Power Delivery Network (PDN) Tool

Here's a screen capture attached, looking from the bottom of a Cyclone IV board.

Red = top layer, yellow = top silk

Blue = bottom layer, sludge = bottom silk

I wasn't pin constrained but don't think I lost many user IO, I would share an odd via for decouplers or pull ups.

Nial

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.

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).

Cheers,

Dave

If I took that to my assembly guy he'd :eek:

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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.

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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). :)

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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).

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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.

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If I took that to my assembly guy he'd :eek:

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I always ask the assembly company what they can handle first :)

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No, that's why I kept saying they mirrored the BGA pads on the bottom of the board.

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Actually, you said two things; you said mirrored and you said on the ends of the vias.

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Sheesh, you academics (my dad was/is one so I know what you're like).

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I live in the middle of nowhere, so have noone to talk to ... sniff sniff ... seriously though, I have noone to talk to about this type of PCB design, so I wanted to clearly understand what you were doing.

Just call me pedantic :)

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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?

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Ah, I think you are right ... sorry 'bout that.

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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.

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I'm sure no stub is better :)

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Is this the sort of thing you could get students to do as project work?

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Alas, I really am in the middle of nowhere. No students in sight ... (none that I'd give a soldering iron to anyway)

http://en.wikipedia.org/wiki/combined_array_for_research_in_millimeter-wave_astronomy

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Another experiment would be incrementally removing decoupling caps from a working board and make noise measurements on the supplies.

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I haven't tried removing caps. I'd be afraid that I might reflow a BGA ball. The boards are 18 layers thick, with a lot of copper.

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Just shows there's no 'correct' way of doing things.

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It is nice to discuss the possible solutions though.

Thanks for sharing your knowledge.

Cheers,

Dave