[SI-LIST] Re: Power/GND and fires
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Shawn Arnold" <shawn@xxxxxxxxxxxxxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Wed, 7 Jun 2006 07:35:28 -0700
Shawn,
I've seen this a dozen or more times. It has always been traced to copper
to copper clearances that are not large enough inside a PCB. Common
problem when allowance isn't made for the normal tolerances in the PCB
fabrication process. I'd check there first.
When these clearances are not large enough a short develops, copper is
vaporized and trapped inside the PCB making a very nice low resistance arc
welder. Continues until a hole is burned into the PCB and the vapors
escape.
> [Original Message]
> From: Shawn Arnold <shawn@xxxxxxxxxxxxxxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 6/6/2006 3:33:58 PM
> Subject: [SI-LIST] Power/GND and fires
>
> 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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