What material are PCBs made of?

PCB Substrate Materials

The substrate is the base material of a PCB, providing mechanical support and electrical insulation for the copper traces and components. The choice of substrate material depends on the specific requirements of the application, such as thermal stability, dielectric constant, and cost. Some common PCB substrate materials include:

FR-4

FR-4 (Flame Retardant 4) is the most widely used PCB substrate material. It is a composite material made of woven fiberglass cloth with an epoxy resin binder. FR-4 offers good mechanical strength, electrical insulation, and flame resistance. It is suitable for most general-purpose applications and is relatively inexpensive.

Property	Value
Dielectric Constant	4.7 @ 1 MHz
Dissipation Factor	0.02 @ 1 MHz
Thermal Conductivity	0.3 W/mK
Glass Transition Temperature (Tg)	130-140°C
Flammability Rating	UL94 V-0

Rogers Materials

Rogers Corporation offers a range of high-performance PCB substrate materials for demanding applications such as high-frequency RF and microwave circuits. These materials have low dielectric loss, stable dielectric constant, and excellent thermal properties. Some popular Rogers materials include:

• RO4000 Series: These hydrocarbon ceramic laminates offer a low dielectric constant and low loss, making them suitable for high-frequency applications up to 18 GHz.
• RT/duroid: These PTFE-based composites provide excellent electrical and mechanical stability, with a low dielectric constant and low loss. They are ideal for microwave and millimeter-wave applications.

Material	Dielectric Constant	Dissipation Factor	Thermal Conductivity
RO4350B	3.48 @ 10 GHz	0.0037 @ 10 GHz	0.6 W/mK
RT/duroid 5880	2.20 @ 10 GHz	0.0009 @ 10 GHz	0.2 W/mK

Polyimide

Polyimide is a high-performance polymer that offers excellent thermal stability, chemical resistance, and mechanical strength. It is often used in flexible and rigid-flex PCBs for applications that require high reliability and durability, such as aerospace and military electronics.

Property	Value
Dielectric Constant	3.5 @ 1 MHz
Dissipation Factor	0.002 @ 1 MHz
Thermal Conductivity	0.2 W/mK
Glass Transition Temperature (Tg)	260°C
Flammability Rating	UL94 V-0

Copper Foil

Copper foil is laminated onto the PCB substrate to create the conductive traces and pads for components. The thickness of the copper foil is typically measured in ounces per square foot (oz/ft²), with common thicknesses ranging from 0.5 oz/ft² to 2 oz/ft². Thicker copper foil is used for high-current applications or to improve thermal dissipation.

Copper Thickness	Metric Equivalent	Typical Applications
0.5 oz/ft²	17 μm	Low-power, high-density designs
1 oz/ft²	35 μm	General-purpose PCBs
2 oz/ft²	70 μm	High-current, power electronics

The copper foil is typically electrodeposited (ED) or rolled annealed (RA). ED copper has a rougher surface and provides better adhesion to the substrate, while RA copper has a smoother surface and is preferred for high-frequency applications due to its lower surface roughness.

Solder Mask

Solder mask is a thin, protective layer applied over the copper traces on a PCB, leaving only the pads and other areas that need to be soldered exposed. It serves several purposes:

1. Prevents solder bridging between closely spaced pads
2. Protects the copper traces from oxidation and corrosion
3. Provides electrical insulation between adjacent traces
4. Improves the aesthetics of the PCB

Solder mask is typically made of a photoimageable polymer that is applied as a liquid or dry film and then exposed and developed to create the desired pattern. The most common solder mask colors are green and blue, but other colors such as red, yellow, and black are also available.

