Printed circuit board materials

Types of PCB Materials

FR-4

FR-4 is the most common PCB material, consisting of a woven fiberglass cloth impregnated with an epoxy resin. It offers good mechanical strength, electrical insulation, and heat resistance. FR-4 is suitable for a wide range of applications, from consumer electronics to industrial equipment.

Property	Value
Dielectric Constant	4.35 @ 1 MHz
Dissipation Factor	0.02 @ 1 MHz
Thermal Conductivity	0.3 W/mK
Tg (Glass Transition Temperature)	130-140°C
CTE (Coefficient of Thermal Expansion)	14-16 ppm/°C

High Tg FR-4

High Tg FR-4 is a variant of standard FR-4, with a higher glass transition temperature (Tg) of 170-180°C. This makes it more suitable for applications that require higher thermal stability, such as automotive and aerospace electronics.

Property	Value
Dielectric Constant	4.35 @ 1 MHz
Dissipation Factor	0.02 @ 1 MHz
Thermal Conductivity	0.3 W/mK
Tg (Glass Transition Temperature)	170-180°C
CTE (Coefficient of Thermal Expansion)	14-16 ppm/°C

Polyimide

Polyimide is a high-performance polymer known for its excellent thermal stability, chemical resistance, and mechanical strength. It is often used in applications that demand reliability under harsh conditions, such as aerospace, military, and medical electronics.

Property	Value
Dielectric Constant	3.5 @ 1 MHz
Dissipation Factor	0.002 @ 1 MHz
Thermal Conductivity	0.2 W/mK
Tg (Glass Transition Temperature)	260°C
CTE (Coefficient of Thermal Expansion)	12-16 ppm/°C

Rogers High-Frequency Materials

Rogers Corporation offers a range of High-Frequency PCB materials designed for applications such as wireless communication, radar, and satellite systems. These materials provide low dielectric loss, stable dielectric constant, and controlled thermal expansion.

Material	Dielectric Constant	Dissipation Factor	Thermal Conductivity	Tg	CTE
RO4003C	3.38 @ 10 GHz	0.0027 @ 10 GHz	0.71 W/mK	>280°C	11-14 ppm/°C
RO4350B	3.48 @ 10 GHz	0.0037 @ 10 GHz	0.62 W/mK	>280°C	10-12 ppm/°C
RT/duroid 5880	2.20 @ 10 GHz	0.0009 @ 10 GHz	0.20 W/mK	>260°C	31-48 ppm/°C

Ceramic PCBs

Ceramic PCBs use ceramic substrates, such as alumina (Al2O3) or aluminum nitride (AlN), instead of organic materials. They offer excellent thermal conductivity, high Dielectric strength, and low dielectric loss, making them suitable for high-power and high-frequency applications.

Material	Dielectric Constant	Dissipation Factor	Thermal Conductivity	Flexural Strength
Alumina (96%)	9.4 @ 1 MHz	0.0004 @ 1 MHz	24-28 W/mK	345 MPa
Aluminum Nitride	8.8 @ 1 MHz	0.001 @ 1 MHz	170-180 W/mK	300 MPa

Flexible PCBs

Flexible PCBs use thin, flexible substrates such as polyimide or polyester to create circuits that can bend, fold, or conform to various shapes. They are used in applications that require compact packaging or dynamic flexing, such as wearable devices, medical implants, and automotive electronics.

Material	Dielectric Constant	Dissipation Factor	Tensile Strength	Elongation
Polyimide	3.5 @ 1 MHz	0.002 @ 1 MHz	231 MPa	72%
Polyester	3.2 @ 1 MHz	0.005 @ 1 MHz	138 MPa	50%

Copper Foil

Copper foil is the conductive layer used to form the circuit patterns on a PCB. It is available in various thicknesses, typically measured in ounces per square foot (oz/ft²). Common thicknesses include:

• 1/2 oz/ft² (17.5 µm)
• 1 oz/ft² (35 µm)
• 2 oz/ft² (70 µm)
• 3 oz/ft² (105 µm)

The choice of copper thickness depends on the current carrying requirements and the desired trace width and spacing.

