|SHENGYI||S1000||Low CTE FR4||/24||100||–||V-0||175||335||4.8||–||0.013||–|
|FR406||FR4||/24, /26, /28||60||1000||–||170||295||–||–||–||–|
|FR4||/24, /26, /98||115||54||V-0||180||350||4.7||4.5||0.016||0.017|
|IS-410||FR4||/24, /26, /28||129||1100||V-0||180||350||–||–||–||–|
|IT600||High CTI FR4||/21||60||–||V-0||140||305||4.8||–||0.018||–|
For more information on Laminates, visit the following websites.
SHENG YI – www.syst.com.cn
ROGERS – www.rogerscorp.com
ISOLA – www.isola-group.com
ARLON – www.arlon-med.com
ITEQ – www.iteq.com.tw
TACONIC – www.taconic-add.com
NAN YA – www.npc.com.tw
DUPONT – www.dupont.com
NELCO – www.parkelectro.com
TAIFLEX – www.taiflex.com check if the entire unmasked area has been etched or not. In case it is not etched, leave it in the solution for some more time.
Printed circuit boards (PCBs) are made up of various raw materials that provide the required electrical, thermal, and mechanical characteristics. Selecting the right base materials is crucial during the PCB design and fabrication process to achieve the desired performance and reliability goals for the end application.
This article will provide an overview of the major types of raw materials used in PCB manufacturing, including:
- Substrate materials
- Conductive layers
- Dielectric layers
- Final surface finishes
- Solder mask and silkscreen
- Through-hole plating
Understanding the properties and trade-offs of common PCB raw materials allows designers to make informed selections when specifying a board stackup.
PCB Substrate Materials
The substrate forms the base laminate material that makes up the core layers of a PCB. Key attributes like dielectric constant and thermal performance are determined by the substrate chosen. Common options include:
The most common and cost-effective substrate material used in PCBs is FR-4 glass epoxy. It consists of woven fiberglass cloth bonded with flame-resistant epoxy resin. FR-4 provides good mechanical strength and manufacturability. It has a dielectric constant of around 4.5.
This variant of FR-4 is formulated with a high glass transition temperature resin system. High-Tg FR-4 has improved thermal and mechanical properties compared to standard FR-4. It enhances high-temperature operation.
Cellulose paper reinforced epoxy laminates defined under the NEMA grade CEM-1. It has similar electrical properties to FR-4 but superior thermal performance. The dielectric constant is slightly higher at 4.7.
This NEMA grade denotes woven glass reinforced epoxy. It offers improved mechanical strength over CEM-1 paper composites while maintaining good thermal conductivity. Dielectric constant remains around 4.7.
Known for excellent electrical performance at high frequencies, PTFE is a fluoropolymer material with a very low dielectric constant of 2.1. It has exceptional thermal stability but relatively poor mechanical strength.
Used when very high temperature operation is required, polyimide laminates retain properties at over 300°C. The dielectric constant of 3.4 is lower than FR-4, with good signal integrity. Polyimide offers high cost and lower fabrication yields.
Additional high-performance substrate materials like polyphenylene ether (PPE), liquid crystal polymer (LCP), and quartz glass are used for specialized applications. But cost and manufacturability are concerns.
The trade-offs between cost, electrical performance, thermal management, and mechanical properties must be weighed when selecting PCB substrate materials.
Copper foils act as the primary conductive layers for traces, planes, and pads in PCBs. Thicker copper and alloy alternatives provide various advantages:
The standard foil used is rolled high purity copper with defined weights measured in ounces per square foot (oz/ft2). Common weights are 1⁄2 oz, 1 oz, 2 oz, and 3 oz. Thicker copper improves current handling and reliability.
Rolled Copper Alloy
Foils made from stronger copper alloys including brass, bronze, and copper-iron allow reduced copper thickness while maintaining durability. This lowers costs and weight.
A very pure copper foil produced through electrodeposition. It provides excellent bond strength to dielectrics but higher cost limits applications mainly to innerlayers only.
ED Copper Alloy
These electrodeposited copper alloys like copper-cobalt and copper-molybdenum offer high tensile strength and temperature resistance compared to standard ED copper foils.
Copper Clad Aluminum (CCA)
CCA uses thin copper foil bonded to an aluminum core for better thermal performance and reduced weight. It trades off electrical conductivity versus pure copper designs.
Matching the conductor materials to the current loads, mechanical requirements, and cost targets of the PCB allows optimization of the stackup.
Dielectric Layer Materials
Dielectrics are the insulating layers between copper foil conductors in multilayer boards. Key material properties impact PCB performance:
Glass fabric pre-impregnated with FR-4 epoxy resin that flows during lamination to bond the copper layers together. This is the most common and cost-effective dielectric option.
High-Tg FR-4 Prepreg
Similar to standard FR-4 but engineered with a high Tg resin system for improved thermal and mechanical properties. This enhances PCB reliability.
Provides exceptional thermal resistance and stable electrical performance at temperatures above 170°C. Typical dielectrics are aromatic polyimides like Kapton®. Cost is higher than epoxy.
