Printed Circuit Board (Printed Circuit Board, PCB)

Printed circuit board is a layered structure in which copper traces are chemically and mechanically processed onto an insulating surface to provide mechanical support and electrical connections for electronic components. Today, PCBs are used in nearly all electronic devices and offer cost and time advantages in mass production due to their suitability for automation in both design and manufacturing stages.

History

Early electronic circuits, constructed by manually connecting components, were built until the mid-20th century using wire wrap and point-to-point techniques. In the 1950s, the concept of patterning copper foil onto an insulating substrate using photolithography was developed, leading to the first industrial PCBs. In the 1960s, the widespread adoption of surface-mount technology (SMT) enabled the development of multilayer designs and higher component densities.

Structure and Materials

PCBs typically consist of the following components:

• Substrate: FR-4 (glass fiber-epoxy) is most commonly used; it provides high mechanical strength and excellent dielectric properties.
• Conductive Layers: Copper foil is transformed into traces through chemical etching or milling processes.
• Solder Mask: A protective layer applied to prevent short circuits and improve aesthetic appearance.
• Silk-screen: White or black ink printing is used to indicate component positions.

Manufacturing Processes

1. Photolithography and Etching: A photo-sensitive resin is applied to the copper-clad substrate, exposed to UV light through a patterned mask, and the unexposed copper is removed in an acid bath.
2. Drilling and Plating (Vias): Holes drilled between layers are made conductive through chemical or electroplating processes.
3. Solder Mask Application: Solder masks are applied using CNC screen printing or inkjet printing systems.
4. Testing and Quality Control: Errors are detected using automated optical inspection (AOI) and electrical testing methods such as flying probe or in-circuit testing (ICT).

Types and Applications

• Single-Layer PCB: Used for simple circuits; offers low cost and rapid prototyping capabilities.
• Dual-Layer PCB: Can accommodate components on both top and bottom surfaces; ideal for moderately complex circuits.
• Multilayer PCB: Can have up to 4, 6 or even 40 layers; preferred for high-density or frequency-sensitive applications.
• Flexible and Rigid-Flex PCB: Provide mounting capability in complex geometries thanks to flexible substrates; used in medical devices and wearable technologies.

Assembly and Soldering

• Through-Hole Technology (THT): Component leads are inserted through holes and soldered on the underside.
• Surface-Mount Technology (SMT): Miniature components are placed directly onto the board surface; common in portable devices.
• Reflow and Wave Soldering: Reflow ovens are used for SMT, while wave soldering machines are used for THT.

Environmental and Legal Regulations

The European Union’s RoHS (Restriction of Hazardous Substances) directive prohibits the use of hazardous elements such as lead and cadmium in PCBs and solder materials. Additionally, the UL 796 standard defines criteria such as flammability and heat resistance.