Printed circuit boards (PCBs) are essential components in modern electronics, providing the framework to connect and support electronic components through conductive pathways. These pathways are etched into a substrate, creating a reliable and organised system for electrical connections. Understanding what printed circuits are made of is crucial for comprehending their performance, durability, and applications across industries.
Layers of a PCB
PCBs are made up of multiple layers, each serving a specific purpose to ensure the board’s functionality and stability. These primary layers include:
Substrate (base material)
The substrate forms the foundation of a PCB, providing mechanical support and insulation for the conductive layers. Common substrates include:
- FR-4 (Flame Retardant 4): The most widely used substrate, made from woven fibreglass cloth impregnated with an epoxy resin. It offers excellent strength, insulation, and thermal resistance.
- CEM (Composite Epoxy Material): A less expensive alternative to FR-4, made from woven glass fabric combined with paper and epoxy resin.
- Flexible Substrates: Materials such as polyimide are used for flexible PCBs, allowing the board to bend and conform to various shapes.
Copper layer
The copper layer is the conductive part of the PCB that forms the circuit pathways. It is typically applied to the substrate through a laminating process and is available in varying thicknesses, measured in ounces per square foot.
- Single-sided PCBs: These have one layer of copper.
- Double-sided PCBs: These have copper layers on both sides of the substrate.
- Multilayer PCBs: These consist of multiple layers of copper and substrate stacked together to achieve complex circuit designs.
Solder mask
The solder mask is a protective layer applied over the copper traces to prevent oxidation and accidental short circuits. It also gives the PCB its distinctive colour, often green, although other colours such as red, blue, and black are also used.
Silkscreen
The silkscreen layer is used to apply labels, symbols, and text on the PCB for easier assembly and troubleshooting. It typically uses non-conductive ink, printed in white or other contrasting colours.
Surface finish
The surface finish protects the exposed copper pads and ensures a reliable connection during soldering. Common surface finishes include:
- HASL (Hot Air Solder Levelling): A lead or lead-free solder coating.
- ENIG (Electroless Nickel Immersion Gold): A thin layer of gold over nickel, providing excellent durability and conductivity.
- OSP (Organic Solderability Preservative): A water-based finish that protects copper pads from oxidation.
Materials used in PCB
The choice of materials in PCB construction significantly impacts the board’s performance, reliability, and cost. Below are the key materials used:
- Fibreglass (FR-4) – Fibreglass is the most common material for rigid PCB substrates. It provides excellent strength, durability, and resistance to heat and moisture.
- Polyimide – Polyimide is used for flexible and high-temperature PCBs. It offers superior flexibility, chemical resistance, and thermal stability.
- Copper foil – Copper foil is used to create the conductive pathways. The thickness of the copper layer depends on the current-carrying requirements of the circuit.
- Epoxy resin – Epoxy resin is used to bond the substrate layers together and provide insulation. It also acts as an adhesive for the copper layers.
- Solder mask ink – The solder mask ink is applied to the board to protect the copper traces and improve its durability. It is typically made from epoxy-based or UV-cured materials.
- Silkscreen ink – The silkscreen ink is used for printing labels and markings on the PCB. It must be non-conductive and resistant to heat and solvents.
Environmental considerations
The production and disposal of PCBs have environmental impacts, prompting manufacturers to adopt more sustainable practices. This includes reducing hazardous materials, recycling old PCBs, and complying with regulations such as RoHS (Restriction of Hazardous Substances) to limit the use of harmful substances like lead, mercury, and cadmium.
Conclusion
Printed circuits are composed of various materials, each chosen to ensure the board’s functionality, durability, and reliability. From the substrate to the surface finish, every layer plays a vital role in the performance of a PCB. As technology advances, the materials used in PCB construction continue to evolve, focusing on improving efficiency, sustainability, and performance.
