Printed Circuit Board

A printed circuit board (PCB), also known as a printed wiring board, is a laminated composite structure designed to support and electrically connect electronic components. It functions as the foundational platform upon which most modern electronic devices are built. A PCB consists of alternating layers of conductive copper and insulating material patterned to create precisely routed electrical pathways. These pathways replace the bulky wiring used in earlier generations of electronic assemblies, allowing for high-density, lightweight, and reliable circuit configurations.

Structure, Function, and Manufacturing Methods

A PCB is formed through chemical milling or etching, in which copper sheets laminated onto non-conductive substrates are selectively removed to produce conductive traces, planes, and pads. Components are commonly mounted on the outer layers using soldering, a process that both electrically bonds and mechanically secures them to the board.
When circuits require interconnection between layers, metallised drilled holes known as vias are incorporated. These openings enable vertical conduction in multi-layer boards and are essential in complex systems where simple single- or double-sided layouts are insufficient. PCBs are used in nearly all modern electronic products, providing a unified method of mounting components and routing electrical connections.
Although alternatives such as wire-wrap or point-to-point construction once dominated electronic assembly, these methods have largely fallen out of use. They lack the repeatability, compactness, and automated manufacturing capability made possible by PCB design and fabrication. Layout processes, now commonly achieved through electronic design automation software, must be completed only once; thereafter, identical boards can be mass-produced quickly and cost-effectively.
PCBs can be fabricated as:

• Single-sided boards, with copper traces on one surface
• Double-sided boards, with copper on both sides of one substrate
• Multi-layer boards, with stacked and laminated substrates containing internal copper layers

Multi-layer PCBs allow for significantly higher component densities, as inner layers provide additional routing capacity. However, such designs are more difficult to repair or modify due to their complexity. By 2024, the global market for bare PCBs surpassed USD 80 billion, reflecting the technology’s centrality to the electronics industry.

Early Construction Methods and Technological Origins

Before PCBs, circuits were assembled using point-to-point wiring on metal or wooden chassis. Components were mounted on insulating supports, and their leads were directly soldered or connected using screw terminals, jump wires, or crimp connectors. Assemblies were bulky, fragile, labour-intensive, and expensive, especially because vacuum tubes were common components.
Efforts to create printed circuits began in the early twentieth century. Innovations included:

• In 1903, Albert Hanson proposed laminated foil conductors in multiple layers.
• In 1904, Thomas Edison experimented with metallising patterns on insulating materials.
• In 1913, Arthur Berry patented a printed and etched circuit method.
• In the United States, Max Schoop developed metal flame-spraying techniques.
• Charles Ducas patented an electroplating process for printed circuit patterns in 1925.

John Sargrove’s ECME system of 1936–47 represented an important precursor, as it automated the spraying of metal traces onto Bakelite panels, producing radio boards at rapid rates.

Development of Early Printed Circuit Boards

The printed circuit as recognised today was developed by Paul Eisler, an Austrian engineer working in the United Kingdom, who incorporated the technology into a radio set in 1936. By 1941, multilayer PCBs appeared in German naval mines, and by 1943 the United States adopted the technology for proximity fuzes during the Second World War. These fuzes required circuits that could survive extreme mechanical stresses and be produced in large numbers.
The Centralab Division of Globe Union developed a suitable approach using ceramic substrates screen-printed with metallic inks for conductors and carbon inks for resistors. The process was patented under military classification, and contributions to printed-circuit technology were later recognised by the IEEE, which honoured Harry W. Rubinstein for foundational advances.

Post-War Expansion and Industrial Adoption

The U.S. government released PCB technology for commercial use in 1948. By the early 1950s, manufacturers were adapting it for consumer products. Motorola announced its adoption of plated circuits for home radios in 1952, and companies such as Hallicrafters soon followed.
Despite these developments, point-to-point wiring persisted in larger products into the late 1960s. Smaller devices, such as portable radios, increasingly relied on PCBs to reduce cost, weight, and size. Early PCBs primarily used through-hole technology, in which component leads passed through drilled holes and were soldered to copper pads. The AutoSembly process developed in the late 1940s advanced automated assembly and wave soldering.
By the 1980s, surface-mount technology (SMT) began replacing through-hole components, offering miniaturisation and simplified automated placement at the expense of repair difficulty. The 1990s saw the widespread introduction of multilayer SMT boards across consumer, industrial, and military electronics.

High-Density Interconnect and Modern Innovations

Technological progress continued with the introduction of microvia fabrication techniques, enabling high-density interconnect (HDI) PCBs from the mid-1990s onwards. HDI designs allow more traces and components within the same area, improving performance and reducing board size. They employ blind and buried vias and sometimes stacked microvias, which enhance structural reliability.
HDI boards are widely used in mobile devices, computers, medical diagnostics, and defence communication systems. A four-layer HDI microvia board can often achieve the same routing capability as an eight-layer traditional through-hole board, contributing to cost reduction and design flexibility. Materials such as Ajinomoto build-up film support these fine-pitch interconnection techniques.
Recent advances in 3D printed electronics permit additive manufacturing of conductive structures, allowing components and circuits to be fabricated layer by layer using functional inks. Some advanced PCBs now incorporate optical waveguides, integrating photonic functions alongside electronic pathways.