Printed circuit board is a layered structure where copper traces are processed onto an insulating surface using chemical and mechanical processes to provide mechanical support and electrical connections for electronic components. PCBs, which are used in the internal structure of almost all electronic devices today, offer cost and time advantages in mass production thanks to their suitability for automation in both design and production stages.

History

The first circuits, which were created by manually connecting electronic components, were built using wire wrap and point-to-point methods until the mid-20th century. In the 1950s, the idea of processing copper foil onto an insulating substrate via photolithography was developed, and thus the first industrial PCBs were born. The widespread adoption of surface-mount technology (SMT) in the 1960s paved the way for multi-layer designs and higher component density.

Structure and Materials

PCBs generally consist of the following components:

• Substrate: Mostly FR-4 (glass fiber-epoxy) is used; it offers high mechanical strength and good dielectric properties.
• Conductive Layers: Copper foil is converted into traces by chemical etching or engraving methods.
• Solder Mask: Applied as a protective layer to prevent short circuits and provide an aesthetic appearance.
• Silk-screen: White or black ink printing is done to show component positions.

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Production Processes

1. Photo-lithography and Etching: Photo-curable resin is coated onto the copper-clad substrate, the desired pattern is exposed with UV light, and the remaining copper is etched in an acid bath.
2. Drilling and Plating (Vias): Holes opened for inter-layer connections are made conductive by chemical or electro-plating.
3. Solder Mask Application: CNC printing machines apply masks using screen printing or jet printing methods.
4. Test and Quality Control: Error detection is performed with automatic optical inspection (AOI) and electrical testing (Flying Probe or ICT).

Types and Applications

• Single-Layer PCB: For simple circuits, it offers low cost and fast prototyping.
• Double-Layer PCB: Can house components on both top and bottom surfaces; ideal for medium-complexity circuits.
• Multi-Layer PCB: Up to 4, 6, or even 40 layers; preferred for high-density or frequency-sensitive applications.
• Flexible and Rigid-Flex PCB: Allows mounting in areas with complex geometries thanks to flexible substrates; used in medical devices and wearable technologies.

Assembly and Soldering

• Through-Hole Technology (THT): Component leads are passed through holes and soldered on the bottom surface.
• Surface-Mount Technology (SMT): Miniature components are placed directly on the surface; common in portable devices.
• Reflow and Wave Soldering: Reflow ovens are used for SMT, and wave soldering machines 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. In addition, the UL 796 standard determines criteria such as flammability and heat resistance.