Never Underestimate the Power of the Footprint

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Right now, the electronics industry is being hit hard by a part shortage. Ordering parts has become increasingly difficult, and parts you could get yesterday now take six months and longer to obtain. It is a constant race to not only keep up with what suppliers have available, but to find replacement parts quickly enough to still get that printed circuit board (PCB) design completed. Customers and designers are reworking layouts repeatedly, trying to keep up, even re-routing entire boards just for one new part. Those parts are gold! With that said, the importance of a working, functional library is critical. Being prepared for every PCB layout starts at the footprint level.

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In general, there are a lot of options for what footprints to use. Not just from services that can build them for you, but from online databases like the Altium Vault. Information can be quickly shared from one designer to the next, and now there are shared digital databases to pull this knowledge from. Mentor PADS, Cadence Allegro and Altium all have their own in-house footprint wizards. These can build footprints right to IPC standards as long as you have the right component dimensions.

For a company that orders a lot of PCBs, they might have their own in-house library that they use. Some of these are tailor made to work with the manufacturing and PCB fab houses they use. If a part has been fabricated before and had no issues on the floor, why change it? Having a board with all the footprints already in the database can also save a lot of time in the data prep stage. If these parts are already tried and true, there is no need to spend the extra time re-checking and re-verifying these parts. All that needs to be done is to check the netlist for any new changes, check the statement of work and then start working with the PCB designer.

For new PCB layouts, or even companies looking to update old designs, pulling in parts from outside sources can come with a lot of uncertainties.

Open source databases are great for having a wide variety of parts already loaded. Many of them will even have some footprints for some of the oddball or more difficult-to-find connectors. However, it is not always known who is creating these parts, who is verifying their land patterns and what their checking process looks like. When using footprints from outside sources, some extra checking is usually done during the design phase. The pinout, package and part numbers all need to match up. And when the board has a lot of parts, an 0402 package being assigned to an 0201 capacitor can go completely unnoticed. Components are in high demand right now, so every possible step should be taken to avoid future re-spins all the way at the library level!

A lot of footprints can vary, even for the same package. Many ECAD tools now have their own built-in generators. Using a component generator that is compliant to IPC-7351 standards is a great way to create footprints. However, these programs will give two parts with the same package the same footprint. What works for one part may not work for the other. Some components require special considerations for solder reflow, thermal performance or keepouts specific to their parts. It is important that the footprint not only lets the part get fully soldered on, but gives it the best performance possible on the board.

Having an established, verified library can mitigate a lot of these risks. At ACDi, our libraries have been built up over the course of 35 plus years. We have used them on over 11,000 designs and these parts get verified by our manufacturing process. Footprints are built not just to IPC standards, but also with design for manufacturing (DFM) and design for assembly (DFA) in mind.

It is one of our biggest advantages, having a direct line of communication between manufacturing and the design departments. If there are any suggestions or ideas to make assembling boards easier, they will let us know. Any connector that needs some extra room for tolerances, any small SMT parts where the recommended pads are a bit too large, footprints can be adjusted to account for this. Any concerns in the design phase can be answered right away by a team that already has years of experience assembling these boards.

Decals are all verified through our checking process. All new parts are double checked by a second engineer to make sure the package, pinouts and footprints match the part number on the bill of material (BOM). Everything goes through this process before being added to our master libraries. If there are any special requests from the customer as well, we can keep these custom footprints for all future projects of theirs.

Improvements can be even be made to footprints at the post-processing level. All of our designs go through both a peer review and a design rule check (DRC) using the Valor NPI software from Mentor Graphics. If there isn’t enough solder mask in between pins, if there isn’t enough space in between pads, that can be caught and adjusted before a design it out the door.

When using a pre-established library, the rest of the post processing can be done quicker and more seamlessly. Parts from outside sources can have different layer setups between each part. How pads and mounting holes are defined, how silkscreen and assembly drawings are defined on each layer, and how rules are setup for solder paste and solder mask layers can change in between footprints. ACDi’s PCB layout software platforms are already configured to export Gerbers and layout files in a way that makes creating deliverables quick and free of errors. Components in our library are already set up for our ECAD tools, so there is no need to reconfigure settings for each design.

Every step of designing, fabricating, manufacturing and assembling PCBs can be improved all the way back at the library level. It is an important stepping stone to making sure the entire process runs smooth. As designs become more complex, and the need to get it right the first time becomes imperative, a working library can remove uncertainties at every stage.

Katherine Bryant
PCB Librarian