Printed Circuit Board Substrates

The majority of today’s printed circuit boards are made up of layers, also referred to as multi-layer printed circuit boards. Most circuit boards have between 1-16 layers and are separated by a substrate which is a laminated insulator that separates the circuitry design. Each layer of the circuitry is connected to each other through a series of drill holes.

Theses holes know as VIAS are plated holes that connect copper tracks from one layer of the printed circuit board to other layers they can be plated or have a rivet . There 2 types of vias used in multi-layer printed circuit boards:

• Blind Vias: This is a vias that is visible on one side only.
• Buried Vias: This is a vias that is not visible on either side

Printed Circuit Board Substrates

Substrates can be classified into five types, each having a unique set of characteristics for specific applications.

FR-2 Paper Printed Circuit Board Substrate

• This lower grade of substrate is a made of impregnated paper, also called Phenolic, that is easy to machine over a fiberglass material substrate. The “FR” refers to the term Flame Resistant. This type of substrate is usually found on more inexpensive consumer electronics.

FR-4 Fiberglass Printed Circuit Board Substrates

• Fiberglass substrates are made up of a woven fiberglass material and impregnated with a flame resistant material. The material is rigid and can also be drilled cut or machined but due to the abrasive nature of the fiberglass, tungsten carbide tools are needed. An FR-4 is a stronger substrate compared to an FR-2 and is more resistant to cracking or breaking and are usually found in higher end electronics.

RF Radio Frequency Printed Circuit Board Substrates

• RF substrates are comprised of low dielectric plastics and used in printed circuit boards for applications in high power radio frequencies. Although the substrate has poor mechanical properties it has exceptional electrical performance properties.

Flex – Flexible Printed Circuit Board Substrates

• Not all circuit boards use rigid core materials. Some are designed to be very flexible or slightly flexible called flex circuits. Thin and flexible plastics and or films are employed as substrates. The manufacturing process is more difficult than using rigid substrates, but offers benefits that cannot be achieved with rigid substrates such as saving space by bending the circuit board to fit a particular space or where repetitive movement requires a flexible layer.

Ceramic/Metal Core Substrates

• Power electronics demand a low-thermal resistivity substrate. A ceramic core or metal core substrate provides the necessary characteristics to handle larger copper tracks and the high electrical currents used with these type of circuit boards.