Pat,

All polyimides are not created equal and they have such a wide variety
of electrical / mechanical / chemical / thermal properties that your
really have to know specifically what material is being discussed. You
can get yourself into real legal trouble assigning all polyimides the
label Kapton.

Kapton is a registered trademark of the DuPont Company and chemically
the most common (HN) subtype is:
N, N' -p, p'-oxydiphenylene pyromellitimide

In a traditional (adhesive based) flex circuit the nonflammable
polyimide acts to help suppress the natural flammability of the
adhesive. Additives can also be integrated into the adhesive formulation
to provide some degree of Fire Retardancy. Adhesiveless flex circuits
are inherently nonflammable. The various flex circuit constructions I've
seen pass UL flammability testing were self extinguishing, not
self-sustaining for combustion.

Andy Magee
Flex Guru - Consulting
[log in to unmask]
(937) 435-3629



Pat Hogue wrote,

     Before getting started I want to say that references to Kapton
should
not be taken to necessarily indicate a specific reference to Du Pont's
brand
of polyimide.

     I have not read all of the postings on this subject but the few I
have
read did not mention the problem of arc-tracking that causes Kapton to
become self-sustaining for combustion.

     My first encounter with Kapton's ability to burn when heated
electrically was during ground operations related to preparing the
Magellan
(also know as Venus Radar Mapper) satellite at Martin Marietta in
Denver.
One of our technicians tried to blind-mate a connector but didn't get
the
clocking right.  As I recall (from around 1991) contact was made between
a
hot battery pin and a ground pin.  The Kapton caught fire.  It wasn't
too
long after this that NASA released a video showing how dramatic the
combustion event could be, and how flame propagation continued after
power
was turned off.

     I think that the problem has to do with the chemistry of the
pyrolysis
event peculiar to Kapton.  I'm not sure I can accurately describe the
sequence of events but I seem to recall that as the molecule breaks
apart it
leaves conductive carbon tracts that assist in the heating and
decompostion
of further material.

     A few months ago 60 Minutes (or it might have been 20 20 or similar

news show) put forth the Kapton self-sustaining combustion theory for
the
crash in Nova Scotia.

     Radiation cross-linked Tefzel (ETFE) wire insulation is the current

favorite here.

Pat Hogue
Materials and Processes Engineering
Honeywell Satellite Systems Operation
Glendale, AZ 85308
(602) 561-3748

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################