Printed circuit board

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.

Contents

1 General characteristics

2 History

2.1 Surface-mount technology

3 Substrate

4 Design

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:

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] (http://www.fullnet.com/u/tomg/gooteepc.htm)
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.
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

Surface-mount technology was developed in the 1960s and became widely used in the late 1980s. Much of the pioneering work in this technology was done at IBM. Components were mechanically redesigned to have small metal tabs or end caps that could be directly soldered to the surface of the PCB. Components became much smaller and component placement on both sides of the board became far more common with surface-mounting than through-hole mounting, allowing much higher circuit densities. Often, only the solder joints hold the parts to the board, although parts on the bottom or "second" side of the board are temporarily secured with a dot of adhesive as well. Surface-mounted devices (SMDs) are usually made physically small and lightweight for this reason. Surface mounting lends itself well to a high degree of automation, reducing labor cost and greatly increasing production rates. SMDs can be one-quarter to one-tenth the size and weight, and one-half to one-quarter the cost of through-hole parts.

Solder paste, a sticky mixture of flux and tiny solder particles, is first applied to all the solder pads with a stainless steel stencil. If components are to be mounted on the second side, a numerically controlled (NC) machine places small liquid adhesive dots at the locations of all second-side components, large and small. The boards then proceed to the pick-and-place machines, where they are placed on a conveyor belt. Small SMDs are usually delivered to the production line on paper or plastic tapes wound on reels. Integrated circuits are typically delivered stacked in static-free plastic tubes or trays. NC pick-and-place machines remove the parts from the reels or tubes and place them on the PCB. Second-side components are placed first, and the adhesive dots are quickly cured with application of low heat or ultraviolet radiation. The boards are flipped over and first-side components are placed by additional NC machines.

The boards are then conveyed into the reflow soldering oven. They first enter a pre-heat zone, where the temperature of the board and all the components is gradually, uniformly raised. This helps minimize thermal stresses when the assemblies cool down after soldering. The boards then enter a zone where the temperature is high enough to melt the solder particles in the solder paste, bonding the component leads to the pads on the circuit board. The surface tension of the molten solder helps keep the components in place, and if the solder pad geometries are correctly designed, automatically aligns the components on their pads. There are a number of techniques for reflowing solder. One is to use infrared lamps; this is called infrared reflow. Another is to use a hot gas. At one time special fluorocarbon liquids with high boiling points were used, a method called vapor phase reflow. Due to environmental concerns, this method is falling out of favor. Today, it is more common to use nitrogen gas or nitrogen gas enriched air in a convection oven. Each method has its advantages and disadvantages. With infrared reflow, the board designer must lay the board out so that short components don't fall into the shadows of tall components. Component location is less restricted if the designer knows that vapor phase reflow or convection soldering will be used in production.

Following reflow soldering, certain irregular or heat-sensitive components may be installed and soldered by hand, or in large scale automation, by focused infrared beam (FIB) equipment.

After soldering, the boards are washed to remove flux residue and any stray solder balls that could short out closely spaced component leads. Rosin flux is removed with fluorocarbon solvents, high flash point hydrocarbon solvents, or limonene, derived from orange peels. Water soluble fluxes are removed with deionized water and detergent, followed by an air blast to quickly remove residual water. Where aesthetics are unimportant, non-corrosive flux residues are sometimes left on the boards, saving the cost of this processing step and eliminating a waste disposal issue.

Finally, the boards are visually inspected for missing or misaligned components and solder bridging. If needed, they are sent to a rework station where a human operator corrects any errors. They are then sent to the testing stations to verify that they work correctly.

Substrate

Low-end consumer grade PCB substrates frequently are made of paper impregnated with phenolic resin. They carry designations such as XXXP, XXXPC, and FR-2. The material is inexpensive, easy to machine by drilling, shearing and cold punching, and causes less tool wear than glass fiber reinforced substrates. The letters "FR" in the designation indicate Flame Resistance.

High-end consumer and industrial circuit board substrates are typically made of a material designated FR-4. This consists of a woven fiberglass mat impregnated with a flame resistant epoxy resin. It can be drilled, punched and sheared, but due to its abrasive glass content requires tools made of tungsten carbide for high volume production. Due to the fiberglass reinforcement, it exibits about five times higher flexural strength and resistance to cracking than paper-phenolic types, albeit at higher cost.

PCBs for high power radio frequency (RF) work use plastics with low dielectric constant (permittivity) and dissipation factor, such as Rogers® 4000, Rogers® Duroid, DuPont® Teflon® (types GT and GX), polyimide, polystyrene and cross-linked polystyrene. They typically have poorer mechanical properties, but this is considered an acceptable engineering tradeoff in view of their superior electrical performance.

PCBs designed for use in vacuum or in zero gravity, as in spacecraft, being unable to rely on convection cooling, often have thick copper or aluminum cores to dissipate heat from electrical components.

Not all circuit boards use rigid core materials. Some are designed to be completely flexible or partially flexible, using DuPont's® Kapton® polyimide film, and others. This class of boards, sometimes called flex circuits or rigid-flex circuits, respectively, are difficult to create but have many applications. Sometimes they are flexible to save space (PCBs inside cameras and hearing aids are almost always made of flex circuits so they can be folded up to fit into the limited available space). Sometimes, the flexible part of the circuit board is actually being used as a cable or moving connection to another board or device. A perfect example of the latter application is the cable connected to the carriage in an inkjet printer

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 certain digital circuits of greater complexity.

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.