3D STEP Model: High-fidelity, standardized files (ISO 10303) that represent the exact physical geometry of electronic components. Unlike basic 2D outlines, these "solid" models allow designers to visualize a board in three dimensions to verify mechanical fit and enclosure integration before manufacturing. 3D models can be used for Clearance & Collision Testing. They enable automated 3D Design Rule Checks (DRC) to detect if a tall capacitor will hit a lid or if a connector is properly aligned with a panel cutout. Footprint Expert autogenerates 3D STEP models for standard component families. Non-standard 3D STEP models are available for free download from the component manufacturer. STEP model names should be identical to the footprint name. The file extension is mostly .STEP but .STP is also used.

Antipad: The circular "no-copper" zone on a power or ground plane that surrounds a via or a plated through-hole. Without an antipad, a signal via passing through a ground plane would be permanently shorted to ground. Fabricators need this "moat" to be slightly larger than the drill size to account for small misalignments during drilling or layer registration. In high-speed designs such as PCIe or DDR4, the size of the antipad significantly affects the via's impedance. A larger antipad reduces parasitic capacitance, which helps maintain signal quality.

Assembly Outline: A specific mechanical layer drawing in a PCB footprint that maps the physical geometry and boundaries of a component to generate precise assembly documentation. Unlike the superficial lines found on the silkscreen layer, the assembly outline serves strictly as a back-end reference for manufacturing validation and is utilized to build Assembly Drawings rather than being printed directly onto the board surface. Assemblers cross-reference the outline to guarantee components are placed in the precise location, especially when silkscreen markings are hidden beneath a component body. Keeping the detailed component contours on a mechanical assembly layer prevents the exterior silkscreen layer from becoming cluttered or overlapping. It maps the maximum material boundaries (including tolerances) and explicitly highlights directional features, such as the location of Pin 1. The assembly outline is a closed polygon drawn to match the maximum physical dimensions of the component body. A distinct graphic indicator (such as a dot, bevel, chamfered corner or bar) mapping the location of Pin 1 or positive/negative terminals. The assembly Ref Des is located inside the assembly outline and stays there forever. Pads are normally not printed with the assembly outline, because the assembly reference designator inside the outline can cause clutter. The average line widths for the assembly outline range from 0.10 to 0.15 mm.

Component Outline: A closed polygon representing the nominal package outline. It has various uses depending on your footprint creation rules. It was originally intended to create an extruded 3D model without terminal leads.

Courtyard: A boundary layer used to define the minimum physical space a component needs on the board, including its body, pins, and a safety buffer. The courtyard is primarily for CAD design and DRC (Design Rule Check) to prevent components from being placed too close to each other. It ensures that pick-and-place machine nozzles have enough room to operate without hitting neighboring parts. It provides enough clearance for a technician to reach a component with a soldering iron or tweezers if it needs to be replaced. It prevents tall components from shadowing smaller ones, which can interfere with Automated Optical Inspection (AOI). The courtyard outline excess maps to the maximum material condition of the component package.

Discrete Component: A single, self-contained electronic component that performs a single, fundamental electrical function and cannot be divided into smaller individual parts. Unlike an Integrated Circuit (IC) – which combines thousands or millions of microscopic transistors, resistors, and diodes onto a single piece of silicon – a discrete component contains only one primary active or passive element inside its packaging. All discrete components are classified into one of two functional categories based on whether they require a power source to operate:

Footprint: The physical layout on a circuit board that matches an electronic component's pin configuration and dimensions. It acts as a digital template that tells manufacturers exactly where to place copper pads for soldering and where to print labels for identification. A footprint contains pads, solder and paste masks, drafting outlines for silkscreen, assembly, courtyard, component, polarity marking, origin marking and terminal leads. A calculator can be used to create standard package footprints. Non-standard packages like complex connectors must use the manufacturer recommended solder pattern because there are no industry standard rules for calculating patterns.

