PCB Footprint Creation Guide for Your Next Design Project

Footprints in PCB design are very important, so circuit engineers are required to adhere to specific guidelines when creating them. The ability to accurately design PCB component footprint is an essential skill for engineers as it provides crucial information about the physical interface between electronic components and printed circuit boards. This information is critical during the assembly and maintenance phase of a PCB and includes details such as component shapes and symbols, pad locations and numbering, reference pins, polarization, and more. Improper design can result in component errors, signal interference, and pin misalignment. In this article, TechSparks will provide PCB footprint design guidelines to enhance the circuit performance of your next project. So, let’s get started!

Table of Contents

PCB Footprint Types

Different electronic applications require different components, assembly methods, and types of footprints. The different footprint types are described below:

Through-Hole/THT: These footprints are designed for components that have leads or pins that pass through the PCB and are soldered on the other side. These footprints consist of circular pads that have holes drilled through the board. However, they are not commonly used in modern electronics and are usually reserved for larger or more expensive components.

Surface Mount/SMT: SMT is the most commonly used technology. These footprints consist of rectangular or circular pads with small solder paste stencils. Components are mounted directly onto the board surface rather than inserted through holes.

Ball Grid Array/BGA: This footprint is designed for components with a series of small solder balls on the bottom. These components are mounted directly onto PCBs and are commonly used in microprocessors, memory chips, and other high-performance ICs.

Dual In-Line Package/DIP, Quad Flat Package/QFP, Small Outline Integrated Circuit/SOIC, and Chip-Scale Package/CSP are some other common PCB footprints.

Each footprint has its own characteristics, and creating a footprint based on project characteristics is the key to ensuring compatibility with the board design and efficient operation of the device.

Elements Included in a PCB Footprint

Pads: A Pad in a PCB footprint is a metallized area that is typically rectangular or circular in shape and is used to connect component leads. Pads are usually made of copper and play a critical role in ensuring a secure electrical connection between the component and the PCB. During assembly, the precise positioning of components is dependent on the location of their respective pads and holes. It is essential to accurately design the size, shape, and spacing of the pads when creating the PCB footprint. This is because the pad dimensions will vary based on the size and spacing of the component leads or pins. An incorrectly sized pad can result in poor soldering connections, incorrect component placement, and potential device failure.

Silkscreen: It is an important layer in PCB footprints that is used to print text, symbols, and graphics to indicate the exact location and parameters of components, such as polarity and pin count. It is created by applying a layer of white ink to the surface of the board. The proper positioning of components during assembly depends on the accurate placement of the pads and silkscreen. Therefore, designers must carefully design the size, shape, and spacing of the pads and silkscreen to ensure precise alignment of component pins and silk screens, thus ensuring the correct assembly and maintenance of the circuit board. It is important to note that for smaller components, such as SOT, resistors, and capacitors, the silk screen layer should surround the entire component body without overlapping the component’s pads.

Copper traces: Also referred to as conductive paths, play a critical role in PCB footprints as they connect the pads and components on the board and provide pathways for electrical signals. Accurate design of the PCB trace width is crucial to ensure that the current flows through the path with minimum loss and interference. Following the strict IPC standard for straight-line design of copper traces can help ensure the reliability and performance of the circuit board.

Drill holes: These are holes drilled into the PCB board to facilitate the insertion of through-hole components or to mount the PCB on a chassis or packaging. The size and position of the holes must be accurately designed to ensure that they do not interfere with other components and nearby traces on the board.

Keepout areas: To prevent components from interfering with each other or with neighboring mechanical components due to heat generated by components and electrical interference caused by traces, no-go areas must be designed on the board. The courtyard is the designated area that acts as a boundary for the entire body of the component, ensuring that it stays clear of other components and mechanical parts.

Component symbol: To aid in the identification of components on the board, special reference marks are included on the PCB layout, schematic diagram, and finished board, serving as unique identifiers for each component. These marks typically consist of a combination of words and numbers. Common reference designations include “R” for resistors, “C” for capacitors, “U” for integrated circuits, and “D” for diodes.

Pin-1 Designator: It is a crucial identifier used in the PCB footprint to indicate the location of a device’s first pin. Its absence can lead to components being installed upside down or rotated, potentially resulting in PCB failure. Therefore, accurate identification of pin locations is necessary during footprint design to prevent assembly errors. Typically, the Pin-1 Designator is represented by a small circle or triangle placed next to the device’s first pin to ensure correct device orientation during assembly. In some cases, such as with chips, the Pin-1 Designator may be a groove or a small bump. For components with a large number of pins, like QFP or DIP packages, marking the position of pin 1 on the package is important to avoid assembly misunderstandings.

PCB Footprint Guidelines

Creating a custom PCB footprint can be a challenging process, but it can be done efficiently if you follow some basic guidelines. In this article, TechSparks provides some tips to help you create your own PCB footprint.

Follow manufacturer recommendations: High-quality manufacturers often have professional design teams, and choosing the right PCB manufacturer can be very helpful for your packaging work. These manufacturers provide datasheets for components on their official websites. Creating a footprint that follows the manufacturer’s recommendations and datasheet information is the first step in your design process.

Design guidelines: Size pads based on component lead shapes; follow component dimensions and lead spacing; use silkscreen marks to indicate polarity, component parameters, and pin numbers; use PCB prototypes to test the design’s footprint, and more.

Avoid design mistakes: Choose the correct type of footprint; refer to the component tolerances provided by the manufacturer; avoid placing components too close to each other; remember to add exclusion zones for mechanical parts.

Component datasheet: This is a form or document that records the characteristics, parameters, and specifications of a component, usually provided by the manufacturer. Designers should consult these datasheets before designing to understand the component’s dimensions, pin layout, electrical characteristics, etc., in order to properly integrate it into the board design.

3D models: These are three-dimensional models of the actual appearance of components, provided by the component manufacturer. They can be directly imported into PCB design software, and used to ensure accurate connection and placement of components.

Online footprint library: PCB footprint libraries contain pre-made footprints for electronic projects. These can be created in-house or downloaded from different online sites, and are useful for designers and engineers to increase design speed and reduce the risk of errors. While using prefab footprints, make sure they are up-to-date and correctly configured. A testing and validation process is often performed. There are many online libraries with pre-designed PCB footprints, such as the SnapEDA library or Ultra Librarian.

PCB Footprint Standard

IPC 7351B: IPC is a widely recognized set of standards in the printed circuit industry. Whether you’re involved in design, manufacturing, or assembly, it’s important to adhere to the guidelines provided by this organization. For PCB footprint design and land pattern, IPC-7351B provides comprehensive coverage for almost all types of SMD components, including chip capacitors, resistors, diodes, transistors, as well as non-SMD components such as sockets and connectors.

ANSI Y32.2: Itis a standard established by the American National Standards Institute (ANSI), which provides guidelines for naming conventions and symbols used in electrical and communication engineering. Despite ANSI Y32.2 being initially published in 1975, the standard has remained relevant due to its specialized nature. Some CAD tools offer the option for designers to conform to the standard, while others do so automatically.

ISO 10303-21: ISO is the International Organization for Standardization. It is an important consideration for electronics manufacturers when selecting PCB manufacturers. Many foreign trade PCB companies lack ISO certification as they do not have factories. ISO 10303-21 is a file format standard for exchanging 3D product data, also known as STEP-File. The standard includes a common data schema, as well as guidelines for file structure and content, which makes it easier for different software programs to interpret and use the data.

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