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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