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January 2004

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Subject:
From:
Brian Toleno <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 30 Jan 2004 11:39:28 -0500
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As a supplier of solder/fluxes and conformal coatings I would just like to
second (third, etc.) what some of our esteemed colleagues have said.
Testing is critical in these cases.  It is certainly possible, we have
several customers who build high reliability (automotive, military, etc.)
who have qualified material sets.  Some of the places to look where there
might be hiccups would be if there is rework - insuring that the flux used
in that operation is compatible with all the other material systems being
used (not only the conformal coating, but also the other flux residues).

In addition to possible adhesion problems between certain flux and coating
systems, there is also a danger with some conformal coating chemistries
have their curing mechanism poisoned by flux residues.  We have observed
this in the past, it is rare, but it will lead to serious problems.

Also, if you do end up cleaning, making sure any saponifer used will clean
the flux used and that you completely remove any saponifier residue, and
dry the PWB well prior to coating.

Basically, it is going to come down to you doing the qualification to
insure that you are comfortable with the reliability achieved when using
the materials you have chosen in your process.  It certainly would be a
good idea to talk to your material suppliers (paste, flux, and coating)
and make sure that there isn't already some data on those combinations.
Even if they say there is no known problems, you still need to test in
your process, because process conditions will play a role in the nature of
the flux residue.  I hope this helps.

Brian J. Toleno, Ph.D.
Sr. Applications Chemist
Loctite Electronics
15051 East Don Julian Rd
Industry, CA 91746
Ph: 626-968-6511 x304
Cell: 626-215-0879

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