Peter tosses the following out into the ether: Doug, who has been sniffing
ether, responds.
Hi, Folks,
*Good morning Peter.
I have a theory about one phenomenon out of two that I have just witnessed,
but seek your sage thoughts on the matter.
*Have you ever wondered if sage thoughts are what Emeril Lagassi has before
he prepares
Thanksgiving dinner?
Our fabrication shop was spray coating an FR4 / HASL-finished board with
Humiseal 1B31, which was masked with some very expensive Kapton tape. The
tape is meant to be a no-static, no-residue type. We noticed that, under
the UV inspection lamp in the spray booth, the coating appears to recede
from the edge of the Kapton tape - at least there is a "line" 1 or 2 mm
wide at the edge of the masking, that indicates (to me) a different
thickness of coating in that zone compared with the rest of the board. We
tried over-spraying the affected areas again once the initial coat had set
a bit, but the line reappeared. It only happens where there is Kapton tape.
*The tape adhesive is the problem. See below.
At the same time, the coating on the solder side of the board is a nice
shiny finish, while the component side has quite large areas that have a
more matte, slightly "rough" appearance. I doubt there is a connection
between the two phenomena, but you never know.
*It is quite possible that your board fabricator may have a cure process
where
top side and bottom side of the board get different cure profiles. The
solder side
may be fully cured and the top side less so (perhaps two different UV bulb
intensities).
In addition, the thermal energy from contact with the molten wave may
impart additional cure
and may change the surface dynamics.
My theory about coating recession from the tape is that there is a static
charge on the Kapton tape, in spite of the "no-static" presumption, and
that the coating particles in the spray are likewise charged from their
passage through the spray gun. The two like charges are then repelling one
another. I noticed that neither board nor spray gun are grounded, and think
that they should be (comments, anyone?).
*Nope. Static tends to bleed off pretty fast. It is not going to survive
in a spray
coating environment long enough to have the effect you describe. Should
the gun be grounded?
It would be prudent, but it won't solve this problem.
My secondary theory is that the coating is reacting with the tape adhesive
at the edge of the tape, creating a line of contamination that the coating
doesn't stick to properly. Kapton tape is a commonly used masking material,
though, and I haven't heard of this problem occurring with it before.
*Bingo. Not all Kapton tapes are created equal. We recently saw the same
thing that you describe.
The adhesive of the tape used was silicone based and everywhere the tape
had been, we had 1B31 dewetting.
We had to resort to a cleaning with Aquanox SSA (our normal in-line
cleaning chemistry), followed by an
isopropanol wipe of the affected area. Then coating stuck just fine. I
have found that many adhesives,
under thermal reflow conditions, will creep out a little, make a zone
around the edge of the tape that is
adhesive and no Kapton. The reason that some of these tapes say they are
residue-free is that it is very difficult
to detect silicone residues analytically (at least on the production
floor).
I haven't come up with an explanation for the matte areas of coating on the
component side of the board. The board had just come from the customary
cleaning and drying that I give all boards just prior to coating.
*See the theory above.
Can you come up with possible explanations and cures for what is happening
here?
*Well, the above is my best estimate, other than my theory of elves
sneaking in every night to cause problems.....
Doug Pauls
Rockwell Collins
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
