Hi Brian,
Thanks for the reply and to everyone else who replied. In response to
one of your comments you wrote: "In reality, I don't see the reason for
the second wash, as your in-line cleaner should be more than
sufficient."
Our boards get handled a lot between the contract manufacturer and
conformal coating. Most of the boards go through receiving inspection
and test prior to conformal coating. We want to make sure that if
anything has been added during that handling that it has been removed
prior to conformal coat.
I believe that the smoking gun is the way that our people have been
loading the batch washer. After I got all the responses from TechNet I
went out and observed them filling every slot on the wash rack leaving
little room between the boards. They were also placed all the way in the
corners of the racks. I've change the way we load the racks and will see
what happens.
One more piece of information, we only see this issue with one of our
contract manufacturers. I believe the reason is that they use a
selective wave solder fixture which prevents a lot of the via's from
being filled. The other contractors do not use pallets and all the via's
are filled. I believe the open via's can entrap saponifier.
We do a ROSE test on one board from each wash batch and depending on
where that board was removed from I guess it could pass or fail. I'll be
checking all of this and see what our results are.
Thanks again,
KennyB
-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: Friday, April 06, 2007 8:35 AM
To: TechNet E-Mail Forum; Bloomquist, Ken
Subject: Re: [TN] Conformal Coat Dewetting
I can make only a guess with the info supplied, but I think there is
more than a 50% chance of my being right. Commercial saponifiers are
typically a blend of an amine, usually monoethanolamine, and one or more
diglycol ethers or similar. The latter are like surfactants and have
"lipophilic ends" which bond easily to things like substrates and
"hydrophilic ends" which don't. Normally, the bond between the latter
and the wash water, combined with the mechanical spray action, is
sufficient to break the hydrogen bond of the ether to the substrate and
the board is thus cleaned. If, for any reason, this doesn't happen, then
you may easily get ether residues on the board surface or in the holes,
where they can creep out during drying. I would suggest that the
incriminated boards have either been shadowed by other ones in the
baskets or have been placed in the corners of the baskets, where the
spray energy is minimal. "Dishwasher" type machines are not noted for
high energy spraying and the spray bars are a compromise between the
jets required for washing and the fine spray required for rinsing.
In reality, I don't see the reason for the second wash, as your in-line
cleaner should be more than sufficient.
As you use a water-soluble flux, a saponifier addition should not be
necessary; have you tried it without? If, for any unstated reason, it is
required, I suggest you keep the percentage down to a minimum, say 2%
max.
Another possibility is that drying is incomplete after the second wash
and you may have residual water in the holes and under the components.
Have you tried baking before coating?
Ionic contamination testing will not detect diglycol ethers which are
totally non-ionic.
Brian
Bloomquist, Ken wrote:
> Good afternoon all,
>
> I've go some assemblies that are not taking conformal coating in some
> areas. The boards are reflowed and then selective wave soldered with a
> pallet. They us a water sol flux and run it through an inline wash
using
> DI. We wash them in a batch washer using a mild saponifier prior to
> coating. The boards are cleanliness tested in a ROSE tester and then
> coated with polyurethane.
>
> It is only on a few boards and it appears to almost be coming from the
> via's. If we try to brush coat over it it still won't coat. If we
clean
> it with IPA then it coats just fine.
>
> We've ruled out silicone and finger grease.
>
> I have a couple of photos so you can see what it looks like. One is
> black light showing the void areas. The other one is where we tried to
> touch up the coating around a via and you can see how it pulled back.
>
> http://www.ema-wa.com/DSCF0845ViaContamAreas.jpg
>
> http://www.ema-wa.com/DSCF0840_Via_Close-up_Y1_WL.JPG
>
> Thanks,
>
> KennyB
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