Peter,
I have experienced no less than three board fires within enclosures in
three different companies that I have worked for over the past twenty
years. My present company excluded. My first one was related to
modules(cards) not being screwed into a chassis. Due to vibration, the
module "walked" out from the connector mating pair enough to build up heat
over time and cause a fire. The second was due to cracked by pass caps.
The third was the most interesting. The pc board was reworked at a field
location to replace a defective chip. The rework technician did not clean
the board properly. Instead of cleaning, he actually spread solder
particles mixed in the flux. When the board heated, the solder particles
came back together and caused a short. Most power distribution systems
handle a hard short. Power supply outputs foldback under short circuit.
Many power systems require high current and the type of short I described
may seek current for ever without ever shutting down. Even forty watts on
a board in a specific area gets hot enough to cause damage. Another case
for overheating or flame outs could be contaminants in the dielectric of
the pc board. My experience would say look at any rework with an electron
microscope. Any QA lab should be able to help.
-John
[email protected] (Peter Merguerian) on 11/05/98 05:16:57 PM
Please respond to [email protected] (Peter Merguerian)
To: [email protected]
cc: (John Loiselle/US/3Com)
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: [email protected]
Visit our Website: http://www.itl.co.il
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