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In electronics, printed circuit boards, or PCBs, are used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate. Alternative names are printed wiring board (PWB),and etched wiring board. Populating the board with electronic components forms a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA).

PCBs are rugged, inexpensive, and can be highly reliable. They require much more layout effort and higher initial cost than either wire-wrapped or point-to-point constructed circuits, but are much cheaper, faster, and consistent in high volume production. Much of the electronics industry's PCB design, assembly, and quality control needs are set by standards that are published by the IPC organization.

History

The inventor of the printed circuit was the Austrian engineer Paul Eisler (1907–1995) who, while working in England, made one circa 1936 as part of a radio set. Around 1943 the USA began to use the technology on a large scale to make rugged radios for use in World War II. After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army.

Physical composition

Most PCBs are composed of between one and twenty-four conductive layers separated and supported by layers of insulating material (substrates) laminated (glued with heat, pressure & sometimes vacuum) together.

The most common substrate for single-sided PCBs is FR-2 (synthetic resin bonded paper) which is cheap and easy to drill but is prone to cracking and is not good for making plated through hole boards. Double sided and multilayer boards tend to use FR-4 (a fiberglass type material). Other substrates include power electronic substrate and Kapton (used to make flexible electronics). The conductive layers are almost invariably made of copper, which sometimes is gold-coated.

Layers may be connected together through drilled holes called vias. To form an electrical connection, the holes are either electroplated or small rivets are inserted. Even though they may not form electrical connections to all layers, these holes are typically drilled completely through the PC board for simplicity of design and manufacture. The exception are high-density PCBs, which may have blind vias (which are visible only on one surface), or buried vias (which are visible on neither).

Design

The patterns on a PCB are usually drawn using electronic design automation software. That PCB design software generates vector graphics output drawings, most often in Gerber file format.

The design process involves moving from the specification at the start, to a plan that contains all the information needed to be physically constructed at the end, this normally happens by passing through a number of stages, although in very simple circuit it may be done in a single step. The process normally begins with the conversion of the specification into a block diagram of the various functions that the circuit must perform, at this stage the contents of each block are not considered, only what each block must do, this is sometimes referred to as a "black box" design. This approach allows the possibly very complicated task to be broken into smaller tasks which may either by tackled in sequence or divided amongst members of a design team.

Each block is then considered in more detail, still at an abstract stage, but with a lot more focus on the details of the electrical functions to be provided. At this or later stages it is common to require a large amount of research or mathematical modeling into what is and is not feasible to achieve. The results of this research may be fed back into earlier stages of the design process, for example if it turns out one of the blocks cannot be designed within the parameters set for it, it may be necessary to alter other blocks instead. At this point it is also common to start considering both how to demonstrate that the design does meet the specifications, and how it is to be tested (which can include self diagnostic tools ).

Finally the individual circuit components are chosen to carry out each function in the overall design, at this stage the physical layout and electrical connections of each component are also decided, this layout commonly taking the form of artwork for the production of a printed circuit board or Integrated circuit. This stage is typically extremely time consuming because of the vast array of choices available. A practical constraint on the design at this stage is that of standardization, while a certain value of component may be calculated for use in some location in a circuit, if that value cannot be purchased from a supplier, then the problem has still not been solved. To avoid this certain amount of 'catalog engineering' can be applied to solve the more mundane tasks within an overall design.

Manufacturing

Patterning (etching)

The vast majority of printed circuit boards are made by adhering a layer of copper over the entire substrate, sometimes on both sides, (creating a "blank PCB") then removing unwanted copper after applying a temporary mask (eg. by etching), leaving only the desired copper traces. A few PCBs are made by adding traces to the bare substrate (or a substrate with a very thin layer of copper) usually by a complex process of multiple electroplating steps.

There are three common "subtractive" methods (methods that remove copper) used for the production of printed circuit boards:

"Additive" processes also exist. The most common is the "semi-additive" process. In this version, the unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. (Unlike a subtractive process mask, this mask exposes those parts of the substrate that will eventually become the traces.) Additional copper is then plated onto the board in the unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped away and a brief etching step removes the now-exposed original copper laminate from the board, isolating the individual traces.

The additive process is commonly used for multi-layer boards as it facilitates the plating-through of the holes (vias) in the circuit board.

Lamination

Some PCBs have trace layers inside the PCB and are called multi-layer PCBs. These are formed by bonding together separately etched thin boards.

