What Is Printed Circuit Board (PCB)? - Definition From

Printed circuit board layers

A PCB can be single-layer, double-layer or multilayer. Single-layer PCBs are typically used for simple electronic devices such as home appliances, while multilayer PCBs tend to be built for more complex hardware, such as computer graphics cards and motherboards. Double-layer PCBs allow for greater density than single-layer PCBs and are used for electronic devices such as automotive dashboards or LED lighting, for example.

A PCB's layer count is commonly based on its number of conductive layers. However, a PCB also includes other types of layers, which are laminated together in alternating patterns of conductive and insulating material. Today's PCBs always contain the following four types of layers:

• Substrate. This is the base, or core, layer. It is typically a rigid insulating material such as FR-4, a fiberglass/epoxy composite. In some cases, the substrate is a flexible material, usually plastic, which can fold or bend to accommodate space requirements. Flexible substrates can also withstand higher temperatures and other harsh conditions. Some PCBs use a combination of both rigid and flexible substrate materials.
• Conductive. This layer is usually made from a thin sheet of copper. On a single-sided (or single-layer) PCB, there is one conductive layer laminated to the substrate. On a double-sided (or double layer) PCB, there are two conductive layers, one on each side of the substrate. A multilayer PCB alternates between the substrate and conductive layers.
• Solder mask. The conductive layer is covered with a solder mask, a nonconductive material that gives PCBs their green color, although other colors can be used. The solder mask acts as an insulator for the underlying traces that are etched in the conductive material. The solder mask is also applied to the bottom of a single-sided PCB.
• Silkscreen. This layer is simply the labeling that is applied to the PCB after all the other layers have been added. The labeling can include numbers, letters, symbols or other information that indicates the various functions of each connection point. The labeling is usually white, but other colors can be used.

A PCB is made up of a mix of these four types of layers. The mix and its organization varies depending on the PCB's purpose. A single-layer PCB includes one of each of these four layers. A double-layer PCB includes one substrate, but two of each of the other types of layers. A multilayer PCB will contain a mix of conductive and nonconductive layers, although the exact organization can vary. In some cases, a multilayer PCB might also use prepreg as a layer. Prepreg is a dielectric material that's sandwiched between two core layers or between core and conductor layers.