Solder Mask

Solder mask is a protective coating applied over the copper traces on a PCB, leaving only the areas intended for soldering exposed. It serves several purposes:

1. Prevents solder bridges between closely spaced traces
2. Protects the copper from oxidation and corrosion
3. Provides electrical insulation
4. Enhances the visual appearance of the PCB

Solder mask is typically available in green, but other colors like red, blue, yellow, and black are also used.

Silkscreen

Silkscreen is a printed layer on the PCB that provides text, logos, and component outlines for easy assembly and identification. It is usually applied on top of the solder mask and is available in various colors, with white being the most common.

PCB surface finishes

Surface finishes are applied to the exposed copper areas on a PCB to protect them from oxidation and enhance solderability. Common surface finishes include:

HASL (Hot Air Solder Leveling)

HASL involves dipping the PCB in molten solder and using hot air to remove excess solder, creating a thin, uniform layer of solder on the exposed copper. It is an economical and widely used surface finish.

ENIG (Electroless Nickel Immersion Gold)

ENIG is a two-layer surface finish consisting of a nickel layer (3-6 µm) covered by a thin gold layer (0.05-0.2 µm). It provides excellent solderability, corrosion resistance, and shelf life, making it suitable for high-reliability applications.

OSP (Organic Solderability Preservative)

OSP is a thin, transparent organic coating applied to the exposed copper to protect it from oxidation. It is an economical and environmentally friendly option but has a limited shelf life compared to other surface finishes.

Immersion Silver

Immersion silver is a thin layer of silver (0.1-0.5 µm) chemically deposited onto the exposed copper. It offers good solderability and electrical conductivity, and is often used as a lead-free alternative to HASL.

Factors to Consider When Choosing PCB Materials

Electrical Properties

• Dielectric constant: Affects signal propagation speed and impedance
• Dissipation factor: Determines the amount of signal loss
• Dielectric strength: Measures the material’s ability to withstand electrical breakdown

Thermal Properties

• Glass transition temperature (Tg): Indicates the temperature at which the material starts to soften
• Thermal conductivity: Measures the material’s ability to dissipate heat
• Coefficient of thermal expansion (CTE): Determines the material’s dimensional stability under temperature changes

Mechanical Properties

• Flexural strength: Measures the material’s resistance to bending
• Tensile strength: Indicates the material’s resistance to pulling forces
• Elongation: Determines the material’s ability to stretch without breaking

Environmental Factors

• Operating temperature range
• Humidity resistance
• Chemical resistance

Manufacturing Considerations

• Ease of processing
• Compatibility with standard PCB manufacturing processes
• Cost and availability

FAQ

Q1: What is the most common PCB material?

A1: FR-4 is the most widely used PCB material, offering a good balance of electrical, mechanical, and thermal properties at a reasonable cost.

Q2: What are the advantages of using ceramic PCBs?

A2: Ceramic PCBs offer excellent thermal conductivity, high dielectric strength, and low dielectric loss, making them suitable for high-power and high-frequency applications.

Q3: What is the purpose of solder mask on a PCB?

A3: Solder mask serves several purposes, including preventing solder bridges, protecting copper from oxidation and corrosion, providing electrical insulation, and enhancing the visual appearance of the PCB.

Q4: What factors should I consider when choosing a PCB material?

A4: When selecting a PCB material, consider factors such as electrical properties (dielectric constant, dissipation factor), thermal properties (glass transition temperature, thermal conductivity), mechanical properties (flexural strength, tensile strength), environmental factors (operating temperature range, humidity resistance), and manufacturing considerations (ease of processing, cost).

Q5: What is the difference between HASL and ENIG surface finishes?

A5: HASL (Hot Air Solder Leveling) is an economical surface finish that involves dipping the PCB in molten solder, while ENIG (Electroless Nickel Immersion Gold) is a two-layer surface finish consisting of a nickel layer covered by a thin gold layer, offering excellent solderability, corrosion resistance, and shelf life.

In conclusion, understanding the properties and characteristics of various PCB materials is crucial for designing and manufacturing reliable and high-performance electronic devices. By carefully considering the electrical, thermal, mechanical, and environmental requirements of your application, you can select the most appropriate PCB material and surface finish to ensure optimal performance and longevity.