This kind of thing usually has to occur over an somewhat extended amount
of time. The current in this area has to be fairly significant and
constant. The board won't flame (generally with the material you
describe) but it will slowly bake and smolder leaving the char marks you
describe. . There has to be some sort of high resistance short or a
carbon trail building up.
If you aren't already doing so you might want to consider a fuse
in the plus stuff on the card if you getting this kind of activity. Know
what your card draws and fuse it ever so slightly above that. The
farther above the demand current the longer the fuse will stay in place.
On the DC side you shouldn't be seeing a whole lot of significantly
spiky current draw so you can us a quick blow fuse if you want - I
probably won't preferring to control the long term draw with the fuse
rating. A fuse rupture chart will show you the approximate time to
rupture for a given current. The closer you are the longer it will take.
You just don't want to be in the region that still allows you board to
crisp up.
Even more important is finding out what is happening. A data logger
measuring current draw will probably be helpful. Then exercise the heck
out of things. If that doesn't show a problem you may want to look into
the physical construction of the PCB's and the vendor lots etc.
Gary
-----Original Message-----
From: Crabb, John [SMTP:jo...@exchange.scotland.ncr.com]
Sent: Friday, November 06, 1998 12:49 AM
To: 'pe...@itl.co.il'; emc-p...@ieee.org
Subject: RE: Burning Card Module
I have come across similar incidents, and "guess" that the
decoupling
capacitors have been installed with reversed polarity. The
capacitors
do not always burst into flame when power is supplied - it may
take
some time - and if the equipment is not powered up for very long
while it is being built and tested in the factory, it can show
up in the
field.
Proves the need for "integrated circuit packages, transistor
packages,
optocoupler packages, capacitors and other small parts to be
mounted
on material of flammability class V-1 or better". (IEC950,
clause 4.4.3.3)
John Crabb, Product Safety Engineer, NCR Financial Solutions
Group Ltd,
Dundee, Scotland.
> -----Original Message-----
> From: pe...@itl.co.il [SMTP:pe...@itl.co.il]
> Sent: 05 November 1998 22:17
> To: emc-p...@ieee.org
> Subject: Burning Card Module
>
> Dear Members,
>
> I am sure some of you can give me ideas on how to analyze a
card
> module which in four different occassions caused charring and
> smoke within the equipment. In all cases, fire did not spread
and
> the charring occurred in all cases, arounf decoupling SMT
> capacitors located between the Vcc and ground.
>
> The subject card sits in an industrial type computer within a
GSM
> equipment. The card is fed by 5 V and 12 Vdc from the computer
> power supply and employs two SE:V interfaces. It employs a
> piggy board with a receiver with direct connection to an
external
> outddor antenna.
>
> Any suggestions how this may have happened in the field, on
four
> different occassions and the charring/burning occured around
the
> decouplng capacitors.
>
> For this let us assume PWB is flame rated 94V-0 and is
> Recognized.
>
> Regards,
> PETER S. MERGUERIAN
> MANAGING DIRECTOR
> PRODUCT TESTING DIVISION
> I.T.L. (PRODUCT TESTING) LTD.
> HACHAROSHET 26, P.O.B. 211
> OR YEHUDA 60251, ISRAEL
>
> TEL: 972-3-5339022
> FAX: 972-3-5339019
> E-MAIL: pe...@itl.co.il
> Visit our Website: http://www.itl.co.il
>
>
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