PCB Manufacturing Glossary: Terms Engineers Must Know

Why the Vocabulary Matters

A printed circuit board passes through a dozen hands before it is finished, and each of those hands uses a slightly different word for the same object. A designer calls it a footprint; a fabricator calls it a land pattern. A buyer asks for one ounce copper; the plant quotes 35 micrometres. An engineer asks for controlled impedance; the fabricator wants to know which layer and which reference plane. The misunderstandings that follow are expensive, because they are usually discovered after the panels have been etched.

This glossary collects the terms that appear most often in a specification, a quote or a quality report, grouped by the stage at which they matter.

Base Materials

• Substrate: the insulating base material that carries the copper and gives the board its mechanical and electrical properties. FR-4, polyimide and ceramic are common families.
• Copper clad laminate (CCL): a substrate panel already faced with copper foil on one or both sides, ready to be patterned. This is what a fabricator buys and cuts.
• FR-4: a woven glass fabric in an epoxy resin, flame retardant, the default laminate for rigid boards.
• Prepreg: a partially cured resin sheet that flows and bonds under heat and pressure. It is the adhesive between the cores in a multilayer stack.
• Core: a fully cured laminate sheet with copper on both sides, used as a rigid element in the stack.
• Copper weight: the thickness of the copper foil expressed in ounces per square foot. One ounce is about 35 micrometres.
• Tg (glass transition temperature): the temperature at which the resin changes from a rigid to a rubbery state. A higher value means more thermal margin through assembly.

Circuit Features

• Trace: a copper conductor that carries a signal or power between two points.
• Plane: a large unbroken copper area connected to a supply or to ground, used for return currents, shielding and power distribution.
• Pad or land: the copper area to which a component lead or a solder ball is attached.
• Via: a plated hole that connects conductors on different layers. A through via passes through the whole board; a blind via reaches an outer layer only; a buried via connects inner layers and is invisible from outside.
• Annular ring: the copper ring around a drilled hole. Its minimum width is a standard fabrication check, because too little copper at the drill breakout produces a cracked connection.
• Solder mask: the protective coating printed over the copper, with openings at the pads. It prevents oxidation and stops solder bridging between adjacent joints.
• Silkscreen or legend: the ink markings that identify components and orientation.
• Surface finish: the metallic or organic coating on the exposed copper that keeps it solderable. Hot air levelling, electroless nickel immersion gold and organic solderability preservatives are the common types.
• Panel: the manufacturing panel that contains several copies of the board, plus tooling strips and borders.

Design and Data Transfer

• Gerber: the standard data format that describes each layer of the board as a set of apertures and flashes. A Gerber package alone is not a complete fabrication drawing.
• Drill file: the data that defines every hole, its diameter and whether it is plated. It is generated with the Gerber set and must match it.
• Netlist: the list of electrical connections the board must implement, used to verify that the artwork matches the schematic.
• DFM (design for manufacturability): the review that checks a design against the plant capability before it is released. It catches the trace widths, clearances and annular rings that cannot be produced.
• DRC (design rule check): the automated check inside the layout tool that compares the layout against the rules the designer set.
• Impedance control: designing a trace so that its characteristic impedance is a specified value, achieved by fixing its width and its distance to a reference plane for a given dielectric constant.
• Stack-up: the ordered list of copper, core and prepreg layers that make up the board, with their thicknesses.
• HDI (high density interconnect): a board built with fine lines, small laser drilled microvias and sequential lamination to achieve a higher wiring density.

Our notes on PCB design and layout cover how these items are used during design, and our notes on PCB capabilities set out the limits a design has to respect.

Fabrication Processes

• Etching: removing unwanted copper with a chemical solution to leave the conductor pattern. The etchant also attacks the sides of the trace, which is why the finished trace is narrower than the artwork.
• Plating: depositing copper in a hole or on a trace, either electroless, which uses a chemical reaction, or electrolytic, which uses a current.
• Lamination: bonding the layers under heat and pressure with prepreg between them.
• Registration: the alignment of each layer to the others, measured as the offset between the layers in the finished board.
• Aspect ratio: the ratio of the board thickness to the drill diameter. A high aspect ratio is harder to plate uniformly.
• Back drilling: removing the unused stub of a plated through hole after lamination to improve the performance of a high speed signal.
• Panelisation: arranging several boards on one panel with tooling holes, fiducials and a routable border.

Assembly Terms

• SMT (surface mount technology): placing components directly onto the surface of the board and reflowing them in place.
• THT (through hole technology): inserting component leads through drilled holes and soldering them, used where mechanical strength is required.
• Solder paste: a mixture of solder powder and flux that is printed onto the pads and melts during reflow.
• Stencil: the stainless steel sheet with laser cut apertures that controls the volume of paste printed on each pad.
• Reflow soldering: heating the board through a controlled profile until the paste melts and forms the joint.
• Wave soldering: passing the board over a standing wave of molten solder to solder the through hole leads in one pass.
• BGA (ball grid array): a package whose connections are an array of solder balls on its underside, leaving the joints hidden beneath the body.
• Conformal coating: a thin protective film applied over the assembled board to resist moisture and contamination.
• Box build: assembling the finished product around the completed board, including the enclosure, wiring and software loading.

The assembly sequence and its controls are covered in our notes on PCB manufacturing.

Test and Quality

• AOI (automated optical inspection): a camera system that compares each assembled board with a reference and reports missing, misaligned or badly soldered components.
• X-ray inspection: imaging that shows hidden joints, essential for ball grid array and other area array packages.
• ICT (in circuit test): a fixture based test that measures components and nets on the assembled board.
• Flying probe: a fixtureless test that moves probes from point to point, used for prototypes and dense boards.
• FCT (functional circuit test): powering the board and exercising its real interfaces.
• Electrical test or e-test: the bare board test for opens and shorts, performed on every board or on a sample depending on the class.
• Coupon: a test structure built on the panel alongside the boards, used to measure impedance, plating thickness or solderability on the production panel.
• IPC standards: the industry specifications that define acceptability for design, fabrication and assembly, such as IPC-2221 for design, IPC-6012 for rigid boards and IPC-A-610 for assembled boards.
• First article inspection: the full dimensional and electrical check of the first board produced from a new design or a new process.
• Yield: the proportion of boards that pass the final test. It is the single number that shows whether a process is under control.

Our notes on PCBA testing explain how these methods are combined, and our notes on quality management describe the system that keeps the numbers stable.

Using the Vocabulary

Three habits prevent most of the confusion. Quote dimensions in units that cannot be misread, so state the copper as a weight and the thickness in micrometres, and give trace widths in millimetres or in thousandths of an inch but never in both. Name the layer in every impedance requirement, because an impedance without a reference plane is meaningless. And write the acceptance criteria into the purchase order by reference to a standard, since an ambiguous phrase such as good quality is not something a supplier can measure.

FAQ

What is the difference between a via and a through hole? A via connects conductors on different layers and is normally filled or tented; a through hole accepts a component lead and is soldered.

What does 1oz copper mean? One ounce of copper per square foot of area, which corresponds to a foil thickness of about 35 micrometres.

Why is impedance control important? Because the geometry of a trace sets its characteristic impedance, and a mismatch causes reflections that corrupt a high speed signal.

What are IPC standards for? They define what is acceptable in design, fabrication and assembly, so that a specification can be enforced with a number rather than an opinion.

Conclusion

A shared vocabulary is the cheapest quality tool available. Learn the terms for the material, the features, the processes and the tests, use them precisely in the specification and the purchase order, and most of the arguments that delay a board never happen. The rest of this site covers each of these areas in more depth.