PCB glossary

An annular ring is the copper surrounding a drilled hole on a PCB pad, measured from the edge of the finished hole to the outer edge of the pad. IPC-6012 sets minimum annular ring sizes: Class 3 requires at least 2 mil (0.05 mm) on external layers and 1 mil (0.025 mm) on internal layers.

AOI (Automated Optical Inspection) uses high-resolution cameras to inspect solder joints, component placement, and orientation after assembly. Modern 3D AOI measures joint height and volume volumetrically, catching defects like lifted leads and insufficient solder that 2D systems miss. Limited to visible surfaces — hidden joints under BGAs require X-ray inspection.

An authorised distributor (also called franchised distributor) has a contractual agreement with the original component manufacturer to stock and sell their parts. Examples include Digi-Key, Mouser, Farnell, RS Components, and Arrow. Components sourced from authorised distributors come with full traceability and no counterfeit risk — unlike brokers or independent distributors.

BGA (Ball Grid Array) and QFN (Quad Flat No-leads) are surface-mount IC packages with hidden connections. BGAs use an array of solder balls underneath the package (typical pitches 0.4-1.0 mm); QFNs use pads on the bottom edges plus a central thermal pad (pitches 0.4-0.8 mm). Both require X-ray inspection to verify hidden solder joints.

A BOM (Bill of Materials) is the structured list of every component needed to build a PCB, mapping reference designators on the board to specific manufacturer parts. It is one of the three core files — alongside Gerber and pick and place — required to quote and assemble a PCBA.

Castellated edges are plated half-holes on the edge of a PCB used for board-to-board mounting, allowing one PCB (typically a module) to be soldered directly onto a larger carrier board. Common on Wi-Fi, Bluetooth, and microcontroller modules. They require specific design rules and add cost.

Conformal coating is a thin polymeric film (typically 25-75 µm) applied to an assembled PCB to protect components from moisture, dust, chemicals, and contamination. Five main types: acrylic (AR), urethane (UR), silicone (SR), epoxy (ER), and parylene (XY) — each with different protection profiles and cost. Common in automotive, medical, marine, and outdoor electronics.

Copper weight is the thickness of copper foil on each PCB layer, specified in µm or ounces per square foot. Common weights are 18 µm (0.5 oz), 35 µm (1 oz), and 70 µm (2 oz). Heavier copper carries more current and dissipates more heat but limits minimum trace width.

A datasheet is the manufacturer's technical document for an electronic component, specifying electrical characteristics, mechanical dimensions, recommended operating conditions, package details, and compliance information (RoHS, REACH, automotive grade, etc.). Always verify component selection against the datasheet — not against distributor summaries.

DFM (Design for Manufacturability) is a structured review of your PCB design against a fabricator's process limits — trace widths, drill sizes, annular rings, solder mask clearances, and stack-up choices — to catch issues before tooling starts. A good DFM review prevents rework, scrap, and missed delivery dates.

ENIG and HASL are two of the most common PCB surface finishes. HASL (Hot Air Solder Levelling) is cheaper and well-suited to through-hole work; ENIG (Electroless Nickel Immersion Gold) is flatter, finer-pitch capable, and lead-free by default — at a higher cost.

An EOL (End of Life) component is one the manufacturer has announced will no longer be produced. Manufacturers issue Product Change Notices (PCNs) with key dates: Last Time Buy (LTB) and Last Time Ship (LTS). Orders placed in this window are typically Non-Cancellable, Non-Returnable (NCNR). Designs using EOL parts need re-spinning or a last-time buy of lifetime stock.

Fiducial marks are small copper reference dots on a PCB (typically 1 mm diameter) used by automated assembly machines to align the board precisely before placing components. Three fiducials per side enable rotation and skew correction. Their absence or incorrect placement can prevent SMT assembly entirely.

FR-4 is a flame-retardant woven glass-reinforced epoxy laminate. It is the default base material for most rigid PCBs, balancing mechanical strength, electrical insulation, thermal performance, and cost. Variants exist for high-Tg, halogen-free, and high-speed applications.

Functional testing powers up the assembled PCB and verifies that it performs its intended function, typically using customer-supplied test scripts or fixtures. Unlike ICT, it tests behaviour rather than individual components. Often combined with programming (loading firmware) and calibration as a final assembly step.

A Gerber file is the industry-standard format that describes each layer of a PCB — copper, solder mask, silkscreen, and drill data — as 2D vector geometry. A complete Gerber package is the minimum a fabricator needs to quote and build your board.

