Honored Contributor
Honored ContributorIf I took that to my assembly guy he'd :eek:
--- Quote Start --- From the image, it does appear as if you have put the decoupling capacitors on round pads in a mirrored location to the BGA pads. The caps are not directly on the vias. --- Quote End --- No, that's why I kept saying they mirrored the BGA pads on the bottom of the board. Sheesh, you academics (my dad was/is one so I know what you're like). :) --- Quote Start --- Here's a photo of the bottom of the PCB on one of the CARMA board FPGAs. Note how the decoupling caps are actually on top of the tented vias. The square pads are slightly offset due to the fact that we initially tried not tenting the vias. However, too much solder wicked into the vias, so we had them tented (and didn't bother centering the cap pads in the vias). --- Quote End --- I'm not sure if we're using 'tenting' to mean the same thing. To me (and most on the Altium forums I think) 'tenting' means the via is covered in solder resist, but if your vias were tented you wouldn't be able to solder your caps on them. Do you mean they've been filled (I think they use epoxy) so the solder that's down for the caps won't wick down the via? It would be interesting to see how having the cap directly connected to the via affects it's impedance, as seen from the BGA pad, compared to having the short stub as in my example. Is this the sort of thing you could get students to do as project work? Another experiment would be incrementally removing decoupling caps from a working board and make noise measurements on the supplies. Just shows there's no 'correct' way of doing things. Nial.
