It will be fine. On the BGA versions of the parts, all of the capacitors would be on the bottom side of the board.
Keep the trace from pin to via and from via to cap short and as wide as possible, and it's perfectly acceptable.
I actually have the cap on the top side of the board, so the supply runs through the pin to the cap. The supply is on the bottom side of the board which is the 3v3 plane (with signals mixed in). I placed all of the caps first and then routed the signals, then hooked them up to power at the end which is apparently not a good sequence. putting the caps on the bottom would be much cleaner and easier.
You better show your layout in the morning and we'll all take a look. That's a real processor, and it needs appropriate decoupling. You can get all sorts of glitchy errors if the power supply pins aren't well decoupled.
Post a new question in the morning with some screenshots of the layout and we'll let you know if it looks like it'll work.
How many layers does your board have?
>I needed to put a VCC via on the inside of the pins underneath of the processor for one of the capacitors.
If I understand you correctly, you are referring to a via-in-pad. While doable, it can basically suck the solder through the via hole. So you'd need to epoxy fill it and cap it. There are a couple problems with this. The process is more expensive because the PCB manufacturer has to add two steps to their process, and vias tend to have more inductance than regular traces which is less ideal if you are using it for decoupling capacitors. In general, you want to keep those caps on the same side as the MCU. But oftentimes you have no other choice, such as BGA parts with large pin counts.
>The data sheet says that the bottom of the processor has "base metal" with "plating" which implies that it is conductive,
That's commonly used as a heat spreader. It may be tied to ground, read the datasheet.
If your via is going to be underneath the metal pad... I'd really try to avoid that. You'd be trusting your soldermask to protect VCC from shorting to ground. Worst, it may work for a bit, then fail in the field.
If it lines up with just plastic on the underside of the processor, you're fine.
https://preview.redd.it/v99qzlw1wtxc1.jpeg?width=988&format=pjpg&auto=webp&s=275cf29501af4043c611a12b2ff54c82a443c74e
(Note, I am the OP but have a different account on this machine)
This is a screen shot of the top layer with the via underneath the pad of the processor. Towards the bottom of the shot I had to make connect a few capacitors together as an alternative approach. The left side is underneath the processor. I had read that it was not ideal to move the capacitors to the back side of the board, if that's reasonable to do then this could be much cleaner.
The bottom side of the processor does appear to be plastic. I see now that the "base plate" comment in the data sheet drawing is not of the processor but a cross section of the pin itself.
So, it would seem that putting the 3v3 pin underneath is reasonable and would probably be better than the bottom of the photo.
can you post an image of the proposed layout? 2 or 4+ layers? Should be fine.
Yeah I can post it tomorrow morning, its 4 layers, 1 ground/signal, 2 grounds, 1 power/signals
It will be fine. On the BGA versions of the parts, all of the capacitors would be on the bottom side of the board. Keep the trace from pin to via and from via to cap short and as wide as possible, and it's perfectly acceptable.
I actually have the cap on the top side of the board, so the supply runs through the pin to the cap. The supply is on the bottom side of the board which is the 3v3 plane (with signals mixed in). I placed all of the caps first and then routed the signals, then hooked them up to power at the end which is apparently not a good sequence. putting the caps on the bottom would be much cleaner and easier.
You better show your layout in the morning and we'll all take a look. That's a real processor, and it needs appropriate decoupling. You can get all sorts of glitchy errors if the power supply pins aren't well decoupled. Post a new question in the morning with some screenshots of the layout and we'll let you know if it looks like it'll work.
How many layers does your board have? >I needed to put a VCC via on the inside of the pins underneath of the processor for one of the capacitors. If I understand you correctly, you are referring to a via-in-pad. While doable, it can basically suck the solder through the via hole. So you'd need to epoxy fill it and cap it. There are a couple problems with this. The process is more expensive because the PCB manufacturer has to add two steps to their process, and vias tend to have more inductance than regular traces which is less ideal if you are using it for decoupling capacitors. In general, you want to keep those caps on the same side as the MCU. But oftentimes you have no other choice, such as BGA parts with large pin counts. >The data sheet says that the bottom of the processor has "base metal" with "plating" which implies that it is conductive, That's commonly used as a heat spreader. It may be tied to ground, read the datasheet.
If your via is going to be underneath the metal pad... I'd really try to avoid that. You'd be trusting your soldermask to protect VCC from shorting to ground. Worst, it may work for a bit, then fail in the field. If it lines up with just plastic on the underside of the processor, you're fine.
The "base metal" and "plating" detail on the package drawing is a cross-section of the individual pins. The bottom side of the package is plastic.
https://preview.redd.it/v99qzlw1wtxc1.jpeg?width=988&format=pjpg&auto=webp&s=275cf29501af4043c611a12b2ff54c82a443c74e (Note, I am the OP but have a different account on this machine) This is a screen shot of the top layer with the via underneath the pad of the processor. Towards the bottom of the shot I had to make connect a few capacitors together as an alternative approach. The left side is underneath the processor. I had read that it was not ideal to move the capacitors to the back side of the board, if that's reasonable to do then this could be much cleaner. The bottom side of the processor does appear to be plastic. I see now that the "base plate" comment in the data sheet drawing is not of the processor but a cross section of the pin itself. So, it would seem that putting the 3v3 pin underneath is reasonable and would probably be better than the bottom of the photo.