Keepout: A designated "no-go" zone embedded within a component's land pattern to prevent specific design elements like copper traces, vias, or other components from being placed in a sensitive area. Unlike a courtyard, which provides a general safety margin for assembly, a keepout is often used for electrical or mechanical safety to prevent shorts, interference, or physical obstructions. Used under connectors with metal housings (like SD card holders or USB ports) to ensure traces don't run directly under sharp metal edges that could pierce the solder mask. Placed under components like crystals or oscillators to keep electrically noisy signals from passing directly beneath sensitive pins. All non-plated holes have a minimum of 0.20 mm annular ring keepout to prevent all copper features from getting near the hole.

Land Pattern: The specific arrangement of conductive pads or through-holes on a printed circuit board (PCB) designed to physically attach and electrically connect a specific electronic component. Created using reference documents like a manufacturer's component datasheet, a properly sized land pattern ensures solid solder joint creation, prevents manufacturing defects, and allows for optimal heat and electrical performance.

Mounting Hole: A mechanical opening drilled through a printed circuit board to secure the board physically to a chassis, enclosure, or standoff using hardware like screws, bolts, or plastic snap-rivets. Beyond mechanical retention, mounting holes are critical interface points for establishing physical earth grounding and controlling electromagnetic compatibility (EMC).

Oscillator: a surface mount (SMD) or through-hole (THD) enclosure that houses a quartz crystal alongside an internal oscillation circuit (including logic gates and load capacitors) to output a clean, standalone clock signal. Unlike bare crystals, which require external capacitors and micro-controller circuitry to oscillate, an active oscillator package only needs a stable power supply and ground to immediately output a square or sine wave.

Pad Stack: The total collection of features associated with a single hole or connection point on a Printed Circuit Board (PCB). A complete through-hole pad stack typically includes the drill size, the finished hole size, capture pads on both inner and outer layers, clearances for solder mask, and power planes. Surface mount pad stacks contain a pad, solder and paste masks. Pad stacks ensure reliable electrical connections and mechanical stability for soldered components.

Package Tolerances: Component body tolerances affect all drafting outlines including the courtyard. If the component manufacturer only provides nominal package dimensions, a 0.10 mm tolerance should be applied. Typically, the component manufacturer datasheets that provide package dimensions and tolerances, the tolerance values are sometimes too robust and unrealistic. A package tolerance larger than 0.20 mm is considered robust. Only very large component packages have tolerances greater than 0.20 mm.

The package and terminal positive and negative tolerances must be the same value. Some component manufactures tolerances are + 0.20, – 0.10. The average is +/- 0.15. If the +/- tolerances are unbalanced, the positve tolerance can be added to the Nominal dimension to calculate the Max value. The negative tolerance can be subtracted from the Nominal dimension to calculate the Min value. But then the Nominal dimension will not match the datasheet. It’s best to use a nominal +/- tolerance for all dimensions.

Paste Mask: Often called the solder paste, defines the specific locations and sizes of openings in a stencil used to apply solder paste to surface mount (SMD) pads. Unlike the solder mask, which is a permanent part of the board, the paste mask is a temporary tool used only during the assembly process to ensure the correct volume of solder is deposited for reflow. Paste masks are almost exclusively used for SMD components. Through-hole components typically do not have a paste mask layer because they are not soldered via the reflow process unless Pin-in-Paste technology is required.

Photo Resist: a photosensitive polymer used to coat printed circuit boards (PCBs). It acts as an insulating, permanent protective layer that insulates copper traces against oxidation, prevents electrical short circuits, and stops solder bridges from forming between closely spaced pads. There are two primary methods for applying a photo-imageable solder mask at home or in prototyping labs. Liquid Photo Imageable (LPI) is the industry standard and most common type. It is a liquid, UV-curable ink (often known as "green oil") that can be painted, screened, or squeegeed onto the board. Liquid Photo Imageable (LPI) conforms to the topography of the PCB traces. The silkscreen is affected by the trace bumps. Dry Film Solder Mask (DFSM) A photosensitive sheet applied using heat (via a laminator). It offers a perfectly even layer and is highly convenient for one-off designs or quick prototypes. Not recommended for mass production runs. Dry Film Solder Mask (DFSM) sits on top of PCB traces. The silkscreen is flat on the Dry Film and very legible.

