PCB Assembly Services - Screaming Circuits: Never Take Pin Numbering For Granted


Never Take Pin Numbering For Granted

Package pin numberingOur all-things-about-electronics manufacturing standards body, the IPC, specifies the proper numbering order for most components. That's a pretty nice thing that they do there, but it's not always enough to prevent layout mishaps. Case in point a line of small PC board mount switches.

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

Given, that, it would be logical to assume that 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 thru-hole, top mount surface mount and side mount surface mount, but the leads have to be accessible from different parts of the package.

The following two footprints are from the same switch. One mounts on its side, and the other, standing up.

Two switches with differnt footprint

The pin one numbering doesn't follow convention, nor does the numbering of pins 4 - 6. And, you may have also noticed that the two are top-to-bottom mirror images of each other. Ugh.

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

Duane Benson
Don't take it for granite either, because granite is too heavy.


For these types of parts, I also like to include helpful info on the silkscreen and/or assembly drawing
C / NO / NC for switches (add COIL for relays)
A/K for diodes (especially LEDs)
This is particularly true for parts that are symmetrical. It helps if there's any confusion in assembly or for the person troubleshooting / repairing your board later on.

As you said it is not advisable to take pin numbers from circuit board because numbering helps us to connect the things in a right way.

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