Property	Value
Dielectric Strength	>1000 V/mil
Insulation Resistance	>10^12 Ω
Flammability Rating	UL94 V-0
Thickness	0.5-1 mil (12.7-25.4 μm)

Silkscreen

Silkscreen is a layer of text and symbols printed on the surface of a PCB for identification and assembly purposes. It is typically white, but other colors are also available. The silkscreen layer is used to display information such as:

1. Component outlines and reference designators
2. Polarity and orientation markings
3. Test points and fiducial markers
4. Company logos and product labels

Silkscreen is usually printed using a screen-printing process with an epoxy-based ink that is cured at high temperatures for durability.

Surface Finishes

Surface finishes are applied to the exposed copper pads on a PCB to improve solderability, protect against oxidation, and enhance the assembly process. Some common surface finishes include:

Hot Air Solder Leveling (HASL)

HASL is a traditional surface finish where the PCB is dipped in molten solder and then blown with hot air to remove excess solder, leaving a thin, uniform layer on the pads. HASL provides good solderability and is relatively inexpensive, but it can result in uneven surfaces and is not suitable for fine-pitch components.

Electroless Nickel Immersion Gold (ENIG)

ENIG is a two-layer surface finish consisting of a thin layer of gold over a layer of nickel. The nickel provides a diffusion barrier and improves the gold’s adhesion to the copper, while the gold offers excellent solderability and corrosion resistance. ENIG is suitable for fine-pitch components and has a long shelf life, but it is more expensive than HASL.

Immersion Silver (IAg)

IAg is a single-layer surface finish that deposits a thin layer of silver directly onto the copper pads. It offers good solderability and is less expensive than ENIG, but it has a shorter shelf life and may tarnish over time.

Organic Solderability Preservative (OSP)

OSP is a thin, organic coating applied to the copper pads that protects against oxidation and improves solderability. It is the most cost-effective surface finish and has a low environmental impact, but it has a limited shelf life and may require special handling during assembly.

Surface Finish	Thickness	Shelf Life	Suitable for Fine-Pitch
HASL	1-25 μm	12 months	No
ENIG	Ni: 3-6 μm, Au: 0.05-0.2 μm	12-24 months	Yes
IAg	0.1-0.3 μm	6-12 months	Yes
OSP	0.2-0.5 μm	3-6 months	Yes

Frequently Asked Questions (FAQ)

1. What is the most common PCB substrate material?

A: FR-4 is the most widely used PCB substrate material due to its good balance of mechanical, electrical, and thermal properties, as well as its cost-effectiveness.

2. What are the advantages of using high-performance substrate materials like Rogers?

A: High-performance substrate materials like Rogers offer lower dielectric loss, more stable dielectric constant, and better thermal properties compared to FR-4. These properties make them suitable for demanding applications such as high-frequency RF and microwave circuits.

3. What is the purpose of solder mask on a PCB?

A: Solder mask serves several purposes, including preventing solder bridging between closely spaced pads, protecting the copper traces from oxidation and corrosion, providing electrical insulation between adjacent traces, and improving the aesthetics of the PCB.

4. What are the differences between HASL and ENIG surface finishes?

A: HASL is a traditional surface finish that provides good solderability and is relatively inexpensive, but it can result in uneven surfaces and is not suitable for fine-pitch components. ENIG, on the other hand, offers excellent solderability, corrosion resistance, and is suitable for fine-pitch components, but it is more expensive than HASL.

5. How does the thickness of the copper foil affect PCB performance?

A: The thickness of the copper foil on a PCB affects its current-carrying capacity and thermal dissipation. Thicker copper foil is used for high-current applications or to improve thermal dissipation, while thinner copper foil is used for low-power, high-density designs. Common copper foil thicknesses range from 0.5 oz/ft² to 2 oz/ft².

In conclusion, PCBs are composed of various materials, each serving a specific purpose in ensuring the proper functioning, reliability, and durability of the board. The choice of substrate material, copper foil thickness, solder mask, silkscreen, and surface finish depends on the specific requirements of the application, such as frequency, power handling, environmental conditions, and cost. Understanding the properties and characteristics of these materials is essential for designing and manufacturing high-quality PCBs that meet the demands of modern electronics.