You can often tell a lot about a person, animal, or object by looking at its footprints. Trackers can look at a footprint and determine exactly what animal made it, as well as their size and how fast they are moving. Detectives can pull information from a footprint or tire mark to help them solve a crime. In these examples, however, footprints are the result of something that has already been there and left that pattern. In PCB design, a footprint is a pattern for an electronic component that will eventually be soldered there.
Whether it is a through-hole connector, a surface mount capacitor, or a large ball grid array (BGA), every part that gets soldered onto or into a printed circuit board needs a footprint. Often referred to as landing patterns, these footprints and their arrays of metal pads are to what the electronic components will be soldered.
As such, the footprint must be built to exacting standards for the correct assembly of the PCB. If the pads are not in the correct location, the part won’t solder correctly. If the footprint outline is incorrect, other parts may be placed too close together for assembly. To build the right-size footprints for your next PCB design, here are some PCB footprint creation guidelines to help.
Problems When PCB Footprint Creation Guidelines Are Not in Place
When PCB design tools first became commonplace, there often wasn’t enough attention put into the creation of their library parts. Since design technology was less advanced and tolerances were more forgiving in those days, designers were able to get by following less-strict guidelines
For instance, through-hole bypass cap footprints would often be created generically with 0.300 inch pin spacing and an outline drawn roughly 0.050 inches around the pins, without consulting a datasheet for the exact component being used. Other through-hole footprints, such as single and dual inline parts (SIPS & DIPS) as well as connectors and headers, were often created the same way.
It is amazing to think of all the design space that was given up due to these footprints being unnecessarily large. These bypass capacitors may have ended up being drawn oversized at twice their actual size, allowing a lot of space to go unused around them. This extra space became obvious when there were several of these parts in a row on an assembled board. Where the placement should have been more compressed for better electrical performance and to give the designer more room to work with was visible.
Today, the industry is more aware of the need to follow PCB footprint creation guidelines because wasted space isn’t the only thing that can happen if a footprint is created incorrectly. Here are some of the other problems that you might see:
- Incorrect pad sizes: Pads that are too small can cause breakout problems for through-hole parts and bad solder joints for surface mount technology (SMT) parts. Pads that are too large can take up room for trace routing or cause SMT parts to float out of position during soldering.
- Pads that are not spaced correctly: Through-hole pads that are too close together or too far apart can cause problems with component insertion during assembly. SMT pads that are too close or too far from each other can cause insufficient solder joints to be formed, as not enough of the component lead is available for soldering.
- Component outlines that are too small: Footprints that have their body outlines drawn too small will not be able to be assembled using automated production lines. Depending on the severity of the error, the parts may be able to be manually assembled, but that could take extra time and expense. For large errors, the boards may be completely rejected by the manufacturer.
As you can see, it is very important to get the size of the PCB footprint correct before it is used in the design. Having to redesign a circuit board because of a footprint error can prove costly from a time and money perspective. Here are some footprint creation recommendations, however, that can help.
PCB Footprint Creation: Start from the Ground Up
There are several ways to create your PCB footprint library, and the first is to use existing library parts. In many cases, the vendor or supplier of the PCB design tools you are using will already have vast libraries from which you can pull parts. There are also many component vendors out there who will offer a footprint of the specific part you are using tailored for the design tools that you are working on. The advantage here is that, since these footprints are created by the component vendor, they are usually very accurate. There are also third-party library services you can purchase library parts from, as well.
The next method you can use is to create your footprints by using the part generators and wizards that are built into the design tools. Most PCB design tools today have functionality like this, and many of them are based on industry standards for footprints so that you get a precisely built footprint. These tools usually give you a way to manually edit the footprint if necessary, which can be very helpful if your design workflow requires specific silkscreen information or other modifications.
The last method is to build the footprint yourself. Although you can usually get reliable PCB library parts from the sources mentioned above, there are times where a special need footprint may require building it yourself. Here are some general guidelines to remember:
- Start with exact information: Ensure that you have the correct specifications for the part and the relevant standards, such as IPC-7351, to use as a blueprint.
- Build the footprint from the ground up: Start first with the pad information. Whether your tools require a separate pad part, or you must add the pad geometry as an attribute, ensure you have what you need first. Next, create the body outline of the component at the maximum material width. Finally, add silkscreen outlines, reference designators, other attributes, and STEP models to finish the footprint.
- Make sure to include all the required data: Often a footprint is left incomplete, which can cause problems during PCB layout. For instance, if your CAD tools require you to enter the part’s height as an attribute, don’t neglect this. Later on, you may wonder why you didn’t get a warning when you find out that your component is sticking up through the device enclosure.
- Be cautious of including too much data: Do your footprints need drawing formats, mechanical data, and dimensions? Remember that each footprint will be replicated for each instance of that part in the layout. If you have included all of this extra data, it will also be replicated. Viewing the CAD layer designated for footprint dimensions will result in a confusion of jumbled data that can be difficult to discern for assembly.
Once your footprint is built, it will need to be reviewed for accuracy. It is best to do this before the design is worked on, but at the very least, these library parts need to be verified before the board is sent out to manufacturing.
Speaking of manufacturing, there is another great resource for help with your footprints—a quality contract manufacturer.
Your Contract Manufacturer Can Help
One of the best things you can do to ensure a successful PCB footprint, design, and assembly is to work with a CM that has a lot of experience working with different PCB design CAD tools and their related library footprints.
While the industry-standard IPC specifications are usually enough for the majority of the parts that you will be using on your circuit board, some component footprints will need fine-tuning for manufacturing. Your CM should be able to help you with component and footprint decisions and guide you in the manufacturing ramifications of those parts.
