When I was a new designer, the issue of footprints was largely an afterthought and I assumed all the footprints I needed were included in my PCB design software. Lo and behold, on one of my first projects, one of the components a client demanded didn’t have an associated PCB footprint or schematic symbol. Thankfully, my naiveté didn’t derail the project and I was able to find the footprint and schematic symbol for the component from a distributor website.
What is the intrepid young designer to do in this situation? Your new PCB won’t make it to the assembly line unless your CAD files include the correct footprints for your components. Unless you’re using a 3rd-party component service to get verified models for your parts, you might be left to create PCB footprints, schematic symbols, and 3D models on your own. Creating component footprints, symbols, and models are valuable skills and it helps to know some PCB component footprint basics before you get started.
PCB Component Footprint Basics
Your PCB component footprints should include several basic pieces of information. Things like placement, assembly, and silkscreen outlines, reference designators and part numbers are all invaluable and should be present in a component footprint. Your CAD tools are invaluable here as you can create your 2D footprints in a variety of file formats. However, it is best to use the CAD tools in your PCB design software to ensure file compatibility. There are four important points to remember when creating PCB footprints:
Keep Your Datasheets Handy
I remember creating my first PCB footprint from a datasheet. A thorough datasheet will show a drawing of the component with all dimensions, pad locations, pad sizes, and any other important features that should be included in a PCB footprint. Some components, such as power MOSFETs, may contain an integrated heat sink or die-attached paddle that needs to be included in the component footprint. This is quite important for high voltage designs and high speed designs as these elements are included in some thermal and electrical simulations.
Watch for SMD Pad/Through-hole Locations and Sizes
For SMD components, your PCB footprints need to contain accurately placed and sized pads around the edge of the component. The mounting pads will be smaller than the land pads on the PCB, but it helps to indicate both in the PCB footprint. For through-hole components, you’ll need to include the hole locations in your footprint. The pads or mounting holes on these components need to be accurately sized.
Understand Standard Package Sizes
Your PCB component footprint includes an outline of the component package. This package outline will overlap the pads or through-holes on certain SMD components. In some cases, these dimensions are not shown in a datasheet as the component comes in a standardized package.
Some examples are PLCC, LQFP, and BGA components. These standard packages have well-documented dimensions which are usually not repeated in datasheets. You can create a new PCB footprint from an existing footprint; however, be mindful that package and pin count will not always be the same for unique components both within a manufacturer and certainly between manufacturers. If you do decide to reuse a PCB footprint, be sure to check for the presence of any exposed ground pads on these packages.
Include Polarity and Pin-1 Indicators
Components like diodes and polarized capacitors have a defined anode and cathode, which needs to be indicated in the PCB footprint. For DIP and quad packages, there needs to be some indicator of Pin-1 in the footprint. The pins are numbered sequentially as you move around the package counterclockwise.
Important PCB Component Footprint Standards
PCB footprints are critical CAD objects for your design, but they aren’t the whole story. When you’re creating a new component, you’ll need to create the corresponding schematic symbol and 3D model. Just like everything else in the electronics industry (and other areas of engineering), there are important industry standards that define PCB footprint, schematic symbol, and 3D model requirements.
ANSI Y32.2-1975 (Schematic Symbols)
According to the ANSI Y32.2-1975 standard (Reaffirmed 1989), schematic symbols need to be arranged by pin number, rather than by pin function. Some PCB CAD software will allow you to choose whether you want your symbols to comply with this standard, while others may enforce it by default.
IPC 7351 (PCB Footprints)
The IPC 7351 standard (specifically, IPC-SM-7351-B) defines a set of equations governing pad dimensions around a component. IPC 7351-compliant components will be designed to these standards. Many PCB footprint generator tools are designed to comply with this standard and will perform the necessary calculation automatically. Otherwise, you’ll have to do these calculations by hand when using standard mechanical CAD tools to create PCB footprints. Note that through-hole parts are not covered in IPC 7351.
ISO 10303-21 (3D STEP Models)
The ISO 10303-21 standard places requirements on STEP models used in 3D ECAD/MCAD software. This standard is more of a file format specification and not a set of requirements placed on how components are drawn in STEP files.
If your ECAD software includes a component footprint generator, you’ll have a much easier time following the PCB component footprint basics and industry standards mentioned above. These footprint generators will also create a schematic symbol and 3D model for your component, and your data will be saved in your ECAD program’s preferred file formats. These generator tools are also designed to be compliant with the industry standards mentioned above.
A free service like Ultra Librarian is a great way to get access to common component footprints for your new design. You won’t need to manually search component distributor websites for your components and you won’t need to constantly recreate new 3D models or schematic symbols from existing components. The component data provided by Ultra Librarian can be imported into popular ECAD applications and you’ll have access to sourcing information from worldwide distributors. This type of service is a great tool to help you comply with PCB component footprint basics and industry standards.
While footprint creation and manually creating design libraries takes most teams dedicated resources and time, Ultra Librarian helps by making this process redundant. Working with Ultra Librarian sets up your team for success to ensure any design is going through production and validation with accurate models and footprints to work from. Register today for free!