by rolling and annealing electrolytic copper
- Lower cost
- Rougher surface limits fine features
- 1⁄4, 1⁄2, 1 and 2 oz thicknesses common
Electrodeposited Copper Foil
- Electroplated onto drum produces smooth surface
- Tighter thickness tolerance
- Supports finerfeatures and spaces
- More expensive
- 1⁄4, 1⁄2, 1, 2 and 3 oz options
Heavier Copper Foils
- 3 to 6 oz foils for high current power boards
- Reduces resistive losses
Very Thin Copper Foils
- 9um (1/4 oz) down to 3um foils
- Allows smaller vias and fine features for HDI
- Prone to breaks and tears requiring handling care
Foil selection balances cost, manufacturability and PCB impedance needs.
PCB Coatings and Finishes
Protective coatings and metallic finishes play key roles:
Solder Mask
- Epoxy overcoat prevents solder bridging
- Also insulates and protects PCB
- Liquid photoimageable (most common)
- Epoxy ink identifies components
- Legends and logos
OSP (Organic Solderability Preservative)
- Organic coating protects copper from oxidation
- Allows soldering after months of storage
Immersion Silver
- Coats copper to prevent oxidation
- Silver layer allows solderability
Immersion Tin
- Thin tin coating avoids oxidation
- Lower cost than silver
ENIG (Electroless Nickel Immersion Gold)
- Nickel corrosion barrier + gold outer layer
- Gold enables wire bonding
These finishes protect, solder, bond and identify the completed PCB.
PCB Bonding Materials
Bonding films and adhesives laminate multilayer boards:
B-Stage Epoxy
- Glass fabric reinforced epoxy prepregs
- Flows under heat and pressure
- Bonds layers while providing insulation
Acrylic Adhesive
- Thermoplastic acrylic bonding film
- Some flexibility after cure for stress relief
- Fast curing for quick lamination
Phenolic Butyral
- Thermoplastic resin film
- No fabric reinforcement
- Low flow for bonding inner layers
Fluoropolymer Adhesives
- Maintains dielectric properties at high GHz frequencies
- Example: Rogers RO4400TM series bondplys
Bonding materials critically impact interconnect reliability in multilayer PCBs.
Via Fill Materials
Filled vias in high density interconnect (HDI) boards avoid stub resonance:
Copper Plating Resins
- Plated copper deposits filled with polymer
- Cured to form solid via plugs
Conductive Pastes
- Silver, copper or carbon filled pastes
- Dispensed then cured or sintered
LDS Materials
- Laser activates polymer for subsequent copper plating
- No additional hole filling steps
This provides continuity of ground connections and avoids stub effects.
PCB Material Manufacturers
Some leading global manufacturers of PCB raw materials include:
Substrate Laminates
Reinforcing Fabrics
- Owens Corning
- Saertex
- Valuetex
- Technic Glass
Metal Foils
- Oak Mitsui
- Furukawa Electric Co
Solder Masks
- Taiyo Ink Mfg. Co
- Tamura Corp
- Coates Screen
- Peters Group
Bonding Films
- Rogers Corp
- Park Electrochemical Corp
- Arisawa Mfg. Co
These companies and others supply the materials that make up finished circuit boards.
Recent Advances in PCB Materials
Ongoing R&D targets new material capabilities:
Thermally Conductive Dielectrics
- Enables improved thermal design and cooling of high power PCBs
Flexible Substrates
- Withstand millions of dynamic bending cycles
Tighter Dielectric Tolerances
- Reduce impedance variability for multi-GHz applications
Lower CTE Materials
- Minimize thermal stresses and warpage
Hydrophobic Substrates
- Repel moisture and fluids for harsh environments
Biocompatible Materials
- Non-toxic, inert materials for medical electronics
Higher Glass Transition Temperatures
- Withstand lead-free soldering and temperature cycling
Advances in materials science support new PCB capabilities and applications.
Impact of Materials on PCB Design and Fabrication
Material selection impacts many aspects of PCB implementation:
- Materials compatibility with fabrication processes
- Drillability, platability, reliability, reparability
- Dielectric properties influence layout density, signal speed
- Coefficients of thermal expansion affect multilayer registration
- Glass transition temperatures limit assembly temps
- Thermal conductivity guides power plane design
- Cost and lead times drive prototype vs. production material choices
Collaboration between designers and manufacturers ensures optimal material usage.
PCB Material Selection Examples
Here are some examples of choosing substrates for specific applications:
High Speed Digital Interface
- Requires impedance control up to 5GHz
- Selected low-loss laminate: Megtron 6-m with Dk of 3.8
- Paired with low-loss prepreg: Megtron 6-p
16 Layer Telecom Switching Board
- Complex board with tight alignment tolerances
- Selected: Getek PCB substrate with low Z-axis CTE
Electric Vehicle Power Inverter
- High thermal loads up to 150°C junction temps
- Selected ceramic-filled fluoropolymer laminate: Rogers RO4835HTC
Flexible Wearable Health Monitor
- Flexible substrate able to conform to body
- Chose single-clad DuPont Pyralux AP flexible laminate
The right materials balance electrical, thermal, mechanical, and fabrication needs.
Frequently Asked Questions
Here are some common questions surrounding PCB materials:
Q: What are the trade-offs between standard FR-4 substrates versus advanced high frequency materials?
FR-4 offers lower cost and wide availability but with higher loss. Advanced materials provide tightly controlled properties but at higher price.
Q: What are pros and cons of rolled annealed vs. electrodeposited copper foils?
Electrodeposited foils allow finer features but cost more. Rolled foils are lower cost but have rougher surfaces limiting resolution.
Q: How are ceramic or polymer fillers used in PCB substrates?
Fillers like silica improve dimensional stability, thermal conductivity and dielectric properties but increase cost.
Q: What parameters indicate good drillability of a PCB material?
Low thermal expansion, high decomposition temperature, and glass transition temperature above 150°C improve drillability.
Q: What are key considerations when selecting bonding films?
Compatibility with substrate materials, ease of processing, bonding strength after lamination, and suitable dielectric properties.
Conclusion
The raw materials integrated into a printed circuit board determine fabrication processes, electrical performance, reliability profiles, and cost factors. Leveraging the comprehensive overview of major PCB materials provided here allows engineers to make optimal selections based on application requirements and design objectives. As the breadth of PCB material options continues accelerating, a strong grasp of material properties, costs and capabilities will prove increasingly invaluable.
