Estimated reading time: 5 minutes

Elementary, Mr. Watson: The Anatomy of Your Component—Footprint

I hate to disappoint you if you expected to get everything about footprints from this modest column, but a short search online results in a long list of technical standards and books on this subject. So, I will only hit the surface of the discussion.

I have often spoken about the parent-child relationship principle in PCB design. In this series, we have learned that we can see how that "relationship" is supported directly by the information in the component itself. As a short review, the parent-child relationship is where you use an input of data, material, or parts (parents) as the foundation or resource for another item or document (child). An example of this concept is a schematic (parent) which is the source for the bill of material (child).

Remember, an item or document can be both a parent and a child. It simply depends on where it is in the process. But how do the different elements of our component support this concept? We know that the first part of our component is information. When a component is in a schematic, the information for each component is what becomes or creates the BOM (child document). We can't leave our models out of the party.

With the footprint, this concept explodes in our printed circuit board. According to the typical design flow (Figure 1), the PCB design is the parent of no less than four “children.” You have the fabrication drawing and the fab data (Gerbers) on the assembly side, the assembly drawing, and the assembly data. That said, the source of all that data is found in your component, specifically your footprint. So, to help us understand the requirements in our footprint, let's keep the deliverables in our documentation in mind; that is what’s used to build the PCB.

As if that wasn't enough to worry about, there are key stakeholders involved in this process waiting for critical information to do their jobs. For example, mechanical designers are looking for the board shape, and the component size details (in the form of 3D models or procurement) need the parts choices in the BOM. If, for some reason, you do not provide that information in your design, it will slow things down. Having too much information in your components and libraries is not a problem. The opposite is true, though. Not including critical information needed later on is crucial.

I hate to use the phrase “bare minimum” because some believe it’s all that's required. Then there is always that one who brings up "what if" or "what about this or that?" So, take what I am about to share with a grain of salt; for those struggling with hypertension, skip the salt and use the Mrs. Dash. Knowing and using the industry standards is a tremendous help to knowing what you should do and why. Standards such as IPC-7251, IPC-7351, and IPC-2612 are a few of the significant guidelines dealing with components.

The PCB Footprint
I feel that Copland's “Fanfare for the Common Man” should be playing in the background when I say that. This is when it gets serious and, I might add, where many mistakes occur because of wrong or missing information on the footprint. So many of us work with substandard footprints every day, hoping that this is not the PCB design that bites you in the butt with problems. Your design got through fabrication and assembly; that doesn't mean it was correct. It could be that you were just lucky.

The first rule with your footprints is that the pins must match precisely by naming the pins to the schematic model. So, for example, if you identified your diode with an anode (A) and cathode (K) identifier, your pins on the footprint must be the same. That transfers the graphical connections in your schematic over to the physical part of the PCB.

The function of the PCB is to provide electrical connections and mechanical support to the electrical components of a circuit. Therefore, the first essential part of your footprint is how those components connect to the PCB. They can be either through-hole or surface mount. Before doing anything, know the type of footprint you are designing and the type of lead used on the component. In Figure 3, that component is a small outline IC 14-pin, better known as a SOIC, which uses the gull wing configuration on its leads. That is essential information; it determines the copper pad's size, shape, and location.

Figure 4: SO14 package information.

Your first resource, but not exclusively, is the component datasheet (Figure 4). Identify the component based on the part number and the correct footprint. I've seen more times than I could count getting the wrong part number and the associating footprint. The seasoned PCB veterans in our audience fully agree with the following comment: Do not blindly trust the datasheet. They have been blatantly wrong, resulting in expensive respins for companies.

The toe, heel, and shoulder identify each type of component lead (Figure 5). The toe and the heel area well is called, what else, the foot.

The copper pad sizes depend on foot size to ensure that the pad holds enough solder paste when reflowed, allowing for clean solder wetting and solder fillets on the toe heel and the sides. I can't get into it here, but formulas determine the correct pad size. I would learn that vital information as a designer.

When the component gets soldered to the PCB, the solder fillet on what is the heel is the most important. This is because most of the stress on the component is in the heel area. Unfortunately, I have seen that the inner part of the pad is often too short, not allowing for a good solder fillet. It was only a matter of time with a poorly soldered component, and it could be down the road to a PCB failure.

The last item in the footprint we can take up in this installment is the solder paste and mask (Figure 6). Both are associated with the footprint pad, which has its own IPC Standard: IPC-7527 Requirements for Solder Paste Printing. It is usually one-to-one with the pad size. Be aware, though, that this is not a hard-set rule. Especially with large areas of copper, it may have a smaller solder paste area or none.

 The other item connected to the footprint pad is the solder mask. A thin polymer layer is put on a circuit board to protect the copper from oxidation and shorts during operation. Most PCB ECAD software handles the copper pad size, the solder paste, and solder mask sort of as a group package, we'll call it. When laying down a pad, it automatically sets up the required solder paste and solder mask.

As the journey continues, I will go deeper into our footprint and look at the items you need.

John Watson, CID, is a customer success manager at Altium.

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