HDI (High Density Interconnect) describes PCBs with significantly higher routing density than conventional boards, achieved through microvias, thinner traces, and sequential lamination. HDI is essential for high pin-count BGAs, compact electronics, and applications in smartphones, medical devices, automotive ECUs, and aerospace systems.

ICT (In-Circuit Testing) verifies the electrical integrity of an assembled PCB by contacting test points with a bed-of-nails fixture and measuring individual components and connections in-circuit. It catches assembly defects (shorts, opens, wrong values, missing parts) but does not verify functional operation. Best for high-volume production where the fixture cost can be amortised.

Immersion silver is a thin silver coating (typically 0.1-0.4 µm) chemically deposited on exposed copper pads as a flat, lead-free, RoHS-compliant surface finish. It supports fine-pitch components and multiple reflow cycles but is prone to tarnishing if mishandled and requires careful storage.

Impedance control is the practice of specifying and manufacturing PCB traces so their characteristic impedance meets a precise target — typically 50 Ω single-ended or 100 Ω differential. Required for high-speed digital, RF, and signal integrity-critical designs, it depends on trace width, dielectric thickness, copper weight, and material properties.

IPC class 2 and class 3 are two acceptance levels defined in IPC standards (A-600 for bare boards, A-610 for assemblies). Class 2 covers dedicated-service electronics; class 3 covers high-reliability electronics where failure cannot be tolerated. Class 3 demands tighter tolerances, stricter inspection, and typically costs more.

IPC-A-600 is the IPC standard that defines visual acceptance criteria for bare printed circuit boards. It specifies what surface, dimensional, and structural conditions are acceptable across three class levels, and is the reference document fabricators use to inspect boards before shipment.

IPC-A-610 is the most widely used acceptance standard for assembled PCBs, defining visual criteria for solder joints, component placement, cleanliness, and damage across three class levels. The current revision is IPC-A-610J (March 2024). It is the companion standard to IPC-A-600 for bare boards.

Lead time is the elapsed time from order placement to delivery. For PCBs and PCBA, total lead time includes bare board fabrication (typically 5-15 days), component sourcing (instant to many weeks depending on availability), assembly (3-10 days), and shipping. The longest single component lead time usually drives the overall project lead time.

A microvia is a small laser-drilled blind via, typically 150 µm or less in diameter (per IPC-T-50M definition), used to connect adjacent layers in HDI boards. Microvias enable routing for high pin-count BGAs and densely packed components that conventional mechanical vias cannot support.

An MPN (Manufacturer Part Number) is the unique identifier assigned by a component manufacturer to a specific part. It is the only reliable way to specify exactly which component to source — generic descriptions like "10k resistor" are ambiguous. Every line in a BOM should include the MPN alongside the manufacturer name.

A multilayer PCB has more than two copper layers separated by dielectric material and bonded together under heat and pressure. Common configurations are 4, 6, 8, 10, and 12 layers. Multilayer designs enable higher routing density, dedicated power and ground planes, and better signal integrity for complex circuits.

NRE (Non-Recurring Engineering) cost is the one-time charge for setup work that does not repeat per unit — tooling, stencils, programming, jigs, test fixtures, first-article inspection, and engineering review. NRE is paid once per design revision and is amortised across the production volume, making per-unit cost lower at higher quantities.

OSP (Organic Solderability Preservative) is a water-based organic coating applied directly to exposed copper pads as a low-cost surface finish. It is flat, RoHS-compliant, and ideal for fine-pitch components, but has a short shelf life of 3-6 months and is sensitive to handling.

Panelisation is the practice of arranging multiple PCBs in a single manufacturing panel to reduce fabrication cost and enable automated SMT assembly. The two main methods are V-cut (straight-line scoring for rectangular boards) and tab-routing with mouse bites (perforated tabs for irregular shapes).

PCBA (Printed Circuit Board Assembly) is the process of populating a bare PCB with electronic components — resistors, capacitors, ICs, connectors, and other parts — to produce a functional electronic assembly ready for testing or integration into a finished product.

A pick and place file (also called centroid file, CPL, or XY data) is a machine-readable list of every component's position, rotation, and board side. It is one of the three core files — alongside Gerber and BOM — needed to quote and run PCB assembly.

