Printed circuit board
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A printed circuit board (PCB) mechanically supports and electrically connects
Printed circuit boards are used in all but the simplest electronic products. They are also used in some electrical products, such as passive switch boxes.
Alternatives to PCBs include
PCBs can be single-sided (one copper layer), double-sided (two copper layers on both sides of one substrate layer), or multi-layer (outer and inner layers of copper, alternating with layers of substrate). Multi-layer PCBs allow for much higher component density, because circuit traces on the inner layers would otherwise take up surface space between components. The rise in popularity of multilayer PCBs with more than two, and especially with more than four, copper planes was concurrent with the adoption of
The world market for bare PCBs exceeded $60.2 billion in 2014. 
A basic PCB consists of a flat sheet of insulating substrate material, which gives it its mechanical strength and integrity, and a layer of copper cladding, i.e. a flat sheet of copper foil, laminated to the substrate. Chemical etching through the copper cuts out areas so that the copper is divided into separate conducting regions, forming a circuit wiring pattern: long, thin, sometimes serpentine lines (called tracks or circuit traces), terminals or pads for connection of component leads or for external connections, contacts for vias, and other features such as solid conductive areas for EM shielding or other purposes. The tracks or traces function as uninsulated electrical wires. Their permanent fixation to the insulating substrate keeps them from touching each other and
The preceding basic PCB structure with one copper layer can be extended to a two-layer or multi-layer structure. A two-layer board simply has etched copper on both sides instead of just one side. This allows wire-line paths to cross each other without the use of jumper wires installed on the board like components, simply through a line on one side to a line being connected to on the other side that crosses over or under the opposing lines. The concept can be extended to multiple layers by dividing the substrate into thinner parallel layers and sandwiching additional copper layers between them. Conductors on different layers, whether there are just two layers or many, are connected with
There are two ways of installing components on a PCB:
Though-hole mounting, the original method and the only popular method until the mid-1980s, entails passing all component leads through holes drilled in the board at precise locations. All or almost all components are typically inserted from one side, (the "component side") and each of their leads is soldered to the board on the other side (the "solder side"), where the etched pattern usually includes copper completely surrounding the holes. If the board has two or more layers, solder normally flows through the holes, around the component leads, to connect the copper of all layers where it exists at each lead; thus, soldered component leads themselves can act as vias. (For this to occur, the lead has to fit the hole closely enough to enable capillary action between the lead and the inner cylindrical surface of the hole.)
Surface mounting, which only became popular in the 1990s, is radically different from through-hole mounting in that components are soldered directly to solder pads (usually rectangular in shape) on which they are laid; no holes are involved in component connections. Surface-mounting cannot collaterally interconnect copper layers, so on PCBs with only surface-mount components, all inter-layer connections are by vias. While surface mounting can be done by point soldering for component sizes and lead pitches above certain practical minima, it is more commonly done by depositing solder paste—a mixture of powdered solder and flux—over the solder pads in a precise pattern and at a precise thickness using a mylar mask, depositing the surface-mount components (usually by automated robotic equipment) onto the PCB over the solder paste, which temporarily adheres the components to the board, and baking the board so adorned in a special oven to fuse the solder and permanently fix the electrical and mechanical connections. This is obviously a large-scale manufacturing process not suitable to prototyping or very small limited production runs. PCBs with only through-hole mounted components are now uncommon, though they were very common in machines built before the 1990s, and ubiquitous where PCBs were used before the 1980s.
PCBs with only surface-mount components are now very common, but PCBs using both types of component mounting are also very common: typically, surface mounting is used for active devices (transistors and IC chips) and small-signal passive components, and through-hole mounting is used for a few larger components (e.g. physically large electrolytic capacitors), some electrical connectors (such as 0.100-inch-centered pin header connectors, or any conector that will be subjected to mechanical stress), and special components not available in a surface-mount form.
In the electronics industry, the 2-dimensional design pattern to be etched into each copper layer of a PCB is called the "artwork". The etching of the artwork into the copper is where the word printed in the term "printed circuit board" originates. The etching is usually done using
In multi-layer boards, the layers of material are laminated together in an alternating sandwich: copper, substrate, copper, substrate, copper, etc.; each plane of copper is etched, and any internal vias (that will not extend to both outer surfaces of the finished multilayer board) are plated-through, before the layers are laminated together. Only the outer layers are optionally coated with a protective coating; the inner copper layers are protected by the adjacent substrate layers between which they are sandwiched.
When a PCB has no components installed, it is less ambiguously called a printed wiring board (PWB) or etched wiring board, as the term printed circuit board tends to imply a complete manufactured circuit on a board. However, the term "printed wiring board" has fallen into disuse. A PCB populated with electronic components is called a printed circuit assembly (PCA), printed circuit board assembly or PCB assembly (PCBA). In informal usage, the term "printed circuit board" most commonly means "printed circuit assembly", though technically it is ambiguous as to whether the board is bare or has components installed and if the latter, whether the reference is to the whole assembly or to only the board itself. The
In addition to (and after) being coated with solder resist, it is common for a PCB to be "silkscreen" printed with a legend, usually in white epoxy ink, identifying the components to be installed at each position, their correct orientations and lead identifications, and sometimes additional text such as the functional purposes of connectors or the settings and effects of switches and pin jumpers (a/k/a shunt blocks). The printing may also identify the function or identity of the PCB, any model and revision numbers, and/or the name or brand of the manufacturer of the product (especially if the PCBA is the product, as in the case of a computer adapter card or a single-board computer, for example). Originally, an actual silkscreen printing process was used for this purpose, but today other, finer quality printing methods are usually used instead, though the result is often still called "silkscreening" or a "silk legend" for historical reasons. Normally the screen printing is purely cosmetic and not significant to the functional of the PCBA.
A minimal PCB for a single component, used for easier prototyping, is called a breakout board.  The purpose of a breakout board is to "break out" the signal pins or leads of an IC or other component, to separate terminals or leads so that manual connections to them can be made easily. Breakout boards are especially used for surface-mount components or any components with fine lead pitch.
Advanced PCBs may contain components embedded in the substrate.[