So your problem is hare today, gone tomorrow.
Dewey
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ted Tontis
Sent: Thursday, February 05, 2009 4:14 PM
To: [log in to unmask]
Subject: Re: [TN] Coating material
Doug,
De-wetting occurs all over the assembly and pools in roughly
the
same pattern. I am pretty sure the consistent pattern is due to the
spray
pattern from our coater. SMT is the process prior to coating all
operators
use gloves throughout the process from start to finish due to coating
and
the assembly.
I just recently read about surface energy and I am in the process of
ordering a dyne pen. This was the first time I ever heard that surface
tension had a direct impact on coating.
Here is what we know
1. We do know that boards from the same lot code might coat fine
while
others show signs of de-wetting
2. Populated and un-populated arrays have the same problems
3. We sent out sample arrays to the coating supplier who cleaned
them
in a vapor degreaser and when returned they coated fine. The material
used
was Leksol AL
4. completed a Zestron flux and resin test and they came back
negative
5. There is no way to tell if the board is going to coat until
coating
is applied, nothing looks different between boards that will coat and
ones
that wont.
6. We have sent samples to a lab, ones that coated and ones that
didn't
for comparison. This is the second time we have sent boards out for
evaluation. The first tests came back with nothing but only one array
was
sent and that was prior to knowing that we had variation in our lots.
7. Today we changed the coating and the carrier from 1B73AP to 1B73
and
had better results. We think it is because of the Methyl Ethyl Ketone in
the
1B73 and it is lifting what ever is on the board allowing the coating to
wet
better. Or because MEK is just at good at killing brain cells then it is
for
thinning coating, we are hallucinating and seeing what we want to see.
Oh
look a bunny
Thank you,
Ted
_____
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Thursday, February 05, 2009 4:33 PM
To: TechNet E-Mail Forum; Ted Tontis
Subject: Re: [TN] Coating material
Ted,
Where do you see the dewetting and what processes come before coating?
In my experience, dewetting comes three general sources:
1. A problem with the surface energy of the component or solder mask.
This
may be due to mold release agents or the slick nature of the plastic
itself.
You want surface energies in excess of 40 dynes/cm. Surface energies of
30-35 dynes/cm are a problem. I use Dyne Pens as a quick check. Has
there
been a change in process or part supplier?
2. A problem with a barrier material. Again, it may be a mold release
agent, many of which are Teflon or sulfur based, or it may be a process
residue. Could be silicone residues, could be fingerprints, could be
any
number of things from second or third shift.
3. Dewey Whittaker.
Doug Pauls
Rockwell Collins
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