Plating refers to the electroplating or electroless deposition of metal onto a PCB, primarily copper plating inside drilled holes to make them conductive, and final surface finish plating (ENIG, immersion silver, etc.) on exposed pads. Plating thickness affects current capacity, reliability, and IPC class compliance.

REACH is the EU regulation (EC No 1907/2006) governing chemicals placed on the European market. For electronics, it requires suppliers to notify customers if any Substance of Very High Concern (SVHC) is present above 0.1% by weight. The SVHC Candidate List currently contains 253 entries (February 2026) and is updated regularly by ECHA.

Reflow soldering is the process of melting solder paste in a controlled oven to form joints between SMT components and the PCB. The oven follows a temperature profile with preheat, soak, reflow (peak typically 245-260°C for lead-free), and cooling stages. It is the dominant soldering method for modern SMT assembly.

RoHS compliance means the PCB and all its components meet the EU's Restriction of Hazardous Substances Directive (2011/65/EU), which limits 10 specified substances including lead, mercury, cadmium, and four phthalates. RoHS compliance is mandatory for electronic equipment placed on the EU market.

Selective soldering is an automated through-hole soldering process that solders specific joints one at a time using a precision nozzle of molten solder. Unlike wave soldering, it requires no fixtures and only applies flux to targeted joints. Ideal for mixed-technology boards with SMT on both sides and few THT components.

Silkscreen (also called legend) is the printed text, symbols, and outlines on a PCB used to identify components, connectors, and orientation marks. Typically white or black epoxy ink, it sits on top of the solder mask. Minimum text height is usually around 0.8 mm (32 mil) for legibility.

SMT (Surface Mount Technology) is the dominant PCB assembly method where components are placed directly onto the surface of the board and soldered using reflow ovens. It enables smaller, denser, double-sided boards and supports nearly all modern component packages including BGAs, QFNs, and 0402 passives.

Solder mask is the protective polymer coating applied to a PCB to insulate exposed copper traces, prevent solder bridges during assembly, and protect against oxidation and contamination. It is what gives most boards their characteristic green colour.

Standard solder mask is green liquid photo-imageable (LPI) lacquer, but black, white, blue, red, yellow, and matte variants are widely available. Colour choice is cosmetic and does not affect electrical performance; matte and dark colours can affect optical inspection contrast during assembly.

A PCB stack-up is the cross-sectional arrangement of copper layers, dielectric materials, and bonding films that make up a multilayer board. It defines layer thickness, copper weight, dielectric properties, and is critical for impedance control, signal integrity, and manufacturability.

A stencil is a thin laser-cut steel sheet with apertures matching the SMT pads on the PCB, used to apply solder paste precisely before component placement. Typical thickness is 100-150 µm. Stencil aperture design affects solder paste volume and is critical for fine-pitch BGAs and QFNs to prevent insufficient solder or bridging.

A surface finish is the protective coating applied to the exposed copper pads on a PCB to prevent oxidation and provide a solderable surface for component attachment. The choice affects cost, assembly compatibility, shelf life, and reliability.

THT (Through-Hole Technology) is an assembly method where component leads are inserted through plated holes in the PCB and soldered from the opposite side, typically using wave or selective soldering. Used for connectors, large capacitors, transformers, and parts requiring mechanical strength or high current capacity.

Trace width is the lateral dimension of a copper conductor on a PCB. It determines current-carrying capacity, signal integrity, and impedance. Standard fabricators support down to 100 µm (4 mil) on outer layers with 35 µm copper; advanced and HDI processes support narrower traces.

Turnkey assembly is a full-service PCBA model where the supplier handles everything: bare board fabrication, component sourcing, assembly, inspection, and delivery. The customer supplies design files and receives finished, tested boards — a single commercial counterparty for the whole build.

A via is a plated hole that connects two or more copper layers in a PCB. Common types include through-hole (passes through the entire board), blind (connects an outer layer to an inner layer), buried (connects only inner layers), and microvia (small laser-drilled via used in HDI designs).

Wave soldering is a bulk through-hole soldering process where the PCB passes over a wave of molten solder, soldering all exposed through-hole joints in a single pass. It requires custom fixtures to mask off SMT-only areas. Faster than selective soldering for boards with many through-hole components, but less flexible.

X-ray inspection is a non-destructive testing method that penetrates the PCB to reveal hidden solder joints under BGAs, QFNs, and other bottom-terminated packages. It detects voids, head-in-pillow defects, bridging, and missing joints that AOI cannot see. Available in 2D (top-down) and 3D CT (full volumetric) variants.