The width and spacing of conductors (or "traces") on a PCB are 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.

 

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