Drilling

Holes, or vias, through a PCB are typically drilled with tiny drill bits made of solid tungsten carbide. The drilling is performed by automated drilling machines with placement controlled by a drill tape or drill file. These computer-generated files are also called numerically controlled drill (NCD) files or "Excellon files". The drill file describes the location and size of each drilled hole.

When very small vias are required, drilling with mechanical bits is costly because of high rates of wear and breakage. In this case, the vias may be evaporated by lasers. Laser-drilled vias typically have an inferior surface finish inside the hole. These holes are called micro vias.

It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called blind vias when they connect an internal copper layer to an outer layer, or buried vias when they connect two or more internal copper layers and no outer layers.

The walls of the holes, for boards with 2 or more layers, are plated with copper to form plated-through holes that electrically connect the conducting layers of the PCB. For multilayer boards, those with 4 layers or more, drilling typically produces a smear comprised of the bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemical de-smear process, or by plasma-etch.

Exposed conductor plating and coating

The pads and lands to which components will be mounted are typically plated, because bare copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed copper was plated with solder. This solder was a tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the EU, which restricts the use of lead. Other platings used are OSP (organic surface protectant), immersion silver, electroless nickel with immersion gold coating (ENIG), and direct gold. Edge connectors, placed along one edge of some boards, are often gold plated.

Solder resist

Areas that should not be soldered to may be covered with a polymer solder resist (solder mask) coating. The solder resist prevents solder from bridging between conductors and thereby creating short circuits. Solder resist also provides some protection from the environment.

Screen printing

Line art and text may be printed onto the outer surfaces of a PCB by screen printing. When space permits, the screen print text can indicate component designators, switch setting requirements, test points, and other features helpful in assembling, testing, and servicing the circuit board.

Screen print is also known as the silk screen, or, in one sided PCBs, the red print.

Lately some digital printing solutions have been developed to substitute the traditional screen printing process. This technology allows printing variable data onto the PCB, including serialization and barcode information for traceability purposes.

Test

Unpopulated boards may be subjected to a bare-board test where each circuit connection (as defined in a netlist) is verified as correct on the finished board. For high-volume production, a Bed of nails tester ,a fixture or a Rigid needle adapter is used to make contact with copper lands or holes on one or both sides of the board to facilitate testing. A computer will instruct the electrical test unit to send a small amount of current through each contact point on the bed-of-nails as required, and verify that such current can be seen on the other appropriate contact points. A "short" on a board would be a solid connection where there should be no connection. An "open" is between two points that should be connected and are not. For small- or medium-volume boards, flying-probe testers use moving test heads to make contact with the copper lands or holes to verify the electrical connectivity of the board under test.

Populating

After the PCB is completed, electronic components must be attached to form a functional printed circuit assembly, or PCA(sometimes called a "printed circuit board assembly" PCBA). In through-hole construction, component leads are inserted in holes. In surface-mount construction, the components are placed on pads or lands on the outer surfaces of the PCB. In both kinds of construction, component leads are electrically and mechanically fixed to the board with a molten metal solder.

There are a variety of soldering techniques used to attach components to a PCB.

Often, through-hole and surface-mount construction must be combined in a single PCA because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Another reason to use both methods is that through-hole mounting can provide needed strength for components likely to endure physical stress, while components that are expected to go untouched will take up less space using surface-mount techniques.

JEDEC guidelines for PCB component placement, soldering, and inspection are commonly used to maintain quality control in this stage of PCB manufacturing.

After the board is populated, the populated board may be tested with an in-circuit test system. To facilitate this test, PCBs may be designed with extra pads to make temporary connections. Sometimes these pads must be isolated with resistors. The in-circuit test may also exercise boundary scan test features of some components. In-circuit test systems may also be used to program nonvolatile memory components on the board.

In boundary scan testing, test circuits integrated into various ICs on the board form temporary connections between the PCB traces to test that the ICs are mounted correctly. Boundary scan testing requires that all the ICs to be tested use a standard test configuration procedure, the most common one being the Joint Test Action Group (JTAG) standard.

When boards fail the test, technicians may desolder and replace failed components.

Protection and packaging

PCBs intended for extreme environments often have a conformal coat, which is applied by dipping or spraying after the components have been soldered. The coat prevents corrosion and leakage currents or shorting due to condensation. The earliest conformal coats were wax. Modern conformal coats are usually dips of dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Some are engineering plastics sputtered onto the PCB in a vacuum chamber.

