Printed Circuit Boards (PCBs)
Printed circuit boards are a vital part of electronics as a means of providing connections between electrical components. Consisting of thin boards made of insulated material, PCBs have copper layers for supporting electronic devices. They serve as a guide for the movement of electrical signals and are used in industrial, commercial, and residential devices. Printed circuit boards enhance and smooth the performance of electronics.
The base of PCBs is made of a non-conductive material that is strong and durable and provides insulation, water resistance, and temperature stability. The main part of printed circuit boards is the pathways, referred to as traces, that carry signals. To ensure unfailing electrical signal transmission, traces are made of copper or aluminum.
The basic components of a PCB are traces, pads, vias or holes, and printed markings. Although there are variations in printed circuit boards, these four components are common to all PCBs. The other aspect of printed circuit boards are their layers, which are single, double, and multi or more than two layers. The difference in the number of layers are in accordance with the use of a PCB and its basic design.
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Construction of Printed Circuit Boards
At the beginning of the 20th century during the introduction of modern electronics, engineers realized that they needed a method for conveying electrical signals between devices. Since wires were the conveyor of electricity, designers developed circuit boards for the era that were big, bulky, and made of wood or metal. The structure of the boards enabled their use in a wide assortment of very large electrical equipment.
As technology progressed through the 20th century, electrical devices became smaller, more compact, and handheld, which challenged the traditional method of producing circuit boards. Although the large version of circuit boards was used into the 1960s, printed circuit boards were developed and designed in the 1930鈥檚 by an Austrian inventor. The introduction of PCBs to the entertainment and electronics industry led to the rapid development of many of the devices we use today.
Substrate
The substrate is the foundation of a printed circuit board. All of the parts and components of a printed circuit board rest upon the substrate, which is durable, long-lasting, strong, and capable of withstanding various environmental factors. Substrates come in different forms with the most popular form being a dielectric material made of fiberglass reinforced epoxy resin copper foil. Other substrate materials are ceramic filled polytetrafluoroethylene (PTFE), cyanate ester, and polyamide. Although reinforced fiberglass is a popular choice, the other materials are chosen for special types of PCBs and unique applications.
A key feature of all substrates is their dielectric properties, which provides insulation between conducting components. The four common properties of all substrates are electrical, thermal, chemical, and mechanical with the electrical properties being a substrate's non-conductivity. Substrates have to be resistant to heat, thermal properties, able to avoid decomposition, thermal expansion, and the existence of any conductive properties. The chemical properties of the substrate refer to its reaction to chemicals, such as moisture absorption and the effects of methylene chloride.
As would be expected, the mechanical properties of a substrate are essential to the performance of a PCB. The main function of a substrate is to ensure the unhindered transmission of electric signals. For most PCBs, this involves high external loads that are constantly moving across the substrate through the traces. It is for this reason that the mechanical properties of the substrate include peel strength, tensile modulus, flexural strength, and overall tensile strength. The mechanical properties of the substrate ensures that it will not bend, disfigure, or break when confronted with a PCB鈥檚 loads.
Prepreg
A necessary part of all PCBs is insulation that helps with conductivity. Prepreg is short for pre-impregnation, which is an insulation layer placed between cores or a core and copper foil. Its purpose is to bind layers together. Like the materials of the substrate, the prepreg is made of dielectric glass fiber material that is soaked and cured in epoxy. In certain circumstances, it is altered, modified, and adjusted to meet the requirements of specialized functions. The essential purpose of prepreg is to protect a PCB against short circuits.
Metal Foil
The metal foil of a PCB is its workhorse and is the conduit for the transmission of electrical signals. In the majority of cases, the metal of metal foil is made of copper due to copper鈥檚 high thermal and electrical conductivity. During the manufacturing process, the copper foil is formed into the traces that are the pathways for the electrical charges. Metal foil is applied to the substrate and produces the circuit patterns by the printing of the protective layers and the etching of the substrate. The varieties of metal foils include single or double sided and self-adhesive.
The low level of surface oxygen associated with copper enables it to easily bond with the insulation layer. In addition to its electrical properties, copper, combined with the substrate, provides electromagnetic shielding.
Coatings
When shopping for a printed circuit board, the one factor that differentiates manufacturers is their method of coating, which comes in a wide assortment of types. In all cases, coating is a lightweight protective layer designed to shield PCBs from a variety of environmental factors. During the initial design phase of printed circuit boards, the negative factors of a PCB鈥檚 working environment are examined to determine the proper type of substrate and coating. The fundamental factors that are crucial to the final choice of a coating are its thermal properties and the substrate of the PCB to which it will be applied.
The general types of coatings include acrylic, silicone, polyurethane, epoxy, and parylene. The basic differences between each of the types is their endurance, ruggedness, method of application, and protective properties. While high endurance coatings are ideal, they make it difficult to repair PCBs.
Vias
In multi-layer printed circuit boards, holes are drilled in order to make connections between the layers. The difference in the types of holes is in accordance with the nature of the connections to be made with some holes going through one layer while other holes go through all of the layers. Holes for PCBs are referred to as vias. The three types of holes are blind, buried, and through hole.
After the holes are drilled in a PCB, they receive coverings, which are tenting, uncovered, and plugged. Tenting covers the ring of a hole with a solder mask. Uncovered holes do not have a solder ring. Plugged holes have a solder mask that prevents solder balls from popping out.
The three types of vias are used for different types of printed circuit boards. Blind vias go part way through the layers of a PCB and allow connection between layers. They are placed on the top or underside of a PCB. Buried holes are in the middle of a PCB, located between layers, but not visible on the surface of a PCB. They connect inner layers and prevent signal interference. Through hole vias go completely through a printed circuit board and provide internal connectivity between all layers.
Bonding Film
The purpose of bonding film is to protect various components of a PCB, such as wiring and thermal insulation. As with other aspects of PCBs, bonding film provides a protective layer against static that could interfere with the function of a PCB. Bonding films are normally a form of plastic, such as polyester, polyimides, acrylic, or epoxy. The strength, durability, and quality of the bonding film is dependent on the conditions under which a PCB will be used. The types of bonding films are forms of adhesives that vary in their properties and strength.
Printed circuit boards are the central part of a wide range of modern devices. They can be as small as a cigarette lighter or as large as a white board. Regardless of the many sizes, types and configurations of PCBs, they are an essential part of computer age technology.