I would disagree with your blind acceptation of isolated bubbles, Inge.
I would insist on knowing the cause of the bubbles before accepting
them, and their location. It would not be oxygen that would worry me;
have you thought about water vapour that could condense? Even if there
is only one now, will there be more in a few months?
Now, let's look at modern high-speed/frequency electronics. An airfilled
microbubble between a transmission line pair will have little effect
because substituting a dielectric constant of 1 for, say, 3 over such a
tiny distance will hardly cause a visible blip on your TD reflectometer.
But are you sure that a dielectric constant of 72, instead of 3, would
have so little effect? Not to mention the capacitive effect from
neighbouring tracks causing crosstalk?
And one little thought for you to ponder: all vesication probably starts
with a single microbubble.
Sorry to upset your complacency.
Brian
Hernefjord Ingemar wrote:
> If you get bubbles that 'look like foam', there is something wrong in
> the process. We use Humiseal acrylic coatings, and have tried Urethane
> before. Single microscopic bubbles are acceptable, which means in the
> range 1 mil or less. We use to say, that the bubbles must not exceed one
> third of the total thickness of the coating.
>
> A bubble is entrapped atmosphere, and the oxygen content can
> theoretically cause oxidation. The oxygen content in such little a
> bubble is not enough to cause any oxidation. When you use the word
> corrosion, one use to think of chemical reactions serious enough to
> damage the whole solder joint, or make the lead fall off the package. I
> would like to see the microscopic bubble that could cause such damage!
> Would be no less than a miracle! Single air bubbles is nothing to be
> worried about, foam of bubbles is something you need overlook.
>
> Inge
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Joel Alexander
> Sent: den 23 april 2007 21:47
> To: [log in to unmask]
> Subject: [TN] Conformal Coating Voids
>
> We are using Urethane coating on an assembly that uses 63/37 solder. We
> are using organic water soluble flux. We are Ionic testing after wash
> and we are well within IPC cleanliness specifications. We installed a
> selective coating machine and in our initial run of product we had some
> small bubbles that look like foam. Our customer does not like it but our
> stand is that it meets IPC-A-610 and J-STD-001 requirements since there
> in no obvious voids between conductors. They were worried about
> corrosion if a bubble was against a lead. The bubbles are fully
> encapsulated. Does anyone think this is a valid concern?
>
>
>
> Joel Alexander
> Quality Assurance Manager
> TT APSCO, INC.
>
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