kicad-developers team mailing list archive

[Andy Peters <devel@xxxxxxxxx>]

> On Jan 29, 2016, at 8:49 AM, Chris Pavlina <pavlina.chris@xxxxxxxxx> wrote:
> 
> Oh, it's definitely a dirty hack - but it's a dirty hack that is somewhat
> necessary, and used to be possible, and now it's not, so... regression, dude!
> :)
> 
> Yeah, yeah, I'm a spacebar heater, I know... :D
> 
> https://xkcd.com/1172/
> 
> I'd argue that while using a footprint as a via is a dirty hack, the simple
> concept of allowing footprints on the PCB that aren't on the schematic is
> *not*. Lots of people want to be able to place things like mounting holes
> without having to put them in the schematic. (Whether or not that's best
> practice is beside the point, it's very common.)

Use case for “footprint as a via”:

You have a big FPGA in a BGA package. Not all pins are used in the design, but you’d like to bring some of the unused pins out to vias so you can hedge against the need for little green wires or just to provide test points.

In Altium (at the day job) we run into this all the time. The problem is that pins that are no-connects on the schematic have no entry in the net list, so you can’t drop a via and connect the pins to it. OK, well, you can, you can force it, and then you run into DRC issues, and the next time you update the pcb from the schematic you risk blowing that all away.

Creating a component that is only the via, and placing it on the schematic and connecting it to the FPGA pin in question, neatly solves the problem. It’s basically grown out of having a physical pin as the test point. Often, we’d just build the boards and not stuff the test pins as probing the hole was sufficient. The reason to just go with the via-size hole is to minimize the space used by the test point. It fits well in the BGA fan-out.

Oh, and to go further into the weeds, imagine a NPTH mounting hole. Clearly there’s some kind of screw or other fastener that goes into that hole, and you’d like that fastener to show up on the BOM. So you create a footprint for the hole, and you create a symbol for the fastener/hole combination, and your part number for the component is that of the fastener. Place the fastener on the schematic, you get a hole on the pcb and a screw on the BOM.

-a