PCB Anatomy
PCBs are made of various materials such as copper layers, prepreg, core, solder mask, and silkscreen. The conducting layers are interconnected by vias (blind, buried, and through-hole). This infographic provides a clear, cross-sectional view to help you understand how these layers come together to create the backbone of modern electronics.
Copper layers
Copper layers are thin sheets of copper laminated onto one or both sides of the core or prepreg. These thin conductive foils, typically 0.5 to 4 oz in thickness, are carefully etched to create signal traces. The etching process precisely removes unwanted copper. They are also used as power and ground planes.
A ground plane is a large, continuous copper area that serves as a common return path for current. It provides a stable zero-voltage reference point throughout the board.
A power plane is a dedicated copper layer (or layers) used to distribute power to all the components on the PCB.
Copper’s exceptional conductivity ensures electrical signals travel with minimal loss, preserving signal integrity and optimizing the device’s performance. Copper traces create the essential pathways connecting the different components on the board.
Prepreg
Prepreg is a pre-impregnated fiberglass, resin-infused material used as a bonding layer between the core and copper layers.
During the lamination process, prepreg bonds copper layers and cores under extreme heat and pressure. The resin flows to fill gaps and create a strong bond, ensuring electrical insulation and thermal resistance between conductive layers.
Prepreg’s resin content and flow characteristics are critical, as they directly impact the signal integrity and thermal properties of your circuit board.
The consistency and quality of the prepreg are vital; any imperfections can lead to delamination or reduced performance.
Core
Core is the central substrate of the printed board, typically made of fiberglass-reinforced epoxy resin (FR-4). It provides structural rigidity and serves as the foundation for all other layers. When selecting a core material, consider its electrical and thermal characteristics.
The core acts as a dielectric barrier between the copper layers. This insulation minimizes electrical interference and reduces crosstalk, ensuring signal integrity.
It provides the necessary mechanical strength to keep the PCB stable, even under thermal and mechanical stress. This stability is crucial for preventing board deformation during assembly and operation.
Solder mask
Solder mask is a protective layer applied to circuit boards. This polymer coating, typically green but available in various colors (red, blue, black, etc.), covers all copper traces except for the designated solder pads. It is applied using methods such as screen printing or photo imageable techniques.
This resistive layer prevents solder bridging between conductive traces during assembly and protects the copper from oxidation and environmental damage. It also helps maintain desired trace spacing to preserve signal integrity and provides additional insulation.
Various types of solder masks exist, including liquid photo imageable (LPI), dry film, and epoxy-based options, each tailored to different manufacturing needs and performance requirements.
Silkscreen
Silkscreen is a non-conductive ink (typically white) printed on the outer surfaces of a PCB. It doesn’t affect electrical performance. However, it’s crucial for assembly, testing, and troubleshooting. As a PCB designer, you need to ensure the silkscreen is not overlapping with the solder mask.
It is used to print component outlines, essential for accurate component placement, and reference designators (e.g., R1, C2) for easy identification. Additionally, it includes polarity indicators, markings for test points, version numbers, logos, and sometimes warning symbols.
Silkscreen is applied via screen printing, LPI, or digital printing. It guides manual assembly, helps locate components during rework, and assists in debugging. Clear silkscreen saves time and reduces errors during manufacturing and repair.
Vias
Vias are plated holes drilled into the PCB to create vertical electrical connections between layers. Commonly used vias include:
- Through-hole vias: These extend from the top to the bottom of the PCB, connecting all layers. They are the most common type and are used for mounting through-hole components and dissipating heat.
- Blind vias: These holes connect an outer layer to one or more inner layers without penetrating the entire board. This design conserves space and reduces the risk of interference, making it ideal for high-density interconnect designs.
- Buried vias: These are completely hidden within the inner layers of the printed board. Buried vias are particularly useful in HDI designs, allowing efficient usage of surface area.
A PCB combines various materials and technologies to form functional circuits. Understanding the function of each layer—core, copper layers, prepreg, solder mask, vias, and silkscreen is crucial for anyone involved in the design, manufacture, or use of printed circuit boards.