For some switches, numbering can vary based on how the part is mounted.

Our all-things-about-electronics manufacturing standards body, IPC, specifies the proper numbering order for most components. That’s a pretty nice thing it does, but it’s not always enough to prevent layout mishaps. Case in point: a line of small PCB mount switches.

IPC calls out pin numbering for dual inline components, with pin one on the upper left (at 0° rotation), counting down, then over to the bottom right, and counting back up, as in FIGURE 1.

Figure 1. IPC-preferred pin numbering for DIPs.

Given that, it would be logical to assume all dual inline components follow the same pattern. Logical, yes. Accurate, no. Multi-color LEDs, connectors and switches are some of the more common offenders.

In this particular switch, it’s not just a case of the numbering not following convention. It’s also different from one variant to another. I understand why. The switch isn’t changed between through-hole, top-mount surface mount, and side-mount surface mount, but the leads have to be accessible from different parts of the package.

The two footprints in FIGURE 2 are from the same switch. One mounts on its side, and the other standing up. The pin 1 numbering doesn’t follow convention, nor does the numbering of pins 4 to 6. And, you may have also noticed the two are top-to-bottom mirror images of each other. Ugh.

Figure 2. Footprints for the DIP reveal variations to conventional pin numbering.

This is why my mantra is always check the datasheet. Always.

Duane Benson is marketing manager and chief technology champion at Screaming Circuits (screamingcircuits.com); This email address is being protected from spambots. You need JavaScript enabled to view it..

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