Access Hole – A series of holes in successive layers of a multilayer board, each set having their centers on the same axis. These holes provide access to the surface of the land on one of the layers of the board.

Additive Process – A process for obtaining conductive patterns by the selective deposition of conductive material on clad or unclad base material.

Annular Ring – The ring of exposed solder or copper around a through hole.

Buried Via – A plated through hole buried within internal layers of a circuit. There is no direct access to the via.

Blind Via – A plated interconnection from one layer to an adjacent layer through a fixed depth LASER drilled opening.

Cantilevered Leads – Unsupported conductors extending from an edge of a flex circuit.

Circuit – A number of electrical elements and devices that have been interconnected to perform a desired electrical function.

Coverlayer – Insulating layer usually bonded with adhesive.

Dielectric – A material with a high resistance to the flow of direct current, and which is capable of being polarized by an electrical field.

Flexible Printed Circuit – A patterned arrangement of printed circuitry and components that utilizes flexible base material with or without flexible coverlay.

FR-4 – Fiberglass reinforced epoxy laminate used as the substrate layer in a rigid flex circuit.

Land – A portion of a conductive pattern usually used for the connection and/or attachment of components.

LCP – Liquid Crystalline Polymer, a relatively new dielectric substrate used in the manufacture of flex circuits.

Micron – A linear dimension equal to 1 x 10-6 meters or 39.4 x 10-6 inches.

Photo Etching – The chemical, or chemical and electrolytic, removal of unwanted portions of conductive or resistive material.

Phototool – A phototool is a physical film which contains the pattern that is used to produce a circuitry image on a photo-sensitive material by way of exposure to light-energy such as UV light.

Plugged Via – When the solder mask on the rigid portion of the circuit covers the drilled via opening from one side of the board only. Optimal design is to pull the solder mask back from the via, however if coverage is required plugging should be considered as the next option before tented.

Polyimide – The synthetic polymer that has more than two imide radicals in the main chain.

Pre-preg – Fiberglass reinforced epoxy adhesive used to bond layers of the flex and FR-4 together in a rigid flex.

PTH – Plated through hole. Used as a means of creating an electrical connection from one circuit layer to another.

Rigid Flex – A circuit with one or more printed circuit boards attached by an integral flexible circuit layer.

SMT – Surface Mount Technology

Solder Mask – Photo-imageable permanent mask used for insulating rigid sections.

Steel Rule Die – A tool used to cut flex circuits (and other materials) from a panel.

Stiffener – A rigid or semi-rigid material that is bonded to a flex to facilitate component attachment. Typically made of polyimide or epoxy glass.

Tented Via – When the soldermask on the rigid portion of the circuit spans the drilled via opening. This is not a desirable feature. Solder mask on the rigid portion of the board should be pulled back from the vias or Plugged (see definition above).

Via – A plated-through hole that is used as an interlayer connection, but in which there is no intention to insert a component lead or other reinforcing material.
Via Fill – Plated through hole in rigid flex which is filled with conductive or non-conductive epoxy and plated over so the resulting pad is flat and supports SMT component assembly.

Windowed Leads – Conductors that are unsupported by insulation. Typically running across a window, the pitch can be quite tight allowing high density mass termination.

ZIF (Zero Insertion Force) Termination – A style of termination that allows a flex circuit tail or tab to be inserted into a circuit board mounted connector. After insertion a mechanical actuator locks the flex in place.