Surface mount device (SMD)

IPC-7351: The IPC Standard for PCB Footprint Creation

There is a song by the band Rush entitled “Natural Science” that discusses the evolution of life. An excerpt from it describes humanity as

“All the busy little creatures

Chasing out their destinies

Living in their pools

They soon forget about the sea…”

This is probably accurate for most people on most days, performing our tasks oblivious to the many people, ideas and developments that came before and enabled us to do so. The importance of this historical guidance, however, cannot be overstated. Without using these footprints as building blocks, progress and advancement in any area would move at a snail’s pace, if at all.

When designing circuit boards there are many times when you can utilize a component footprint that was previously created. By doing so, the layout of the pads will most likely satisfy the SMD pad layout standard, IPC-7351. However, there are also times when you will have to create a PCB footprint yourself. Let’s take a look at when this is necessary and how to ensure that your design meets the regulatory requirements of the IPC standard for PCB footprint creation.

When Is PCB Footprint Creation Necessary?

Designing a circuit board or a PCB architecture certainly requires creativity and ingenuity. In fact, board design is nothing less than an artistic expression of your unique perspective bounded by the board’s design intent, performance objectives and manufacturing limitations as specified by your CM’s PCB DFM guidelines. Arguably, the most important PCB layout decisions are where to place components, which define everything from trace widths and spacing design to thermal dissipation and distribution during manufacturing and operation.

Component placement begins with determining the best PCB footprint and there are times when creating this layout item must be created, as listed below.

Reasons for PCB Footprint Creation

  • Component library is not in the PCB design program’s database

Probably, the most common reason to create a component footprint is that the PCB design software you are using does not have the selected component’s library (schematic symbol and PCB layout footprint or land pattern) in its database. If not, you can use the manufacturer’s datasheet as a guide to create the library for your software.

  • No manufacturer land pattern is available

Your component may not be listed in your program’s database and the manufacturer's datasheet may not be available. This can be a major issue if component selection optimization is not practiced. In this case, it may be possible to utilize an online components service to create the component library.

  • Creating a new subassembly

PCB footprints are not limited to singular components. Today, the use of pre-built modules is increasingly used to shorten the design-build-test (DBT) development cycle. As these packages may include primary components, such as ICs and associated circuitry or even multiple circuits, footprint design for new modules is generally required.

Regardless of whether the motivation for your PCB footprint design is one of the most common listed above or there is another reason, the process should be guided by the IPC standard for footprint creation, IPC-7351. Before establishing a means to ensure compliance with this standard, let's clearly define what the standard provides.

What Is the IPC-7351 Standard?

IPC-7351 is one of the many standards developed and published by IPC Association Connecting Electronics Industries. Since its founding in 1957, IPC has been at the forefront of standardization for the design, manufacturing and testing of PCBs and PCBAs. With IPC-7351B (published in 2010), the organization provides guidance on the creation of land patterns or footprints for SMDs with the following stated intent.

The purpose IPC-7351

As shown, the motivation for the standard is to maximize solder joint quality by making sure there is adequate space for inspection, testing and rework, if necessary. Now, let's see how to utilize and support the standard during PCB footprint design.

How to Use the IPC Standard for PCB Footprint Creation for Your Board Layout?

IPC-7351 covers a lot of territory. The instructions and guidelines included provide direction on how to leverage common land patterns (which can simplify your design process) and how to generate footprints from scratch. You can make the best use of this standard by following a few simple rules, as listed below.

Using the IPC Standard for PCB Footprint Creation 

  • Use a common package type, if possible, as a starting point for your design

For new component library designs, it is a good idea to begin with a standard package specification for the package type--i.e. SOIP, SOIC, DIP, etc.--as these have standard specifications and tolerances that are common and sufficient for manufacturing. Simply, starting from scratch may lead to pin pitch errors, insufficient spacing between pins, etc.

  • Follow IPC standard dimension and tolerance guidelines

In most cases, the IPC standards will be in agreement with the manufacturer specifications. However, if there is a conflict, defer to the manufacturer recommendations as they may include special considerations, such as soldering reflow instructions, keepouts, etc.

  • Incorporate testability into your design

Make sure to provide test points for functional testing or ICT—Flying Probe or Bed of Nails—if there is no surface connection to component terminations. For BGAs, it may be necessary to aid vias to accomplish this.

  • Ensure that standard specs adhere to your CM’s DFM

As the capabilities of your CM’s equipment and processes ultimately determine manufacturability, your specifications must be within the tolerances of DFM specifications.

  • Allow access space for rework 

Generally, enough space to access--remove and/or maneuver--the component and insert a soldering iron tip without making contact with other joints or components should be sufficient. However, for larger components; such as transformers, power supply elements, etc., more space may be necessary.

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Undoubtedly, you will encounter situations where you will have to create a PCB footprint(s) for your design. IPC-7351 is provided to aid you in this endeavor. However, realizing your IPC-7351 based land pattern can only be achieved if you work with a CM capable of building boards that accurately reflect your design intent. Tempo Automation, the industry leader for fast, high-quality and precise PCBA manufacturing, will work with you throughout design to ensure your board accurately reflects your design and is manufacturable to this quality standard.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on using the IPC standard for PCB footprint creation on IPC-7351, the IPC standard for PCB creation, contact us.

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