Understanding The Composition of Printed Circuit Boards

pcb-assembly A printed circuit board or PCB is a very important part of electronic equipment such as computers. This is often called printed wiring cards or printed wiring boards. It is basically a small piece of layered boards with pad and lines that link different points and components together. Components such as connectors and resistors are mounted on the surface of the PCB with a metal solder. Signals and power pass through the connections, which allow the entire circuit to function.

Basically, a printed circuit board is composed of several alternating layers of various materials. These materials are joined together through a lamination process, using adhesives and heat. The layers include the following:


The FR4 is the substrate of base material of the board. This is the middle layer. Fiberglass is the most common material used for this layer, which is designated as FR4. This solid core provides the PCB thickness and rigidity. Flexibility, as seen in some boards, is achieved by using high temperature plastics that are very flexible such as Kapton.

The thickness of this layer varies, from very thin ones to thicker boards. Most PCBs are at 1.6 millimeter or o.o63 inches thick. To know more about it visit www.pcbnet.com.

FR4 or fiberglass can be very expensive. Some manufacturers opt to use cheaper substrate materials such as phenolics and epoxies. These are inexpensive but are much less durable compared to FR4. Also, these inexpensive materials emit a distinctive unpleasant smell when soldered. Phenolics have the tendency to smoke, char and delaminate when exposed to the soldering iron for too long. This is because the thermal decomposition temperature of phenolics is low. PCBs with phenolics and epoxies for substrates are often used in the manufacture of low-end types of consumer electronics.


After the substrate is a layer of thin copper foil. This material is laminated into the substrate using adhesive and heat. A double-sided printed circuit board has copper foils attached to both sides of the base material. This type of board is often referred to as 2-layer board or double-sided PCBs. Cheaper boards, often seen in low-cost electronics, only have 1 side that contain a copper layer. More expensive ones can have as many as 16 copper layers.

The thickness of the copper layer varies, which is determined by weight as oz/ft2 (ounces per square feet). This would be equivalent to 35 μm (micrometers) or about 1.4 thousandths of an inch (1.4/1000 inches) of copper. Most boards have copper layers of 1 ounce per square foot. Some boards have as thick as 2-3 ounces of copper per square foot. These boards can handle very high power and are often used in high-end electronics.

Solder mask

PCBThe layer of solder mask is next. This layer gives the board’s distinctive color. For most boards, the solder mask layer gives its green shade. This is placed on top of the copper layer in order to insulate copper traces that may accidentally come in contact with the solder, conductive bits or with other metals. The solder mask is spread over most of the board to cover all the small traces but leaves the SMD pads and silver rings exposed where components are soldered to.

The solder mask layer also allows users to properly attach and solder components to the board and prevent solder jumpers. Green is the most common solder mask layer color but other colors are also available such as white, purple and red.

Silk Screen

This is the white layer applied over the solder mask layer. The silkscreen layer adds the numbers, symbols and letters to the surface of the printed circuit board. These allow easier assembly and enables users to understand the board better. The function of each LED or pin can be indicated through the silkscreen labels.

The common color for this layer is white. Other colors are available as well, such as yellow, black, red, and gray. However, on most boards, only one color is used for this layer.

Article by Paige Chen

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