Pin-in-Paste: Through-Hole Reflow (THR) or intrusive reflow, is a manufacturing process that allows through-hole components to be soldered using the same standard reflow oven process as surface mount devices. Instead of using a separate wave soldering machine or hand soldering for through-hole parts, solder paste is applied directly into and over the plated through-holes. A stencil prints solder paste onto the SMD pads and into the through-holes simultaneously. The entire board passes through the reflow oven. The paste melts and wicks down into the barrel of the hole, forming a solid mechanical and electrical joint.

 

Polarity Marking: Visual indicators on the silkscreen and assembly drawing layers that ensure polarized components, such as diodes, electrolytic capacitors, and ICs, are installed in the correct orientation. Proper marking is critical; installing a polarized component backward can lead to circuit failure, short circuits, or even component explosions. Components use standardized symbols to help assembly technicians and automated machines. Semiconductors use a dot to identify Pin 1. Diodes and LEDs are typically marked with a thick bar or the schematic symbol, on the assembly drawing, pointing toward the cathode (negative side). For post-assembly inspection, position markings outside the component body so they remain visible after the part is soldered. For through-hole parts, using a square pad for Pin 1 and round pads for the rest.

Reference Designator: A unique alphanumeric code assigned to each component on a printed circuit board (PCB). These labels serve as "geo-tags" that link the physical board to its schematic and Bill of Materials (BOM), making them essential for assembly, testing, and troubleshooting. Pick-and-place machines use these identifiers to match the correct components from the BOM to specific board coordinates. Technicians use Ref Des labels to quickly locate faulty parts described in service manuals or schematics. They ensure consistency across all design files, from the initial schematic capture to the final assembly drawing. There are normally two Ref Des designators in a footprint library, one for the silkscreen and one for the assembly drawing. The assembly Ref Des stays inside the assembly outline while the silkscreen Ref Des is placed outside the footprint and does not violate the solder mask openings.

Semiconductor: any solid-state electronic component – such as microprocessors, operational amplifiers, diodes, or transistors – that is mounted onto a printed circuit board to control, amplify, or switch electrical signals. In the industry, the intersection of PCBs and semiconductors focuses heavily on packaging technology, thermal management, and high-speed signal routing.

Silkscreen Legend: Non-conductive ink layer on a Printed Circuit Board (PCB) used to label components and provide vital information for assembly and troubleshooting. Silkscreen can include Reference Designators, Polarity Markings, Component Outline shapes that indicate where the physical component should be placed, Logos and compliance such as RoHS, CE, and FCC alongside manufacturers or brand logos. Note: silkscreen should not violate the solder mask area.

Solder Mask: Also known as solder resist, is the protective polymer coating applied to the copper traces of a PCB to prevent oxidation and accidental electrical shorts during assembly. It creates a non-conductive barrier that keeps molten solder confined to the pads where components are meant to be attached, effectively preventing "bridging" between closely spaced traces. Shields the copper from environmental factors like moisture, dust, and oxidation. Acts as a dielectric layer to ensure electrical isolation between signals. Prevents solder from short circuiting pads and traces during the reflow process.

Solder Mask Web: The terms solder mask web, solder mask sliver, and solder mask bridge all refer to the same thing: the thin strip of solder mask material that remains between two adjacent pads or mask openings. This "web" is critical because it acts as a physical dam that prevents molten solder from flowing between pads during assembly, which would otherwise cause a solder bridge (short circuit). Most PCB fabricators require a minimum web width of 0.10 mm for standard green solder mask to ensure it adheres properly without peeling off. Some fabrication shops have a minimum solder mask web of 0.075 mm. If the web is designed too thin (less than 0.075 mm), it is called a "sliver." These thin strips are prone to cracking or detaching during manufacturing, which can contaminate the board or lead to assembly failures.

