Close-up photo of one side of a motherboard PCB, showing conductive traces 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.
A printed circuit board consists of "printed wires" attached to a sheet of insulator. The conductive "printed wires" are called "traces". The insulator is called the substrate, and is often made of fiberglass-reinforced epoxy composite.
A few printed circuit boards are made by adding traces to the substrate. The vast majority are manufactured by gluing a layer of copper foil over the entire substrate (creating a "blank PCB"), then removing unwanted copper, leaving only the copper traces.
After the circuit board has been manufactured, components are attached to the traces, usually by soldering.
There are three common methods used for the production of printed circuit boards:
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Photoengraving, the use 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. Some people produce low-resolution photoplots by printing a design to a laser printer, printing on the sheets used to make transparent presentations.[1] http://www.fullnet.com/u/tomg/gooteepc.htm
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PCB Milling, the use of a 2 or 3 axis mechanical milling systems to mill away the copper foil from the substrate. A PCB milling machine (referred to as a 'PCB Prototyper' operates similar to a plotter, it receives commands from the host software package that controls the position of the milling head in the X/Y and sometimes Z axis. Data to drive the Prototyper is extracted from files generated in PCB design software.
- PCB Printing , the use of conductive ink or epoxy to form traces directly on substrate material. Is 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 Americans 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 to 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 wires are merely cut off.
surface-mount technology
In the 1960s, a technique called surface mount was invented. 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-mount components 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. 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.
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).
Some PCBs for high-frequency RF work use plastics with special characteristics in order to avoid detuning the radio. PCBs in vacuum or spacecraft often have solid copper or aluminum cores to carry away components' heat.
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 plane s detune accidental antennas, and provide efficient distribution of power. Multi-layer boards are needed for complex digital circuits such as computers.
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 file s.
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 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, including placement of bypass capacitors, used for filtering power supply noise and usually placed near the integrated circuit; and bulk capacitors, used to store current for short-term consumption in high-speed integrated circuits and usually distributed 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.
Manufacturing information (21st century technology)
Laser drilling/routing is a new method for removing material from circuit boards. Typically it is used to create very small holes or create precise routes. Some have suggested it could be used to remove copper, creating the circuit images more precisely then chemical etching.
Another modern approach to PCB manufacturing is PCB milling, which uses a series of production steps (drilling/ trace isolation/ cutting board outline) to create single and multilayer boards on a single machine. This technology offers all the advantages of modern photimaging without the use of chemicals, or the production of toxic fumes. In some cases, such as RF and Microwave board production, it provides the best solution as it is not affected by over- or under-etching of traces, which is very hard to control in a chemical bath. Although not recommended for high volume board production, it is an ideal technology for in-house R&D and small batch production. With the ability to mill 4 micrometre traces it holds great potential for designers creating high density boards.
A new improvement in photoimaging, called direct imaging, can do away with the need for developed film. The photoimaging process involves creating a film on a clear plastic in a laser photoplotter, and then shining light through that film onto a PCB coated with photosensitive material which will create the desired image. However, there are new machines which use a direct digital image to expose the photosensitive material with a laser. They work in a different manner than photoimaging because no film is needed, and are considered to be more accurate. However, they are generally slower than then older film process.
Buried vias are internal copper-plated holes which connect layers within the board electrically. These are quite common in higher technology boards, and are achieved by creating part of the board first, and then laminating (gluing) it onto other layers. For instance if a via connects layer 2 and layer 5 in a 10 layer board, layers 2,3,4 and 5 will be laminated together to create a sub-assembly, the holes are created by a drilling process and then it is laminated together with the rest of the layers. If this type of board is intended for extreme environments, these holes will frequently be filled with a conductive substance to ensure electrical connectivity, as well as prevent pockets of air from becoming trapped within the board.
Certain circuit boards are used as antennae to create radio frequencies. These boards require very tight tolerances, and are usually completely plated on the outer layers, as well as the sides of the board. To create proper frequencies, parts of the board are routed down layers to create plated cavities.
On-board resistors are sometimes necessary in circuits which would be unreachable from the outside. These resistors can be created right on the circuit board. Two general methods of creating resistance are used. The first uses a special core material which is conductive. This core material has a layer of copper, a thin layer of non-conductive plastic and then the resistive material. The circuit image for the layer is imaged onto the core material's copper, and etched, and then the resistors are etched by removing boxes of the thin non-conductive plastic which allows an electrical connection between two circuits via the resistive material. The other method is to use a liquid which is electrically resistive, and screen the resistors onto the layer between the circuits.
Controlled impedance is another method complex circuit boards use to create resistance in a circuit. This method involves very precise circuit widths and vertical distances between layers. The electro-magnetic waves from a powered trace are reflected by powered copper on a reference layer, and used to create a resistance in the trace. Careful monitoring and testing of this property is a high priority for manufacturers because it is impossible to fix if incorrectly made.
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, and sometimes the flexible part of the circuit board is actually being used as a cable, or connection to another board or device.
Printed circuit manufacturing Guides
- http://www.fullnet.com/u/tomg/gooteepc.htm
- http://www.ee.washington.edu/circuit_archive/text/design.html
- http://www.precisioncircuits.com.au/cid/hm_cid.html
- http://amscourseware.com/guidelines.htm
- http://www.filtranmicro.com/design.html
- http://www.goldengategraphics.com/pcgloss.htm
- http://www.elchempub.com/files/electroc2.htm
- http://www.pcbprotech.com/Dh3/DH3right.htm
- http://www.pcbprototyping.com/html/html_edu.htm
Last updated: 05-03-2005 17:50:55
