A PCB (printed circuit board) forms the backbone of an electronics assembly, providing the foundation on which components are mounted and interconnected. The materials and construction of PCBs are precisely engineered to provide a robust, reliable platform for building circuits.
This article will provide an in-depth look at what makes up a PCB by exploring the key material layers, manufacturing processes, and design considerations involved.
PCB Substrate Materials
The PCB substrate provides the base laminated structure onto which circuits are built. The most common PCB substrate material is FR-4:
FR-4 is a composite of woven fiberglass cloth infused with an epoxy resin binder. The fiberglass cloth provides mechanical strength and stability, while the epoxy resin offers electrical insulation and bonds the layers together.
The epoxy is known as “flame resistant 4” (FR-4), hence the name. FR-4 makes up the core of the PCB sandwich. It has become the standard substrate for cost and performance reasons.
Alternatives to FR-4 include:
- CE-grade paper
- FR-2 phenolic cotton
- PTFE (Teflon)
But FR-4’s optimal balance of mechanical, electrical, and cost factors has led to its dominance as the substrate of choice for over 90% of PCBs today.
Copper Foil Layers
Copper forms the conductive layer of PCBs. Thin sheets of copper foil, typically 0.5 to 2 oz weight, are laminated to the top and bottom surfaces of the FR-4 core.
This foil is photoetched to form the traces, pads, vias, and patterns that make up the circuit wiring. Most PCBs use 1 oz copper foil, providing a good balance of weight and conductivity.
Some applications utilize thicker 2 oz or 3 oz copper for:
- High current carrying capacity
- Better heat dissipation
- Reduced resistive losses
Fine line PCBs may use thinner 0.5 oz foil to enable tightly spaced traces.
More complex PCBs sandwich additional copper layers between multiple FR-4 substrate cores to form multilayer boards.
Having more layers enables increased circuit density by allowing traces to cross between layers. This is key for packing complex circuits into smaller PCBs.
High-speed design also relies on multilayer boards to provide controlled impedance signal routing and isolated ground planes. The layer count for advanced PCBs can reach 20 or more.
Solder Mask Layer
The solder mask is a coat of polymer lacquer that is applied over the outer copper layers. It coats the entire board except for exposed contact pads.
Solder mask serves several purposes:
- Insulates copper traces to prevent shorting
- Protects circuits from environmental factors
- Controls solder flow when components are attached
- Improves PCB aesthetics with branding or markings
Green is the most common solder mask color, but blue, red, yellow, black, and white masks are also used for specific applications.
The solder mask completes the basic set of materials that make up a finished PCB.
PCB Material Properties
Why are these particular materials used in PCBs? Let’s look at some of the key properties:
FR-4 Epoxy Resin
- Excellent electrical insulator with high resistivity
- Provides mechanical robustness and bond strength
- Relatively low cost and widely available
- Good thermal performance and stability
- Flame resistant rating improves safety
- Reinforces the epoxy resin mechanically
- Provides dimensional stability
- Absorbs stresses and resists warping
- Excellent thermal conductivity
- Low thermal expansion
- Highly conductive allowing low resistance traces
- Solderable and corrosion resistant
- Highly conductive to aid heat dissipation
- Ductile enough to be photoetched
- Available in a range of weights and sizes
Solder Mask Lacquer
- Electrically isolates copper from environment
- Protects circuits from oxidation and moisture
- Withstands flux and cleaning chemicals
- Provides markings for branding and components
- Color coding aids circuit identification
The synergistic properties of these materials allow PCBs to provide the ideal foundation for electronic circuits.
PCB Fabrication Process Overview
The raw PCB materials must go through an extensive fabrication process to form a finished board. Here is an overview:
Sheets of copper foil are arranged in a stack alternating with FR-4 substrate cores. The cross-section is built up to match the desired layer count.
The stack is fused together under heat and pressure of up to 400psi. The FR-4 cores bond with the copper foils to form a solid laminated block.
Small holes called vias are drilled through the laminated stack to interconnect buried copper layers. Larger holes may be drilled for component leads.
The walls of the drilled holes are plated with copper to form a conductive interconnection between layers.
A photosensitive resist is applied to the copper layers. Ultraviolet light is used to transfer the desired circuit pattern onto the resist. Chemical etching then removes unwanted copper, leaving just the resist-protected traces.
The solder mask layer is applied over the entire PCB surface except for exposed solder pads. Screen printing, inkjet, or photolithography techniques can pattern the mask.
Printed markings and component outlines are added using the silkscreen process. Epoxy-based inks are transferred through a stencil onto the solder mask.
This sequence of steps transforms the raw PCB materials into a functional board ready for electronics assembly.
PCB Design Considerations
To leverage the capabilities of PCB materials, designers must consider a number of factors:
Layer Stackup Sequence
Arrange signal layers strategically to maximize routing capabilities and minimize crosstalk. Follow stackup guidelines for characteristic impedance.
Trace Width and Spacing
Match to current levels and density requirements. Allow for thermal expansion and manufacturing tolerances.
Use thicker copper for high power, high frequency, or multilayer boards. Reduce foil weight for tight trace spacing.
Solder Mask Expansion
Account for mask alignment tolerance and expansion when masking exposed copper features.
Select via size and plating based on layers, currents, and manufacturability. Minimize void potential.
Finish and Plating
Choose surface finish based on soldering requirements, environmental conditions, and contact durability needs.
Ensure marking sizes and colors provide sufficient contrast and clarity on the chosen solder mask color.
In summary, PCBs are made from:
- FR-4 substrate – epoxy resin and fiberglass cloth layers
- Copper foil conductors
- Solder mask lacquer coating
- Silkscreen markings
These materials are fabricated through an intricate sequence of processes to produce the finished board. PCB designers must account for the capabilities and limitations of these materials and processes to translate circuit schematics into a rugged, reliable PCB.
The combination of engineered materials, precision manufacturing, and considered design enables PCBs to provide the foundation for almost every modern electronic device we rely on.
Frequently Asked Questions
What is the main material used in making PCB substrates?
FR-4, a composite of fiberglass cloth and an epoxy resin binder, is the predominant PCB substrate material used today.
What are the key advantages of FR-4?
It offers an excellent balance of electrical insulation, mechanical strength, thermal performance, dimensional stability, and cost effectiveness.
How are the conductive traces formed on PCBs?
Copper foil sheets are laminated onto the substrate, then patterns are photoetched away to leave just the desired conductive traces and pads.
What is the purpose of the solder mask layer?
The solder mask insulates and protects traces while controlling solder flow when components are mounted. It can be colored for aesthetics or circuit identification.
What fabrication processes are used to manufacture PCBs?
Major processes include layer lamination, drilling, plating, photolithographic patterning, mask application, and silkscreen printing to transform raw materials into circuit boards.