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Printed circuit board

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Close-up photo of one side of a motherboard PCB, showing conductive traces, vias and solder points for through-hole components on the opposite side.

A printed circuit board or PCB interconnects electronic components without discrete wires. Alternative names are printed wiring board or PWB.

Table of contents

General characteristics

A printed circuit board consists of "printed wires" attached to a sheet of insulator. The conductive "printed wires" are called "traces" or "tracks". The insulator is called the substrate.

A few printed circuit boards are made by adding traces to the substrate. The vast majority are manufactured by plating a layer of copper over the entire substrate, sometimes on both sides, (creating a "blank PCB"), then removing unwanted copper, leaving only the copper traces. Some PCBs have a trace layer inside the PCB ('multi layer').

After the circuit board has been manufactured, components are attached to the traces by soldering.

There are three common methods used for the production of printed circuit boards:

  1. Photoengraving, the use of a photomask and chemical etching to remove the copper foil from the substrate. The photomask is usually prepared with a photoplotter from data produced by a technician using computer-aided PCB design software. Laser-printed transparencies are sometimes employed for low-resolution photoplots.[1]
  2. PCB Milling, the use of a 2 or 3 axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine (referred to as a 'PCB Prototyper') operates similar to a plotter, receiving commands from the host software that control the position of the milling head in the x, y, and (if relevant) z axis. Data to drive the Prototyper is extracted from files generated in PCB design software and stored in HPGL or Gerber file format.
  3. PCB Printing, the use of conductive ink or epoxy to form traces directly on substrate material. Similar to PCB milling in terms of hardware and data used.

PCBs are rugged, inexpensive, and can be highly reliable. They are harder to repair than wire wrap boards. They require much more design than either wire-wrapped or point-to-point constructed equipment.

History

The inventor of the printed circuit was probably the Austrian engineer Paul Eisler (1907 – 1995) who, while working in England, made one in about 1936 as part of a radio set. In about 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.

Before printed circuits, point-to-point construction was used. For prototypes, or small production runs, wire wrap can be more efficient.

Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of each component. The components were then soldered into the PCB. This method is called through-hole construction. This could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. Through-hole mounting is still useful in attaching physically-large and heavy components to the board.

However, the wires and holes are wasteful. It costs money to drill the holes, and the protruding wires are merely cut off.

surface-mount technology

In the 1960s, the surface-mount technology was developed. It became widely used in the late 1980s. Components were mechanically redesigned to have small metal tabs or pads that could be directly soldered to the surface of the PCB. Often, only the attaching solder holds the part to the board. Surface-mounted components (or devices: SMD) are usually made as physically small and lightweight as possible for this reason.

Often an automated machine removes the parts from reels, and sticks them to the PCB. A silk-screened application of solder paste, a mixture of solder and flux, holds the parts in place.

The board is pre-heated, passed through an oven containing infrared lamps whose heat melts the solder, then the board is slowly cooled. Today infrared ovens are not used because they do not heat the PCB and its components evenly, thus causing defects. So convection ovens are utilized in modern day manufacturing facilities. The pre-heating and controlled cooling prevent the parts from cracking when one edge of the part is cold and another edge is hot from the solder.

The parts and pads of the PCB are designed so that the surface tension of the molten solder centers the parts on their copper pads.

The result is components that are one quarter to one tenth of the size and weight, and half to a quarter of the cost of wire-mounted parts.

See also: electronics, wire wrap, point-to-point construction.

substrate

The vast majority of PWBs use a substrate made of "FR4" or paper FR-2 substrates.

'Pertinax' (a phenol formaldehyde resin), or a fiberglass-reinforced epoxy composite material.

Some PCBs for high-frequency RF work use plastics with special characteristics in order to avoid detuning the radio, such as Rogers 4000, Teflon, Duroid, and Polymide.

PCBs in vacuum or spacecraft often have solid copper or aluminum cores to carry away components' heat.

Not all circuit boards use rigid core materials. Some are designed to be completely flexible or partially flexible. This class of boards, sometimes called "flex" or "rigid-flex" respectively, are difficult to create but have many applications. Sometimes they are flexible to save space (PWBs inside cameras and hearing aids are almost always made of flex circuits so they can be folded up and crammed into the limited space available). Other times the flexible part of the circuit board is actually being used as a cable, or connection to another board or device.

Design

Usually an electrical engineer designs the circuit, and a technician designs the PCB. PCB design is a specialized skill. There are numerous tricks and standards used to design a PCB that is easy to manufacture and yet small and inexpensive. (see PCB layout guidelines).

The width and spacing of conductors on a PCB is very important. If conductors are too close, solder can short adjacent connectors, and the PCB will be difficult to repair. If too far apart, the PCB may be too large and expensive.

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

Most PCBs have between one and sixteen conductive layers laminated (glued) together. In more complex PCBs, two or more of the layers are dedicated to providing ground and power. These ground planes and power planes detune accidental antennas, and provide efficient distribution of power. Multi-layer boards enable construction of complex digital circuits.

Ground and power planes are rectangular sheets of conductor 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. Specialized conduction-cooled designs rely on the PCB to conduct away all the waste heat, unlike the air-cooling method more commonly used.

Multilayer PCBs have alignment marks and holes (called fiducials) to align layers and permit the PCB to be mounted in equipment that automatically places and solders components. Some designs place alignment and etch test-patterns on break-off tabs that can be removed before installation.

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.

Holes are drilled with tiny carbide drill-bits or by lasers. The drilling is performed by drilling machines with computerized placement using a "drill tape" or "drill file." A drill file is a computer file describing the location and sizes of all drilled holes. These files are also called numerically controlled drill (NCD) files. You may also see them called Excellon files.

Component leads are inserted in holes or mounted on the surface "pads" and electrically and mechanically fixed to the board with a molten metal solder.

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. Most solder mask is green, but it is also available in red, black, and other colors).

A silkscreen legend on the top or bottom surface of the board provides readable information about component part numbers and placement that aids in manufacturing and repair. New technology allows for the component designators to be printed directly onto the board surface, saving time and money by doing away with expensive and tedious silkscreens. This is essentially done by a giant inkjet printer. A similar process can be used for soldermasks, but it should still be considered developmental.

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

Mass-production PCBs have small pads for automated test equipment to make temporary connections. Sometimes the pads must be isolated with resistors.

PCB designers often design power supply circuits. They usually place a bypass capacitor near each IC, to filter power supply noise and to store energy for short-term consumption in high-speed integrated circuits. They usually place bulk capacitors fairly evenly throughout the PCB.

PCB designers must often renumber components.

To aid manual repair, diodes, capacitors and integrated circuits should be oriented in the same way.

See also

Printed circuit manufacturing Guides and GNU programs








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