PCB terms / Glossary
1U - Electronics and computing devices come in rack-mounted packages. This includes servers, test instruments, telecommunications components, tape drives and audio and video equipment. Units are bolted to the side frames. The height of a rack-mounted device is specifi ed in a unit (U) measure or rack unit (RU). IU (or 1RU) measures 1.75” from top to bottom
2U - The height of two rack mounted devices. See 1U above
20H RULE - A rule of thumb that states that the Vcc plane in a PCB should be recessed in from the ground plane by 20 times the separation between the planes in order to minimize EMI. This rule has no basis in science and has been shown to be false
ADDITIVE PROCESS - A method of producing printed circuit boards that begins with bare insulating materials onto which copper is deposited by plating up. Due to the fact that there is no metal foil onto which to electroplate the copper, this is usually an electroless process.
ADMITTANCE - The reciprocal of impedance.
AMPERE (AMP) - A unit of electrical current flow equivalent to the motion of one coulomb of charge or 6.24 X 1018 electrons passing any cross section in one second.
ANALOG - A term that describes a signal that can have any wave shape and any voltage value. Examples are audio signals and control signals for controlling equipment such as rudders on airplanes. There are usually small changes in value in analog signals.
ANALOG GROUND - That point in an analog circuit that serves as the reference point from which all analog voltage measurements are made. This “ground” should always be connected to the “digital ground” directly under a component that contains both analog and digital functions.
ANSI - American National Standards Institute, (www.ansi.org) an organization based in Washington DC that maintains a variety of industry standards.
ANNULAR RING - The ring of copper formed by a pad used to connect a trace to a plated through hole that surrounds that hole after drilling. See the paper “Anatomy of a Drilled Hole” at the end of this book.
ANTIPAD - This term refers to the opening in a copper plane of a PCB. This opening is placed in a plane to allow a signal or component pin to pass through the plane without shorting to it. The term “antipad” derives from the original method used by PCB design systems to create the artwork of plane layers. At one time, the photoplotters used to plot plane layers were only capable of flashing pads and painting lines. As a result, painting a plane layer as a positive piece of artwork was not possible. Therefore, the plane layer was plotted as a negative with the holes flashed as “antipads”. This resulted in plane layers that were negative images. Once the plane layer was plotted, it was photographically turned into a positive. Another name for antipad is “clearance pad.”
AOI - (Automated Optical Inspection) - Automatic laser/video inspection of traces and pads on the surface of inner-layer cores or outer-layer panels. The machine uses cam data to verify copper feature positioning, size and shape. Instrumental in locating “open” traces, missing features or “shorts”.
AQL - (Acceptance Quality Level) - The maximum number of defectives likely to existwithin a population (lot) that can be considered to be contractually tolerable; normally associated with statistically derived sampling plans.
Array - A group of elements or circuits arranged in rows and columns on a base material.
Artwork - An accurately scaled configuration of electronic data used to produce the artwork master or production master.
Artwork Master - The photographic image of the PCB pattern on film used to produce the circuit board, usually on a 1:1 scale.
APPE - Allylated Polyphenylene Ether, is a resin system formulation produced by Nelco, that is used to manufacture high performance PCBs. The main benefit of this material is a lower loss tangent.
ARTWORK - The plotted film that represents the copper patterns to be etched into each layer of a PCB. This artwork also includes the silk screen or legend and the solder mask. It can also be plotted on paper to provide a “hardcopy” for review and record keeping.
ASIC - Application Specifi c Integrated Circuit, also referred to as custom silicon, is an integrated circuit custom designed to perform a specifi c logic function. Often called custom silicon. As ASICs are custom made for individual companies, they are typically not available on the open market.
ASPECT RATIO - The ratio of a drilled hole’s length to its diameter. The higher the aspect ratio, the more diffi cult it is to uniformly plate copper in a hole. Aspect ratios greater than 6:1 are not considered candidates for volume production.
ASSP - Application Specific Standard Product, a standard product that is designed to perform a complete function. A microprocessor or serdes (serializer-deserializer) is an example of such a component.
ATE - Automatic Test Equipment, ATE are computer-driven testers adapted to test specific products. The most common form of this product is a PCB tester or an IC tester.