STM32F779AIY6 - What if NC pins are connected to GND?

[Exit0815]

Hello,

i just received some custom boards with a STM32F779AIY6 on it.

3V3 net and GND net have a short circuit when MCU is soldered, without MCU there is no short circuit.

I checked everything, it should be fine.

But what i just found was that the datasheet says:

 NC ball must not be connected to GND nor to VDD.

But they are. They are all GND in my design. Big mistake.

Can this cause the short circtuit or should this be unrealistic and the problem must be somewhere else?

I hope someone can help me, it will be hard to remove those balls to test it.

Thanks a lot.

[S.Ma]

There is not much that can be done when the datasheet recommendations are not followed.

It voids any warranty as well.

[Exit0815]

Right, i don't care about warranty actually. I just need to get the pcb up and running.

And yes, i know it is wrong and against the recommendations, but too late now.

The question is if someone knows if the NC pins (3 in each corner) will cause a short circtuit between VSS and VDD if they are connected to GND.

Maybe someone else did the same big mistake than me and can say that my problem is somewhere else.

Design will be changed anyway, but it makes no sense to waste more money if the NC is not internal connected to anything and there are more problems.

[S.Ma]

Well use an ohmmeter, flip the BGA balls up (or in a socket), and check each NC ball against GND and VDD. The other challenging way is to put resist over the ball not to solder it (no solder paste as well).

Acrobatic.

[Exit0815]

Thanks.

The problem is the 0.4mm pitch.

Not easy to measure and with a standard low budget smd station it is also hard to place a new small bga to the exact position without the NC but I will give everything a try.

Hopefully the ordered usb microscope is useful to measure NC against VDD and GND.

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