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 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, including placement of bypass capacitors, used for filtering power supply noise and usually placed near the integrated circuits 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.
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.