Electronics > Microcontrollers

How far away would you place your bypass caps before you considered a problem?

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NaDobraNich:
I’m laying out in stm32 part, using up some excess inventory on the side project.

https://www.st.com/resource/en/datasheet/stm32h725ie.pdf

No ADC, no DAC, just some digital UART, SPI, IO, USB, Ethernet. It’s a BGA package, 176+25. Plan to run 260Mhz or so.

I don’t have a great access to the bottom of the board. Not enough to put the bypass caps as close to the pins as I would like. This part needs like 13x 100nF and another 6x various for the internal LDO and etc. I can got down to 0401 and 0201 parts.

How far away would you route these before getting concerned?

In an ideal world, I think I can get the traces to be about 1mm with via.  Would 4mm be OK? 6mm? I know there is no hard answer here, just gut, how far would until it’s an issue?

uer166:
Presumably the die in the BGA is the same as the LQFP parts which are very large. The internal lead frame and bond wires are likely quite far from the die, and have no magical properties vs. traces on a PCB. I suspect you'll be okay placing all decoupling at the periphery of the BGA, just prioritize them and you'll probably be ok on emissions with a good stack up.

NaDobraNich:
… how have I never thought about it like that!

I agree, the die probably is the same, and it’s just wires to it…. Huh.

I can get most on the bottom nicely but some would have to be a little longer and it was bugging me.

NaDobraNich:
So thinking about it a little, if you had a high power usage, that the BGA parts should preform “better” than the large LQFP parts in terms of how much closer you can get the bypass caps? Does anyone recommend BGA parts for that reason? I’ve never seen it.

The alternative is that 10mm isn’t going to hurt anything very little of it matters despite being a large topic of discussion.

Psi:
Have you considered capacitor array packages. The kind that have like 4 caps in one package.
They cost more, but are a little smaller.

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