Overview: The article explores printed circuit boards and their structure, types, materials, and applications, highlighting their important role in modern electronics.

Early electronic components were connected manually using point-to-point wiring, leading to complex large structures that were error-prone and difficult to repair. Miswiring and poor soldering increased challenges and made repairing a challenging process.

As a major breakthrough, Paul Eisler developed the first printed circuit board (PCB) around 1936 by creating conductive pathways on an insulating substrate. They have become an essential component of all modern electronic devices. PCBs have become smaller, more integrated, and more diverse due to the quick advancement of electrical methods. Compared to traditional layouts, its processing capacity allows it to complete tasks with much-reduced dimensions.

What is a printed circuit board?

A PCB consists of conductive traces, typically made of copper, etched onto a non-conductive substrate. These traces serve as pathways for electrical signals, enabling components like resistors, capacitors, and transistors to communicate and function together effectively.

Structure of PCB

Accurately identifying and placing PCB components is more important during production and assembly. As shown in Fig. 1, the key layers of PCB include

  • Substrate
  • Copper
  • Solder mask
  • Silkscreen layer

Diagrammatic illustration of structure of PCB.

Fig. 1 Diagrammatic illustration of the structure of PCB. Source: Rakesh Kumar, Ph.D.

Substrate

FR4 is a widely used PCB substrate made of flame-retardent fiberglass-reinforced epoxy. It provides mechanical support, electrical insulation, rigidity, and thickness to PCBs. Its properties, such as high dielectric strength, thermal stability, moisture resistance, and cost-effectiveness, make it ideal for various applications.

Copper

This layer consists of thin copper foils that are laminated onto the PCB substrate using heat and adhesive. It involves placing complex copper traces on an insulating layer that acts as conductive wires for the circuit. Copper is selectively removed using chemical or mechanical methods, leaving the desired circuit paths. These traces can be single-sided or double-sided, with thickness varying based on design needs. The thickness typically ranges from 0.5 oz to 13 oz.

Solder Mask

The solder mask is a thin epoxy or polymer protective coating that protects underlying copper traces from oxidation and accidental short circuits. This layer is insulating in nature and gives PCBs their characteristic green color (also available in other colors). The solder mask layer protects copper lines on a PCB, allowing only the areas intended for soldering electronic components to remain exposed.

Silkscreen Layer

Lastly, the various components on the PCB's top and bottom surfaces may be identified using a silkscreen printing technique. The silkscreen layer on a PCB is the topmost layer that provides essential information, such as component names, logos, symbols, designs, and values, to ensure correct component placement. It is created using a process that applies special non-conductive ink to mark symbols and text on the board.

Via

In the case of multilayer PCBs, ‘via holes’ play a vital role in providing electrical interconnects between copper traces of different layers and thermal channels for heat dissipation. These vias pass through some of the layers or all of the layers, as seen in Fig. 2.
2A: Double layer PCB comprising of substrate layer sandwiched between copper layers. 2B: Plated Through-Hole (PTH) Vias, Non-Plated Through-Hole (NPTH) Vias, 2C: Through Vias, Blind Vias, Buried Vias

Fig. 2A: Double layer PCB comprising of substrate layer sandwiched between copper layers. 2B: Plated through-hole (PTH) vias, non-plated through-hole (NPTH) vias, 2C: Through vias, Blind vias, Buried vias Source: MDPI

Based on the layers it passes through, there are several types, which include

  • Plated Through-Hole (PTH) Vias (Fig. 2B): These are plated with conductive material (e.g., copper) to establish vertical signal paths.
  • Non-Plated Through-Hole (NPTH) Vias (Fig. 2B): Mechanically drilled holes without conductive plating, used for alignment or mounting.
  • Through Vias (Fig. 2C): Links all layers from top to bottom, enabling electrical and thermal connections across the entire board.
  • Blind Vias (Fig. 2C): It links external layers (top/bottom) to intermediate layers.
  • Buried Vias (Fig. 2C): It connects internal layers without surfacing.

