Editing Printed circuit board (section)

== Composition ==
[[File:Hand Etched PCB.png|thumb|An example of hand-drawn etched traces on a PCB]]

A basic PCB consists of a flat sheet of insulating material and a layer of copper [[Foil (metal)|foil]], laminated to the substrate. Chemical etching divides the [[copper]] into separate conducting lines called tracks or ''circuit traces'', pads for connections, [[Via (electronics)|vias]] to pass connections between layers of copper, and features such as solid conductive areas for [[electromagnetic shielding]] or other purposes. The tracks function as wires fixed in place, and are insulated from each other by air and the board substrate material.  The surface of a PCB may have a coating that protects the copper from [[corrosion]] and reduces the chances of solder [[Short circuit|shorts]] between traces or undesired electrical contact with stray bare wires. For its function in helping to prevent solder shorts, the coating is called solder resist or [[solder mask]].

The pattern to be etched into each copper layer of a PCB is called the "artwork".  The etching is usually done using [[photoresist]] which is coated onto the PCB, then exposed to light projected in the pattern of the artwork. The resist material protects the copper from dissolution into the etching solution.  The etched board is then cleaned. A PCB design can be mass-reproduced in a way similar to the way [[photograph]]s can be mass-duplicated from [[film negative]]s using a [[photographic printer]].

[[FR-4]] [[fibre-reinforced plastic|glass epoxy]] is the most common insulating substrate. Another substrate material is [[cotton paper]] impregnated with [[phenolic resin]], often tan or brown.

When a PCB has no components installed, it is less ambiguously called a ''printed wiring board'' (''PWB'') or ''etched wiring board''.<ref>[https://www.chipsmall.com/Blog/info/6-reasons-why-we-choose-printed-circuit-boards.html 6 Reasons Why Choose Printed Circuit Boards]</ref> 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" (with components). The [[IPC (electronics)|IPC]] preferred term for an assembled board is ''circuit card assembly'' (''CCA''),<ref>IPC-14.38</ref> and for an assembled [[backplane]] it is ''backplane assembly''. "Card" is another widely used informal term for a "printed circuit assembly".
For example, [[expansion card]].

A PCB may be printed with a legend identifying the components, [[test point]]s, or identifying text. Originally, [[silkscreen printing]] was used for this purpose, but today other, finer quality printing methods are usually used.  Normally the legend does not affect the function of a PCBA.

=== Layers ===
A printed circuit board can have multiple layers of copper which almost always are arranged in pairs. The number of layers and the interconnection designed between them (vias, PTHs) provide a general estimate of the board complexity. Using more layers allow for more routing options and better control of signal integrity, but are also time-consuming and costly to manufacture. Likewise, selection of the vias for the board also allow fine tuning of the board size, escaping of signals off complex ICs, routing, and long term reliability, but are tightly coupled with production complexity and cost.

One of the simplest boards to produce is the two-layer board. It has copper on both sides that are referred to as external layers; multi layer boards sandwich additional internal layers of copper and insulation. After two-layer PCBs, the next step up is the four-layer. The four layer board adds significantly more routing options in the internal layers as compared to the two layer board, and often some portion of the internal layers is used as [[ground plane]] or power plane, to achieve better signal integrity, higher signaling frequencies, lower EMI, and better power supply decoupling.

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 need be coated; the inner copper layers are protected by the adjacent substrate layers.

=== Component mounting ===
[[File:Resistors (1).jpg|thumb|Through-hole (leaded) resistors]]
[[File:MOS6581 chtaube061229.jpg|thumb|Through-hole devices mounted on the circuit board of a mid-1980s [[Commodore 64]] home computer]]
[[File:Box of 02in pcb bits.jpg|thumb|A box of [[drill bit]]s used for making holes in printed circuit boards. While tungsten-carbide bits are very hard, they eventually wear out or break. Drilling is a considerable part of the cost of a through-hole printed circuit board.]]
[[File:Surface Mount Components.jpg|thumb|Surface mount components, including resistors, [[transistor]]s and an integrated circuit]]
[[File:Mouse printed circuit board both sides IMG 0959a.JPG|thumb|A PCB in a [[computer mouse]]: the component side (left) and the printed side (right)]]

"Through hole" components are mounted by their wire leads passing through the board and soldered to traces on the other side. "Surface mount" components are attached by their leads to copper traces on the same side of the board.  A board may use both methods for mounting components. PCBs with only through-hole mounted components are now uncommon. Surface mounting is used for [[transistor]]s, [[diode]]s, [[Integrated circuit|IC chip]]s, [[resistor]]s, and capacitors. Through-hole mounting may be used for some large components such as [[electrolytic capacitor]]s and connectors.

The first PCBs used [[through-hole technology]], mounting electronic components by lead inserted through holes on one side of the board and soldered onto copper traces on the other side. Boards may be single-sided, with an unplated component side, or more compact double-sided boards, with components soldered on both sides.  Horizontal installation of through-hole parts with two axial leads (such as resistors, capacitors, and diodes) is done by bending the leads 90 degrees in the same direction, inserting the part in the board (often bending leads located on the back of the board in opposite directions to improve the part's mechanical strength), soldering the leads, and trimming off the ends. Leads may be [[Soldering|soldered]] either manually or by a [[wave soldering]] machine.<ref>{{cite book |first= |last= |chapter=Electronic Packaging:Solder Mounting Technologies |chapter-url= |editor-first=K.H. |editor-last=Buschow |title=Encyclopedia of Materials: Science and Technology |publisher=Elsevier |location= |date=2001 |isbn=0-08-043152-6 |pages=2708–9 |url=https://books.google.com/books?id=TyVVAAAAMAAJ }}</ref>

[[Surface-mount technology]] emerged in the 1960s, gained momentum in the early 1980s, and became widely used by the mid-1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the PCB surface, instead of wire leads to pass through holes. Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies with much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labor costs and greatly increasing production rates compared with through-hole circuit boards. Components can be supplied mounted on carrier tapes. Surface mount components can be about one-quarter to one-tenth of the size and weight of through-hole components, and passive components much cheaper. However, prices of semiconductor [[surface mount device]]s (SMDs) are determined more by the chip itself than the package, with little price advantage over larger packages, and some wire-ended components, such as [[1N4148 signal diode|1N4148]] small-signal switch diodes, are actually significantly cheaper than SMD equivalents.

=== Electrical properties ===
Each trace consists of a flat, narrow part of the [[copper]] foil that remains after etching. Its [[Electrical resistance|resistance]], determined by its width, thickness, and length, must be sufficiently low for the current the conductor will carry. Power and ground traces may need to be wider than [[signal trace]]s. In a multi-layer board one entire layer may be mostly solid copper to act as a [[ground plane]] for shielding and power return. For [[microwave]] circuits, [[transmission line]]s can be laid out in [[planar transmission line|a planar form]] such as [[stripline]] or [[microstrip]] with carefully controlled dimensions to assure a consistent [[Characteristic impedance|impedance]]. In radio-frequency and fast switching circuits the [[inductance]] and [[capacitance]] of the printed circuit board conductors become significant circuit elements, usually undesired; conversely, they can be used as a deliberate part of the circuit design, as in [[distributed-element filter]]s, [[Antenna (radio)|antennae]], and [[Fuse (electrical)|fuse]]s, obviating the need for additional discrete components. High density interconnects (HDI) PCBs have tracks or vias with a width or diameter of under 152 micrometers.<ref>{{cite web |url=https://www.freedomcad.com/2018/08/21/why-use-high-density-interconnect/ |title=Why Use High Density Interconnect? |date=August 21, 2018 }}</ref>