Standoff Height: The vertical distance between the top surface of a PCB and the underside (bottom) of a mounted component. Unlike "component height," which measures to the top of the part, the standoff height defines the air gap beneath it, which is critical for cleaning, thermal management, and electrical insulation. The standoff height is used for cleaning. A minimum gap is required to allow cleaning agents to flow under components and remove flux residues after soldering. Extremely low standoffs of less than 0.03 mm can trap contaminants that lead to electrochemical migration and failure. Taller standoffs can provide better stress relief during thermal cycling, improving the long-term reliability of the joint.

Thermal Pad: A large metal area on the bottom of a surface mount component normally used with QFN, SON, SOP and QFP packages. It is designed to pull heat directly away from the silicon die and into the PCB. To maximize cooling, this pad must be soldered to the board. It often includes an array of thermal vias to carry heat to internal ground planes. To prevent the component from "floating" on a pool of molten solder, the paste mask should be broken into a grid of smaller windows (covering roughly 50–80% of the pad area).

Thermal Relief: A specialized pad connection used when a through-hole component pin connects to a large copper pour or plane. It consists of a small copper ring connected to the plane by thin "spokes". Without these spokes, a large copper plane acts as a massive heat sink during soldering, making it nearly impossible to melt the solder correctly. This often results in "cold solder joints". The spokes provide enough thermal resistance to allow for easy soldering while still maintaining a strong electrical connection.

Terminal Leads: Conductive pins or wires used to establish electrical connections between a printed circuit board and external components, wires, or other boards. They are categorized based on their physical style, how they secure connections, and their intended power or signal application. Through-hole terminals are round, square or rectangle. Surface Mount terminal leads are Gullwing, J-Lead, Flat, Concave, Ball, Column, End Cap (chips), Cylindrical (MELF). See chapter 12 for comprehensive explanation of all terminal leads.

Terminal Outlines: These outlines represent where the terminal leads to land on the pad. These outlines are an important guideline for assembly attachment QC. All terminal outlines must reside 100% on a pad for the best soldering results. If a terminal outline falls off a pad, there will be assembly attachment problems.

Terminal Tolerances: PCB terminal lead tolerances ensure that components can be physically inserted or placed correctly, despite small variations in component manufacturing. Terminal tolerances impact the pad stack calculations to ensure the resulting pattern will accommodate packages in the minimum, nominal and maximum material condition. However, terminal tolerances vary with the terminal lead form. Normally, the recommended terminal tolerances published in package dimensioned datasheets are slightly larger than reality.

Varistor: A Varistor, or surface mount device (SMD) varistor, is a voltage-dependent resistor (VDR) used to protect electronic circuits from sudden, transient overvoltages such as electrostatic discharge (ESD) and lightning-induced surges. It acts as a safety device by automatically diverting excess current away from sensitive components.

Voltage Regulator: An active electronic component that accepts a variable or unstable input voltage and outputs a steady, precise, and clean voltage regardless of changes to load current or input supply fluctuations. They are fundamental to power management on any PCB, ensuring sensitive digital ICs, microcontrollers, and analog sensors receive safe, predictable power rails.

Zero Orientation: The default "0-degree" rotational angle for a component's footprint in a CAD library. It ensures that automated pick-and-place (PnP) machines interpret centroid data (X-Y-Rotation files) consistently, regardless of how a component was originally drawn. The industry follows two primary standards for defining where Pin 1 is located when a part is at zero rotation. IPC-7351B has Pin 1 is in the upper-left corner for multi-pin components. This was a complete change from the IPC-SM-782 standard that was the leading document for 18 years since 1987. IPC-7351 replaced IPC-SM-782 in 2005. In 2007 the IEC 61188-7 Standard was released with Pin 1 is in the lower-left corner. Pin 1 Lower Left is the default Zero Component Orientation in Footprint Expert.