Many assembled PCBs are static sensitive, and therefore must be placed in antistatic bags during transport. When handling these boards, the user must be earthed; failure to do this might transmit an accumulated static charge through the board, damaging or destroying it. Even bare boards are sometimes static sensitive. Traces have gotten so fine that it's quite possible to blow an etch off the board (or change its characteristics) with a static charge. This is especially true on non-traditional PCBs such as MCMs and microwave PCBs.

Safety Certification (US)

Safety Standard UL 796 covers component safety requirements for printed wiring boards for use as components in devices or appliances. Testing analyzes characteristics such as flammability, maximum operating temperature, electrical tracking, heat deflection, and direct support of live electrical parts.

The boards may use organic or inorganic base materials in a single or multilayer, rigid or flexible form. Circuitry construction may include etched, die stamped, precut, flush press, additive, and plated conductor techniques. Printed-component parts may be used.

The suitability of the pattern parameters, temperature and maximum solder limits shall be determined in accordance with the applicable end-product construction and requirements.

PCB layout guidelines

Usually an electronics or electrical engineer designs the circuit, and a layout specialist designs the PCB. PCB design is a specialized skill. There are numerous techniques and standards used to design a PCB that is easy to manufacture and yet small and inexpensive.

Most PCBs have between one and twenty conductive layers laminated (glued) together in a sandwich with insulating plastic. PCBs with more than two layers help construct complex or dense circuits. They are not always used because they are more expensive, and the inner layers are more difficult to inspect and repair.

In more complex PCBs, two or more of the layers are dedicated to providing ground and power. These ground planes and power planes distribute power well. They also prevent radio waves from antennas unintentionally formed by tracks. These planes are rectangular sheets of foil that occupy entire layers (except for small holes to avoid unwanted connection to vias and through-hole components). They distribute electrical power and heat better than narrow traces. Sometimes solid metal PCBs with thin layers of insulation are used. The power electronic substrate carries away waste heat when air cooling is impossible.

Four-layer PCBs with a ground and power plane are often used in high-quality, but cost-conscious audio, avionics and medical electronics. Most consumer products have one or two layers.

The width and spacing of conductors (or "traces") on a PCB is very important. If the traces are too close, solder can short adjacent traces, and the PCB will be difficult to construct or repair. If too far apart, the PCB may be too large and expensive. When a PCB carries high frequencies, traces may need to be exact widths and lengths to control the characteristic impedance of the trace.

Some designs cut the ground plane or the entire PCB in strategic locations to control the return paths of currents. The usual desire is to keep high voltages or frequencies away from sensitive portions of a circuit. The actual properties of the design are critical, because in some cases, cutting the ground plane makes the PCB into an antenna that radiates radio noise into nearby equipment.

Removing large areas of copper wastes etchant and increases pollution. Also, a PCB etches more consistently and tends to resist warping if all regions have the same average ratio of copper to bare board. Therefore, designers may widen connectors, leave unconnected copper in place, or cover large areas of what would otherwise be bare board with arrays of small, electrically isolated copper diamonds or squares.

Most PCBs have alignment marks (called fiducials) and tooling holes to align layers. These permit the PCB to be mounted in equipment that automatically places and solders components. Some designs also have quality control patterns to measure soldering and etching processes. In some cases, the test patterns are on break-off tabs that can be removed before the PCB is installed.

Layers may be connected together through drilled holes called vias. Either the holes are electroplated or small rivets are inserted. High-density PCBs may have blind vias, which are visible only on one surface, or buried vias, which are visible on neither, but these are expensive to build and difficult or impossible to inspect after manufacture. Good designers minimize the number of vias to reduce the cost of drilling. On older, two-layer PCBs, it was common to solder a wire through the hole.

A solder mask is a plastic layer that resists wetting by solder (the solder is said to "bead up"), and keeps islands of solder from running together. It also protects the outside conductors layers from abrasion and corrosion. Without the solder mask, the fiberglass-reinforced epoxy appears a translucent off-white. Solder masks are usually green, but they may be found in other colors.

A silkscreen legend on the top or bottom surface of the board provides readable information about component part numbers and placement. This aids in manufacturing and repair. To aid manual construction and repair, diodes, capacitors and integrated circuits are sometimes oriented in the same direction.

New technology allows for the component designators to be printed directly onto the board surface, saving time and money by doing away with silkscreens. This is sometimes done by a special inkjet printer. A similar process has experimentally produced solder masks.

PCB layout Basic guidelines:

PCB layout guidelines for RF circuits on a 2-layer or multilayer board: