PCB Terminology: A Working Glossary for Engineers and Buyers

PCB terminology is a shared language between two groups that rarely sit in the same room: the engineers who define what a board must do and the buyers and suppliers who decide whether it can be made. When a term is used loosely, the cost appears as a concession, a rework or an order that has to be repeated.

This glossary covers the terms that cause the most confusion in practice, with the meaning that matters at the point where a decision is being made rather than a dictionary definition.

Copper and Layer Terms

An annular ring is the copper surrounding a drilled hole, measured from the edge of the hole to the edge of the pad. Its minimum width is set by drill tolerance plus registration, and too small a ring produces a breakout that fails thermal cycling.

An aspect ratio is the ratio of hole depth to hole diameter. A 1.6 mm board with a 0.3 mm hole has an aspect ratio of about 5:1, which is comfortable for plating. Above roughly 10:1, plating uniformity becomes difficult and suppliers publish their limit.

Copper weight is expressed in ounces per square foot, where one ounce corresponds to roughly 35 microns of thickness. The term appears in every stackup table, and it matters because current capacity and impedance both depend on it.

Fabrication and Tolerance Terms

Registration is the alignment between layers after lamination. It is quoted as a radial tolerance, because the shift can occur in any direction, and it is the reason annular rings and solder mask dams are sized with margin.

The etch factor describes the lateral removal of copper during etching, which leaves a trace with a trapezoidal cross section narrower at the top than at the base. It explains why a nominal trace width is really a band.

Surface finish refers to the coating over exposed copper that keeps it solderable. Common finishes include hot air solder levelling, electroless nickel immersion gold, immersion silver and hard gold, and each has a different shelf life and cost.

Design and Verification Terms

A design rule check is the automated comparison of a layout against a set of constraints: minimum trace width, minimum clearance, annular ring, mask dam and hole size. A clean check means the design is consistent with the rules that were loaded, not that it is correct.

Impedance control means that specific nets are built to a defined characteristic impedance with a stated tolerance, verified by coupons on the production panel. The term applies to a net list, not to a whole board.

A stackup is the ordered description of layers, materials, dielectric thicknesses and copper weights. Everything about impedance, thickness and lamination is derived from it, which is why it belongs on the fabrication drawing.

Assembly Terms

A stencil is the metal foil that defines where solder paste lands on the pads. Its thickness and aperture design determine paste volume, and therefore the yield of everything that follows.

The bill of materials lists components with reference designators, manufacturer part numbers and quantities. A bill of materials without manufacturer part numbers is not a finished document, because it does not define what may be purchased.

First article inspection is the detailed verification of the first assembled unit against the assembly drawing and the bill of materials. It is the checkpoint that catches a wrong part or a rotated component before the rest of the batch is built.

Test and Board-Level Terms

In-circuit test verifies component values and connections by probing the assembled board with a fixture. Flying probe test performs the same measurements without a fixture, which is slower per board but avoids tooling cost on small quantities.

A coupon is a test structure built on the production panel alongside the boards. It allows measurements such as impedance, copper thickness and plating quality to be taken without destroying a finished board.

Panelization is the arrangement of individual boards on a production panel, including the rails, tooling holes and fiducials that the assembly line needs. Poor panelization raises cost even when the board itself is well designed.

Terms Buyers Should Ask About

Lead time is the interval from order to shipment, but it should be qualified: is it working days, does it include material procurement, and does it cover the surface finish and any special process the design requires.

Minimum order quantity, tooling charges and expedite fees are commercial terms, but they are driven by technical facts. Tooling charges reflect the stencil and any fixture, while expedite fees reflect queue position.

Quality documentation is the last term worth clarifying. A certificate of compliance states that the supplier followed the specified process; a test report shows measured results. Buyers should know which one the order includes before placing it.

Terms That Are Frequently Confused

Design rule check and design review are not the same thing. The check compares geometry against numerical constraints; the review is a human examination of whether the design matches its intent. A board can pass every rule and still be wrong.

Panel and board are also used interchangeably, which causes errors in assembly instructions. The panel is the production format that carries several boards plus rails and tooling features; the board is the finished unit that is routed out of it.

Trace and track mean the same thing in different regions of the industry. Consistency within a document matters more than which term is chosen, because a drawing that uses both invites a question at exactly the wrong moment.

Terms That Define Cost

Layer count, board area, copper weight, minimum trace and gap, and surface finish are the technical variables that determine fabrication price. Where a design sits relative to the supplier capability determines whether it is routine work or a special process.

On the assembly side, the cost drivers are placement points, unique part numbers, panel utilization and inspection requirements. Knowing which term the quotation is responding to is the fastest way to understand why two prices differ.

Tooling appears in both lists. A stencil belongs to assembly, while a test fixture serves both fabrication test and in-circuit test, which is why the same word can appear twice in one quotation with different amounts.

Using the Vocabulary Well

The purpose of consistent PCB terminology is to remove assumptions. When an engineer writes impedance control on a drawing, the supplier should be able to reply with a stackup, a geometry and a tolerance rather than a question.

The same applies inside a company. When design and procurement agree on what a first article inspection covers, or what a coupon report contains, fewer orders stop halfway through for clarification.

Related reading: PCB design quality characteristics, PCB circuit board explained, and multilayer prototype requirements.

FAQ

Is copper weight the same as finished thickness? No. The nominal weight describes the base foil, and plating adds copper to the outer layers during processing. Finished thickness should be measured rather than assumed from the foil specification.

What does a clean design rule check prove? That the layout is consistent with the rules that were loaded into the tool. It does not prove the schematic is correct or that the stackup matches the fabrication drawing, both of which are separate checks.

Which term causes the most disputes with suppliers? Impedance control, usually because the target is stated without a stackup or a net list. Providing both removes almost all of the ambiguity.