Hello Doug
I think the via size will be based on coating viscosity, but in general one starts to see issues with greater than 10 mil diameter based on previous experience. The coating viscosity range can still be an issue with single component coatings, especially ones having moisture as the secondary cure mechanism. Even with a N2 pressurized tank, the initial exposure to air during fill up is enough to start curing mechanism so that the viscosity needs to be measured and adjusted after 2-3 days.

Regards,
Amol

From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Thursday, May 09, 2013 10:02 AM
To: TechNet E-Mail Forum; Amol Kane
Subject: Re: [TN] Automating Conformal Coating

Thank you Amol.  I would agree with the vias.  We have that happen on a number of our assemblies now where the wicking action causes problems.  When you say "if the vias are too big" is there a general range you are talking about?  Something like vias less than 8 mil diameter are not a problem with wicking but 15+ mils are?

Fortunately, we are only working presently with single component conformal coatings and have not had to go to two part coatings, which I agree would make things more difficult for process control, as Graham Collins said.  Nothing quite like trying to get set/cured coating out of a complex spray valve.

Doug Pauls



From:        Amol Kane <[log in to unmask]<mailto:[log in to unmask]>>
To:        <[log in to unmask]<mailto:[log in to unmask]>>
Date:        05/09/2013 08:32 AM
Subject:        Re: [TN] Automating Conformal Coating
Sent by:        TechNet <[log in to unmask]<mailto:[log in to unmask]>>
________________________________



A couple of things come to mind:

1. Design of thru vias with regards to the size and location when the assembly is designed for an automated spray process. If the vias are too big or close to keep out areas on the other side of the board, then you will have coating related defects on that side (usually coated first) during spraying the opposite side. Depending upon the inspection sequence, they may or may not get identified and corrected. Even if they are identified, they may make the assembly unsuitable for automated spraying without significant manual masking!

2. with regards to coating material, while a longer pot life is important, stable viscosity over the pot life is equally important to be able to control coating thickness and coverage for a given recipe setting.

Regards,
Amol




-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: Thursday, May 09, 2013 8:03 AM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [TN] Automating Conformal Coating

Hi Doug
No keep-out areas is a bit difficult, the board presumably has to connect to something, but what I have asked for (but not gotten) is that connectors have clear areas around them so that the measurement tolerance on the coated area can be high, and that the connectors be easily masked - preferably something we could put a rubber boot on, next best is easily taped to cover pins and mating area, worst are open body connectors.

Component height restrictions will depend on how versitile your coating machine is - the one I had was a 4.5 axis machine (x-y-z, theta, and nozzle tilt) and so for us height issues were a programming problem - not so much a running problem.

I would also add to the wish list a coating material that is agreeable with an automated process.  At a previous employer we had a high volume product that was coated - so an automatic candidate for automated spraying.  But the customer specified a coating material that had a 4 hour pot life, after which we had to flush the machine with toluene or it would seize up and need a full overhaul.  With an in-house project I'm sure you have more control over that factor.

regards,

Graham Collins
Senior Process Engineer
Sunsel Systems
(902) 444-7867 ext 211

On 5/9/2013 9:40 AM, Douglas Pauls wrote:
> Good morning all,
>
> Another conformal coating related question.  What, from an assembly
> point of view, would you tell a board designer are good design
> practices to make an assembly coatable from an automated standpoint.
>
> I think we would all agree that having an assembly with no keep out
> zones and a constant height for components to be the ideal.  The more
> holes we poke in the film and the farther we get from the ideal, the
> less we can automate coating.
>
> So, what would you tell a designer to do?
>
> Or, to put the shoe on the other hand, what kinds of things should
> they NOT do, or bad practices to avoid?
>
> And, yes, I know It Depends.
>
> Doug Pauls
>
>
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