Structure and process of main printed wiring boards

Introduction to Printed Wiring Boards

Printed Wiring Boards (PWBs), often referred to as Printed Circuit Boards (PCBs), are essential components in virtually all electronic devices.
These boards mechanically support and electrically connect electronic components using conductive pathways, tracks, or signal traces.
Understanding the structure and process involved in creating PWBs is crucial for anyone interested in electronics and technology.

The Structure of Printed Wiring Boards

Printed wiring boards come in various forms based on their complexity and usage.
However, most PWBs share a standard structural format.
Let’s examine each layer and its role in the board’s functionality.

Base Material

The foundation of any PWB is the substrate, commonly made of fiberglass or epoxy composites.
The base material provides mechanical support and insulation to the board.
A widely used substrate is FR-4, a composite material made from woven fiberglass cloth with an epoxy resin binder that is flame resistant.
The choice of substrate depends significantly on the application’s environmental and mechanical requirements.

Copper Layers

Copper layers are crucial for PWBs because they conduct electricity.
These layers are installed above the substrate and are usually very thin copper foil adhered to the base material.
Single-layer boards have one copper layer, while double or multi-layer boards can have more than one.
The complexity of the PWB dictates the number of copper layers required.

Solder Mask

Above the copper layer lies the solder mask, typically a green layer, although other colors are available.
The solder mask serves mainly as a protective layer for the copper circuits.
It ensures that there is no accidental solder bridging between pathways and protects against environmental factors that might degrade the copper.

Silkscreen

The silkscreen is the topmost layer that adds labels to the PWB.
These labels can be symbols, numbers, or letters that identify different components and test points on the board.
Although not functional in terms of electronic performance, the silkscreen is essential for assembly, maintenance, and repairs.

The Process of PWB Manufacturing

Creating printed wiring boards involves a meticulous process that combines both mechanical and chemical techniques.
Let’s go through the main steps involved in manufacturing PWBs:

Design and Layout

The first step in the PWB manufacturing process is the design and layout.
Electrical engineers use specialized software like CAD (Computer-Aided Design) to layout the PWB’s schematic.
During this phase, they place the components and routing tracks.
Design considerations include minimizing interference, optimal positioning, and meeting size constraints.

Printing the Circuit Pattern

Once the design is finalized, the circuit pattern is printed directly onto the copper layers.
This is done using a photographic method for precision.
A photoresist layer, sensitive to ultraviolet light, is applied, which helps in transferring the circuit pattern onto the panel.

Etching

Etching is a crucial step in creating the PWB.
During this phase, chemical solutions are used to remove excess copper, leaving only the desired circuit paths.
Controlled etching ensures that the paths are precise, free of defects, and maintain the design integrity.

Lamination

In multi-layer PWBs, an additional process known as lamination is required.
Lamination fuses multiple layers of substrate and circuits together under heat and pressure.
The alignment during lamination is critical to ensure the electrical connections line up across layers.

Drilling

After lamination, holes need to be drilled into the PWB to prepare for component placement.
These holes serve as the connection points for through-hole components or inter-layer connections, referred to as vias.
Advanced machines using CAM (Computer-Aided Manufacturing) technology ensure precision and speed in this process.

Plating and Coating

The exposed surfaces and holes are plated with copper, providing a conductive layer that connects different board layers.
After plating, the boards are usually coated with a protective layer like solder mask to protect against oxidation and ensure reliability when components are soldered onto the board.

Testing and Quality Control

Once manufacturing is complete, the PWB undergoes thorough testing to ensure functionality and reliability.
Quality control checks, such as automated optical inspection (AOI) and electrical testing, assess the board for any defects or design discrepancies.
Only those that pass these stringent tests proceed to the assembly phase.

Conclusion

Understanding the structure and process of printed wiring boards is fundamental in the world of electronics.
The intricacies involved in their creation ensure that electronic devices operate efficiently and reliably.
Whether you’re an engineer, hobbyist, or simply curious, this knowledge provides insight into the backbone of modern electronic technology.