Atul,

You may have two problems causing this insulative fault.  We have seen this
in some other troubleshooting projects we have worked on.  I believe that the
insulative film that you are experiencing is a weak organic acid (WOA) from
the no clean flux.  Most low solids fluxes are WOA-activated.  The WOAs are
an insulating material when dried on a surface, but they are not a very
durable film.  That is why you get contact after 3-4 insertions, you have
worn through the WOA film.

The source of the film is two-fold.  First, if you are spraying and wave
fluxing, you are putting high amounts of flux on, probably flooding the area.
 As the no-clean fluxes usually have a high level of isopropanol, capillary
action can carry the flux to places you never intend, such as the mating
surfaces of a ZIF socket.  We have had customers who have had the same
problem inside of membrane switches.  Secondly, you may be getting WOAs as a
result of pyrolization.  When a low solids flux burns off, either in pre-heat
or in wave solder (or hand soldering), where to the evaporated / burned
material go?  In many cases, the smoke or fumes are very pervasive and can
"vapor deposit" on the inner surfaces of components, such as sockets.  Such
deposited layers are also WOAs, but are usually more fragile, causing
intermittant failures, rather than the more consistent layer you are
experiencing.

To combat these problems, you must be more careful about where, and in what
amount, you are applying your flux.  More is not better.  Second, you need to
be more efficient in your fume removal.

Hope this was helpful.

Doug Pauls
[log in to unmask]