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what is the reason of Voltage drop on PCB

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I have a stack PCB with top PCB and bottom PCB.

The +5V on bottom board is powered from top board.

Current situation is, when the top board and bottom board are attached, the +5V dropped down to +3.6V. I have confirmed the +5V power plane is not shortcut to ground.

After all components connected with +5V power plane are removed, the +5V is still dropped to +3.6V.

Please let me know what is this problem?

Thanks.
 

If you said all components have been removed, disconnect the bottom PCB that causes trouble and using an ohm-meter measure resistance between +5V and 0V ..
There has to be “something” that causes this voltage drop, for example a solder or copper bridge ..

IanP
:wink:
 
Those boards are assembled by machines. How this can happen?

Thanks.
 

I guess contamination during manufacturing is one possibility but the most common cause I have come across is splashing of the tin during hot air leveling causing an almost invisible short.

Keith
 
Thank you for you answers.
 

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