Recent advancements like surface mount technology make automatic component assembly accurate and at a rapid speed. Moreover, early PCB defect detection significantly reduces the cost of quality control in manufacturing.

Types of PCB

There are several types of PCBs for specific applications, which are discussed below.

Single-Layer PCB

A single-layer PCB has only one conductive copper layer on one side of the substrate, as shown in Fig. 3. It is simpler in design, more commonly employed by beginners, and less expensive to manufacture but cannot mount components on both sides.
Diagrammatic illustration of single layer-PCB, double layer PCB.

Fig. 3 Diagrammatic illustration of single layer-PCB, double layer PCB. Source: Rakesh Kumar, Ph.D.

Double-Layer PCB

A double-layer PCB has two conductive copper layers on both the top and bottom of the substrate, separated by an insulating material, typically FR4, as shown in Fig. 3. These layers are interconnected through plated-through holes (vias), enabling more complex circuit designs compared to single-layer PCBs. Components can be mounted on either or both sides of the board.

Multilayer PCB

A multilayer PCB consists of three or more conductive copper layers separated by insulating materials, as shown in Fig. 4. These PCBs are widely used in advanced electronics due to their compact size, rigid substrate, and improved performance. More complex circuits, such as the circuits in laptops and smartphones, make use of multilayer PCBs to power numerous components.

Diagrammatic illustration of multi-layer printed circuit board composed of four copper layers (two internal layers), five dielectric layers, and the top and bottom mask layer

Fig. 4 Diagrammatic illustration of multi-layer printed circuit board composed of four copper layers (two internal layers), five dielectric layers, and the top and bottom mask layer.
Source: MDPI

Rigid PCB

They are made up of inflexible substrates (e.g., FR4). Based on the number of copper layers, they can be single, double, or multilayered PCBs.

Flex PCB

It comprises a flexible substrate (e.g., polyester film). The manufacturing process is time-consuming and technique-sensitive, and it is used in places where rigid PCBs cannot be used. They can be single, double, or multilayered PCB. Flexible PCBs use substrates like polyimide (PI), polyester (PET), and polyethylene (PE), which are selected based on application requirements.

Rigid-Flex PCB

It combines rigid and flexible substrates for specific applications.

Applications

PCB serves as a platform for connecting electronic components and ensures signal quality, timing, and several other functions. PCBs are integral to modern devices across various sectors, including medical equipment, industrial machinery, automotive systems, aerospace, consumer electronics, maritime applications, military and defense applications, and so on. They enable complex electronic functions in diverse applications.

Summarizing the Key Points

  • Printed circuit boards reduce challenges caused by manual wiring methods by providing structured, reliable connections for components.

  • The essential layers of a PCB include a substrate, copper traces, a solder mask, and a silkscreen layer, ensuring effective assembly and operation.

  • PCBs come in various types, including single-layer, double-layer, multilayer, rigid, flex, and rigid-flex, each for specific applications.

  • PCBs have a wide range of applications, including medical, automotive, aerospace, and consumer electronics, etc.

Reference

Perdigones, F., & Quero, J. (2022). Printed circuit boards: the layers’ functions for electronic and biomedical engineering. Micromachines, 13(3), 460. https://doi.org/10.3390/mi13030460

Shamkhalichenar, H., Bueche, C. J., & Choi, J. (2020). Printed circuit board (PCB) technology for electrochemical sensors and sensing platforms. Biosensors, 10(11), 159. https://doi.org/10.3390/bios10110159

Nassajfar, M. N., Deviatkin, I., Leminen, V., & Horttanainen, M. (2021). Alternative materials for printed circuit board production: an environmental perspective. Sustainability, 13(21), 12126. https://doi.org/10.3390/su132112126

Glučina, M., Anđelić, N., Lorencin, I., & Car, Z. (2023). Detection and classification of printed circuit boards using YOLO algorithm. Electronics, 12(3), 667. https://doi.org/10.3390/electronics12030667