Printed Circuits Glossary of Terms

Printed circuit boards (PCBs) are found in practically all modern electronics. They serve as the mechanical foundation for all of the components in modern electronics, as well as providing all of the circuitry to connect those components electronically to one another.

A PCB consists of the following components:

• The base material. The base material can be either rigid or flexible. Typical rigid materials include FR4 and polyimide, while typical flexible materials include high-temperature polyimide films, polyester and PEN plastic laminates. This substrate serves as a non-conductive base for the conductive circuits. Some PCBs use both rigid and flexible materials.
• The conducting material. Copper is the most commonly used conductive material because of its high electrical conductivity. To form the conductive circuits, copper foil is first laminated to one or both sides of the base material. Then, the PCB fabricator etches the circuit as per the design specifications provided by the designer. The number and thickness of conductive layers depend on the intended application.
• The soldermask. Circuits located on the surface layers of the PCB, or outerlayers, are coated with an insulative material called soldermask that protects these circuits from oxidation and controls the flow of solder during the assembly process, when components are soldered to the PCB. While soldermask material is typically green, it also comes in other colors, such as clear, black, blue, red, and white.
• The nomenclature. The nomenclature refers to the characters – usually numbers and letters – printed on the soldermask that provide component identification and other important information. They help ensure the PCB is accurately assembled.
• The electronic components. Any necessary electronic components are either attached to the surface of the board (surface-mount technology) or threaded through pre-drilled holes (thru-hole technology).

The majority of PCBs – about 90% – are made from a rigid substrate, known as rigid board or hardboard. The inflexible material provides a very stable and inexpensive foundation for component assembly. It can be used in single-sided, double-sided, and multilayer PCBs.

Flexible PCBs—also known as flex PCBs comprising about 90% of worldwide manufacturing —are made from flexible substrates. The flexible material enables the board assembly to bend to fit into tight or compact spaces. They can also accommodate dynamic flex applications, where the circuits can be bent for hundreds of thousands of cycles without failure. They are available in single-sided, double-sided, and multilayer variations.

Rigid-flex PCBs are a hybrid of rigid PCBs and flex PCBs; they combine the advantages and eliminate the disadvantages of the other two PCB types by connecting flexible circuits to rigid boards. The resulting design is a board assembly with the versatility of flexible PCBs and the strength and stability of rigid PCBs. Additionally, it makes rigid-flex PCBs suitable for use in much more mechanically challenging and complex electronic designs.

Prepreg refers to a thin sheet of glass fabric that is coated in resin and then dried. The glass core stabilizes the fabric, while the resin, under heat and pressure, will flow out to encapsulate the internal circuitry. Applying heat and pressure to a stack of prepreg sheets melts the resin and creates a sturdy laminate, which forms the core of a rigid board.

A no-flow prepreg is a composite material in which the resin is cured for longer, reducing the resin’s ability to flow under heat and pressure. No-flow prepregs are critical for rigid-flex designs since they ensure the resin remains contained to the rigid portion of the board. Standard prepregs are not suitable for these hybrid PCB applications, as the normal state of resin flow could compromise the flexible portions of the rigid flex PCB.

FR4 is the most popular PCB substrate, due to its good physical, mechanical, thermal, and electrical performance available at a low price point. These factors make it suitable for use in a wide range of electronic applications. The FR4 designation is used for materials that meet certain requirements as outlined by NEMA LI 1-1998 standards.

This material consists of glass cloth impregnated with epoxy resin. The epoxy resin can be mixed with additives to further improve the material’s performance for more demanding applications.

Electrical impedance refers to the measure of resistance and conductivity in an electronic circuit indicated in Ohms. Resistance is the opposition the circuit presents to current flow due to the inherent characteristics of the material. Controlling impedance in PCB designs helps the PCB provide critical components with the correct signal at the correct moment, improving communication between your components.

In PCBs, controlled impedance occurs when the PCB exhibits the expected impedance value throughout the board. This is essential to ensuring repeatable performance especially in high-frequency applications. The substrate material, and its electrical performance, must be carefully matched to the trace dimensions to ensure the value of the trace signal is within a certain range of the expected value.

PCB stiffeners are mechanical support components for flex PCBs. They add rigidity to areas of the flex circuit that need it.

The substrate refers to the base material of a PCB. It serves as the base for the other PCB components, including the conductive layer, soldermask, and electronic components. It can be either rigid or flexible.

A via is a hole in a PCB board that allows for a conductive connection between layers. Vias connect inner layers to one another and, in some cases, to outer layers. Blind vias connect inner layers to one of the external surfaces without penetrating the entire board. Buried vias only connect internal layers; they do not reach the outer layers.

For more information about any terms listed (or ones that were not addressed here), contact the experts at Printed Circuits! We would be happy to answer any questions you may have about PCBs so that you have the context and confidence to inquire about the circuitry you need.