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Multilayer Circuit Boards 101

Nearly every electronic today contains PCB technology. But what is it? PCBs (printed circuit boards) are made up of different layers of insulators and conductors. These circuit boards contain an electric circuit, with lines and pads connecting various points. A PCB directs and redirects signals to help power flow through devices. Circuit boards can be single-sided, double-sided, or multi-sided.

We want to help you understand more about PCBs with this guide to multilayer circuit boards.

What is a multilayer PCB?

sketch of a PCB core

A double-sided PCB has two layers, a single-sided PDB has one layer, and a multilayer PCB has at least 3 layers of conductive material. Theoretically, a multilayer PCB could have infinite conducting layers inside that are separated by insulating layers. However, multilayer circuit boards typically have an even number of layers because having an odd number of layers can result in issues within the board.

When were they invented?

Single-sided printed circuit boards were first created in the 1920s. These circuit boards were made from just about anything, including sheets of wood. These were incorporated into new forms of technology and continued to be developed. They were used in radios and gramophones.

In 1947, the first double-sided circuit board was created. During the 1950s and 1960s, the materials used in PCBs evolved and became more advanced and sophisticated. As material science advanced, scientists developed a more standard printing system for circuit boards. But it wasn’t until 1960 that the first multi-layer printed circuit board was produced, with 4 layers.

Since then, PCBs have gotten smaller and more efficient with the development of new plating and printing processes. Computer programs have helped greatly in the design and manufacturing processes so that circuit boards can be produced cheaply and quickly. Today, multilayer circuit boards are very common in electronics.

How are multilayer circuit boards made?

sketch of a multilayer PCB

Multilayer PCBs are made up of a core and alternating layers of material. The core is made from a rigid laminate that is sandwiched between copper foil. Cores can very in thickness between .038 inches and .005 inches, depending on the design and intended function.

The additional layers surrounding the core alternate between prepreg (pre-impregnated bonding sheet) and copper foil. Prepreg is lamanite, like the core, but is has not been cured, making it adjustable. It acts like glue to hold the layers together.

The layers of prepreg and copper foil are laminated together in a vacuum under high heat and pressure to ensure that there isn’t any air trapped between the layers. During this process, resin in the prepreg layers is activated and melts, boding the layers together.

Then, holes of different sizes are drilled into the PCB. The location of these holes depends on the design of the circuit board. The boards go through a desmearing process to remove any resin from the inner walls of the holes, which may have been activated by the heat and friction of drilling. Removing the resin improves the electrical connectivity of the PCB.

A thin coat of copper is then placed on the hole walls. A light-sensitive film, the “resist”, is placed over the outer layers of the PCB, covering the holes as well. This film is laminated and covered by another piece of film that has a traced image of the PCB design, showing where the paths and pads should be. The PCB is exposed to UV light and clear areas in the image film allow light to pass, hardening the resist. This process, similar to a negative and a photograph, creates the image of the circuit pattern and once again exposes the drilled holes.

Copper is plated onto the exposed surfaces and into the holes, and tin is plated over the copper. The developed film and resist layers are removed from the panel, uncovering the non-plated areas. The exposed copper is removed from the outside of the panel so that only the copper covered by tin is left on the panel surface. The tin strip is removed, leaving bare copper pads and paths on top of laminate.

Finally, an ink is applied to coat the panel to protect the components. However, the holes and copper pads are left exposed.

What are the benefits of multilayer PCBs?

Because a multilayer PCB has several layers, there are multiple conductor patterns within the board, increasing the area available for wiring. Additionally, the multiple layers allows the boards to be more flexible and have a higher assembly density. They are also typically smaller than single- or double-sided circuit boards. Multilayer circuit boards also have more features, resulting in higher control.

What are the disadvantages of multilayer PCBs?

sketch of a circuit board

Multilayer circuit boards are more expensive to produce because of the multiple layers that have to be laminated together. They take more time and materials to create them. Because they contain multiple layers, they are also more difficult to repair if anything breaks or isn’t functioning properly.

What are the applications of multilayer PCBs?

Multilayer PCBs can be used in many different types of electronics, including the following:

Quick Turn Circuits creates quality multilayer circuit boards at a competitive price. For more information about how PCBs work, contact us today!