In an integrated circuit, the flat material or board to which circuit components – transistors, resistors, etc. – are mounted is called printed circuit board. Sometimes, the term “printed circuit board” (often abbreviated as PCB) is wrongly used to mean the entire assembly of a PCB including the mounted components. The correct term to designate the board alone is “printed wiring board” or “etched wiring board.” A printed circuit board mounted with electronic components is called “printed circuit board assembly” (PCBA) or “printed circuit assembly” (PCA).
Nevertheless, most people use the term PCB to designate both unassembled and assembled PCBs.
Printed circuit boards are characterized by plated-through holes and lines of etched copper foil. Each electric component has wire or lead ends. This lead ends are inserted into the holes on one side of the PCB and secured on the other side by soldering.
The electric components are connected to one another through the etched copper foil that serves as conductive metal pathways. The printed copper foil functions like the wires in a traditional electric circuit.
Printed circuit boards are made of layers of conducting and non-conducting materials. The innermost layer is called substrate or base metal. It must be made of non-conducting material.
The substrate is usually made of fiber or woven glass, but sometimes plastic or cotton paper is used. The popularly used glass in PCB industry is FR4 which is essentially fiberglass. Other materials that can be used for substrate are FR6 (polyester and matte glass), FR5 (glass and epoxy), FR3 (cotton paper), FR2 (phenol cotton paper). A known plastic used as substrate is Kapton.
The substrate is the main part of a printed circuit board. It constitutes more than 85 percent of the entire board. The quality and rigidity of the entire printed circuit board lies in the type of material used as substrate.
High-temperature plastic and cotton paper is less expensive but less durable than fiberglass. Cotton paper has a tendency to delaminate when exposed too long to the soldering iron. Thus, cotton paper and plastic are commonly used to produce low quality circuit electronics.
The second layer of all printed circuit boards is a conducting copper foil. This layer is carefully etched on the surface of the non-conducting substrate. It can be etched manually but modern technology allows efficient etching with the use of photolithography.
Copper foil is where electric current flows. It connects the electric components with one another. Holes are intentionally drilled on most lines and fields of copper foil since leads of components are inserted there for connection.
Printed circuit boards are either single or double-sided. Single-sided ones have copper foil on one side, while those that are double-sided have copper on both sides. The copper foil placed may range from 1 to 16 layers. Most PCBs have an ounce of copper foil per square meter. Printed circuit boards used for high powered applications have 2 to 3 ounces of copper foil to handle more electric current.
After the copper foil layer is the soldermask layer. Its purpose is to insulate the copper foil layer against unwanted contacts. Remember that copper foil traces and fields are intended to contact only the leads or wires of the electric components. Accidental contact with unintended objects may cause circuit problems.
The soldermask also guides the user or assembler where to solder by giving a green color to the PCB. Areas intended for soldering is not covered in green such as the SMD pads and silver rings. Green is the widely known color of soldermask but other colors such as red and purple are also used.
Silkscreen is the last and outermost layer of a PCB. It’s used to paint letters numbers and other symbols to help the assembler know where to place the components. Text printed using silkscreen is usually white.
Given these facts, one thing should be clear to you by now – PCBs, despite being thin and simple-looking, are among the most complicated components of modern devices.