Overview of PCB Layers

Printed circuit boards consist of multiple layers of conductive copper traces to connect electronic components. By stacking multiple layers of copper traces and insulating dielectric layers (core layer and prepreg layers), more interconnecting traces and thus more functions of the PCB can be enabled. Depending on the number of copper layers, they are arranged in the following manners:
● 1 copper layer - single-sided copper layer on a core layer
● 2 copper layers - copper layers on both sides of a core layer
● >2 copper layers - outer copper layers + further inner copper layers on several core layers, which can be bonded by means of prepreg.

Layered Structure

1. Copper layers: The thin copper lines on the outer as well as inner sides connect electrical components.
2. Core layers (substrate): The core layers insulate the conductive copper traces and provide the PCB with mechanical stability. During manufacturing, the adjacent copper tracks are applied to the core layers.
3. Prepreg: The fiber layers pre-impregnated with resin ("pre-preg") fulfill an insulating function between the conductive copper layers just as the core layers (substrate). To manufacture a PCB, prepreg is used in alternation between two core layers as an adhesive, which melts under the heat of the lamination process.
4. Solder mask: A thin layer that covers the outermost copper layers to prevent solder from flowing to places where it is not desired.
5. Solder Paste Stencil: Optional production template for applying solder paste to specific areas of the PCB.
6. Position Printing (Silkscreen): An imprint on the outer sides, where information on component positioning, component designations, symbols or other inscriptions are printed.
7. Plated Thru Holes: Small holes drilled through the PCB and coated with copper to make electrical connections between the different layers.
8. Non-plated Thru Holes: Simple drilled holes that are not copper plated e.g. for THT components.
9. Peelable solder resist: A temporary protective coating to protect certain areas such as pins and pads from solder during the soldering process.

Manufacturing Steps

Pre-process: Before the actual lamination process, the individual layers are prepared. This includes etching the copper foils to create the desired track patterns and drilling vias to make electrical connections between the layers.
Lamination: The prepared layers are laminated into a single multilayer structure using heat and pressure. During the lamination process, the dielectric between the copper foils is heated and solidified.
Finish: After lamination, the multilayer PCBs are further processed to achieve the final properties. This includes cutting or milling the PCB to the desired size, applying soldermask and electroplating the surface to improve solderability.

Board Thicknesses

There are different types of PCB thicknesses used depending on the application and design requirements. Here are some common types of PCB thicknesses:

Standard thickness 1.6 mm (0.063 inch): This is the most used PCB thickness. It provides a good balance between mechanical strength and flexibility. The standard thickness is often used for rigid PCBs as it provides sufficient hardness for most applications.

Thin PCBs: These have a thickness less than standard and are often used for applications where weight reduction or flexibility is important. Typical thicknesses for thin PCBs can be 0.8 mm (0.031 in.) or even thinner. They are often used in mobile devices such as smartphones or tablets.

Thicker PCBs: These provide increased strength and are suitable for applications where greater mechanical strength is required. Typical thicknesses for thicker PCBs can be 2.4 mm (0.093 in.) or even thicker. They are often used in applications such as power amplifiers or industrial control systems.

Flexible printed circuit boards (flex PCBs): These consist of thin substrates that allow bending and twisting movements. They are used in applications where flexibility, low bend radius or 3D formability are required. The thicknesses of flexible PCBs can vary, typically in the range of 0.1 mm (0.004 in.) to 0.5 mm (0.020 in.).