FreeLists / si-list / [SI-LIST] Power/GND and fires

["Shawn Arnold" <shawn@xxxxxxxxxxxxxxxxxxxxxxxx>]

Hi All,
 
I am in the process of diagosing a backplane to plug-in card fire in a
rackmount chassis which erupted after the system was left running
overnight and would appreciate any inputs.
 
The backplane has/had discrete power planes for +3.3V and +5V along with
multiple GND planes. The powers were delivered to the backplane from
Vicor type power bricks and solidly attached with multiple power tabs
and power bars. As this is a large backplane, the total current
available is 120A.
 
At the time of the "incident", the 13 slot system was lightly loaded
with only 1-2 cards but all power bricks were supplying voltage to the
backplane. There are no indications of capacitor failures and the fire
seems to have begun on the plug-in card, a single-board-computer (SBC)
immediately at/after the press-fit 2mm hard metric connectors and before
the first row of ASICs. There were no obvious pin shorts due to bent or
misaligned pins 
 
The immediate suspicion is run-away voltage due to thermal/resistance on
the SBC and the lack of current limiting on either the power modules,
backplane, nor SBC, of course this will be addressed. We intend on
measuring the pin-pin resistance at low power and full power for clues
to satisfy customer concerns.
 
We will also be concentrating on the PCB design/fabrication in this same
area looking for potential shorting mechanisms. 
 
All comments and enlightening fire stories welcome.
 
Regards,
Shawn   

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