PCB Assembly Services - Screaming Circuits: Pinouts, Package Changes and Subs


Pinouts, Package Changes and Subs

With so many components on allocation, having extended lead times or simply not available, minor re-design is the popular task of the day. MCUs can often be replaced with another family member and most passives and power components have options for near-equal substitutions.

However, nothing should be assumed to be a drop-in replacement. Case in point a line of small PC board mount switches.DIP switch pinoutsIPC 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.

DIP pinoutsGiven, 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.

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 and never assume anything. And once you have said that mantra three times, head over to Screaming Circuits and get a quote.

Duane Benson
If we would just go back to point to point wiring, pinouts would not matter


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