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I think that I can best describe my question by giving two examples:

  1. This is a TRS audio jack: https://uk.farnell.com/lumberg/1503-03/connector-receptacle-3-5mm-phono/dp/1216980. It has two plastic "bumps" (I don't know how those things are really called) and the datasheet clearly shows their location and dimensions.

  2. This is a card edge connector: https://nz.rs-online.com/web/p/edge-connectors/6812238. It has four plastic "bumps" at the corners and the datasheet doesn't mention almost anything about them.

So I assume that in the first case, the plastic things are for mechanical stability, they need to fit inside the pcb, and when I'm creating the footprint for the component I need to add (possibly NPTH?) pads for them.

And in the second case, I assume that the plastic things are for something else (spacing? I don't know?) and I don't need to include them in the footprint.

Is my line of reasoning correct?

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Yes, on both counts.

Pegs, like the ones on the audio jack, are common on surface-mount parts that undergo stress, like switches, pots, and jacks. Basically, you don't want to count on the solder pads to hold the thing in place; the pegs give it that much more mechanical stability.

Standoffs, like the ones on the edge-card connector, are not uncommon. The're there to give the solder room to wick up the legs of the connector a bit.

It's unfortunate that these things don't come with layout suggestions -- it's common for parts like this to come with an example footprint, including any guide holes. I'd be inclined to do a bit of digging for other sources -- that Lumberg part may have a footprint on the next page of the catalog or in a different document; the Tyco datasheet looks like part of a larger document that Radio Shack just hasn't bothered to show you.

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This gets handled typically as follows:

  • Part mounting features that are not soldered, like board locks, pegs, notches and keep-outs are defined as shapes on the part, usually as non-plate through. These features aren’t pins, and they’re not associated with any net.

  • Features that take solder are usually plate through or solder pads (that is, they’re part of the copper.) These not only need to be defined on the shape, but also as pins on the schematic even if they are no-connect. They’re part of the net list.

As you create the part you will distinguish between these two types of features, and ensure the schematic symbol calls out all the soldered and none of the non-soldered features.

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