PTFE (Teflon®) Prepreg
Pure PTFE prepregs have excellent electrical attributes but require special laminating processes like fusion bonding due to limited resin flow properties. Blended versions improve manufacturability.
Cyanate Ester Prepreg
Features low loss, moisture resistance, and good dielectric strength. It has high costs, limited suppliers, and specialized processing requirements.
Ceramic Filled Prepregs
Pre-impregnated dielectric materials with ceramic particle fillers to achieve higher thermal conductivity while maintaining electrical isolation between copper layers.
Selecting compatible dielectric materials with suitable electrical, thermal, and mechanical characteristics allows engineering of robust multilayer PCB stackups.
PCB Surface Finishes
Multiple surface finish options protect exposed copper traces from oxidation and provide improved solderability:
Organic Solderability Preservative (OSP)
A widely used coating that provides good shelf life and minimally impacts assembly process performance. Not suitable for mating connectors.
Immersion Tin (Sn)
Deposits a thin layer of tin which quickly oxidizes to provide an excellent solderable surface. Relatively low cost but prone to whisker growth over time.
Immersion Silver (Ag)
Silver coatings prevent oxidation and maintain consistent solderability long term. Adds cost but provides excellent shelf life with minimal assembly impact.
Electroless Nickel Immersion Gold (ENIG)
A nickel corrosion barrier layer is plated first, followed by a thin gold coating. This finish provides the best wire bondability but higher material expenses.
Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
Matching the PCB finish to the soldering process, assembly duration, and performance requirements optimizes the board stackup.
Solder Mask and Silkscreen
Solder mask and silkscreen coatings protect boards and components during assembly and operation:
Liquid Photoimageable Solder Mask (LPISM)
The predominant solder mask consists of epoxy, acrylic, or silicone resins that are selectively exposed and developed to form openings around pads. LPISM provides the best durability, resolution, and performance.
Dry Film Solder Mask
An alternative approach laminates photo-sensitive dry films onto PCBs using heat and pressure. Dry film masks offer faster processing but lower feature resolution versus LPISM.
Epoxy Ink Silkscreen
Solvent-based epoxy inks screened onto the PCB through stencils produce the printed component identifications and polarity markings. These permanent prints withstand fabrication and assembly.
Durable solder mask and markings are essential for PCB protection, legend visibility, and facilitating automated assembly.
Plated Through Hole (PTH) Materials
Metallization of drilled holes electrically connects layers and allows component leads to pass through and mount to boards:
Electrolytic copper is the standard PTH plating. Thickness ranges from 0.5-2.0 mils depending on hole size and current loads. Copper alone leaves holes prone to oxidation.
Applying a solder coating over copper PTH plating facilitates component hole insertion and provides environmental protection. Common alloys are tin-lead or tin-silver-copper.
A thin gold plating over nickel or palladium enhances connectivity and inhibits corrosion for high-reliability PTH boards. Gold also benefits hole-wall solderability.
Choosing the right PTH plating affects hole conductivity, solderability, and reliability when inserting and soldering component leads.
Bonding Films and Adhesives
Various adhesive films bond layers together and provide critical mechanical reinforcement:
FR-4 Bonding Films
Semi-cured FR-4 prepreg layers that flow and cure during lamination to bond core and prepreg layers together into an integrated board.
Reinforced Core Bonding Films
Filled epoxy and polyimide films with fiberglass cloth reinforcement for enhanced layer-to-layer adhesion strength compared to unfilled bond films.
Acrylic and Urethane Adhesives
Used for bonding sheet metal stiffeners, heat sinks, and other enhancements onto board surfaces when high shear and tensile strength is required.
Bonding materials tailored for the substrate materials and intended operating conditions are key to producing multilayer PCBs able to survive thermal cycling, shock, and vibration exposure.
The raw materials used in each layer of a PCB stackup determine the electrical, thermal, and mechanical characteristics that enable the board to function reliably under application operating requirements. Utilizing the right combination of substrate laminates, conductive foils, dielectric prepregs, surface platings, masks, and adhesives allows PCB designers to optimize the materials selection for performance, durability, and cost goals.
Frequently Asked Questions
What are the most important criteria when selecting PCB substrate materials?
Key considerations include dielectric properties, loss characteristics, thermal conductivity, coefficient of thermal expansion, and mechanical stiffness. The requirements depend on the specific application.
How do I know what copper weight to use for my PCB layer stackup?
Thicker copper improves current capacity and thermal performance but increases cost. 1 oz. copper is typical for outer layers and 0.5-1 oz. for inner layers depending on the circuit current demands.
What are the trade-offs between standard and high-Tg PCB materials?
High Tg resins improve thermal performance, dimensional stability, and reliability but have higher costs and more complex lamination requirements.
When would ENIG versus immersion silver be used as a PCB finish?
How do I ensure good bonding between PCB layers?
Use compatible prepreg and lamination processes suited for the materials. Filled bonding films enhance mechanical adhesion. Carefully follow lamination press cycles with defined temperature, pressure, vacuum, and ramp rates.