Phil,

I think you are right. If  you are too generous with the aerosol,  you get 
so much solvent, that the acrylic gets too low surface tension, the board 
repels the intruder and makes the acrylic to  'close around itself' instead 
of wetting correctly on the board. If the board happens to have some 
nanometers of a fat stuff, the situation will lead to de-wetting.   Let me 
give you some examples (Pa exp -1) :
-glass 3800
-aluminum 1000
-solder 800
-fat 300
 -water 75
-polyester based glue 60
-epoxy adhesive 40
-cyano acrylat glue 30
-alcohol 20
-ethylen ether 10
-teflon 5

the more solvent,  the lower the surface tension, i-e. the board ('glassy') 
repels the coating.  Allegory:  water on a goose, where  the goose 
represents the fat. Easy peasy.

In technical terms, the adhesion forces must exceed the cohesion forces  to 
wet, which old man Dupré found two hundred years ago or so.

(Hope I do right, many decades since school....Brian use to correct me 
otherwise.)

Inge



--------------------------------------------------
From: "Kinner, Phil" <[log in to unmask]>
Sent: den 23 March 2010 06:55
To: <[log in to unmask]>
Subject: Re: [TN] Conformal Coating Issue?

> Rob,
>
> It looks to me to be the phenomenon of capillary flow rearing its ugly 
> head again.  Materials with low surface tensions (like silicones) tend to 
> be subject to capillary forces from vias and low standoff components which 
> tend to pull the material into thicker local areas (high fluorescence) 
> leaving behind lower volumes of material (lower fluorescence)... think the 
> principle of how the level of water in a straw is usually higher than the 
> level of fluid it is in.  This is usually made worse by the application of 
> heavy wet films.
>
> You are probably correct when you say that there is coating material 
> present in the darker areas, the fluorescent tracer is typically 
> ineffective below a certain minimum concentration and so you may not have 
> enough tracer present to fluoresce efficiently, resulting in the darker 
> areas you see.  This is usually a cosmetic issue, but if your customer has 
> issues (and cosmetic appearance is usually a key requirement), then a 
> simple fix may be to apply a super thin first coat (almost as a primer), 
> dry and then apply a normal coating thickness, or else simply apply a 
> second coat to what you already have assuming a rework scenario.
>
> Typically, aerosols are formulated to be heavy on solvent (low solids 
> content) to overcome the poor atomizing ability of aerosol valves and 
> switching to HVLP (or similar) spray application instead would likely 
> enable your contractor to apply a thinner wet film at higher solids 
> content and the issue may mysteriously vanish...
>
> That would be my first step at corrective action.  If that doesn't work, 
> feel free to contact me offline.
>
> Hope of some help.
>
> Best Regards,
>
> Phil.
>
> Phil Kinner
> Business Director
> Chase Electronic Coatings
>
> PLEASE NOTE NEW ADDRESS & PHONE #
>
> Chase Electronic Coatings
> 295 University Av, Westwood, MA 02090 USA
>
> Cell:+1  781.789.4986
> DDI:+1 781.332.0741
>
> www.humiseal.com
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
> Sent: Tuesday, March 23, 2010 8:53 AM
> To: [log in to unmask]
> Subject: Re: [TN] Conformal Coating Issue?
>
> Rob,
> What exactly is the customer's beef?  Are they expecting to see a 
> completely uniform film everywhere across the board?  I don't really see 
> anything wrong with the coating job.
>
> What you may be seeing is residual material in the holes that can weep 
> out.  Do you blow out the holes with compressed air after batch cleaning?
> Do you bake the boards after cleaning?
>
> Doug Pauls
>
>
>
> Rob Strecker <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 03/22/2010 02:46 PM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to Rob Strecker 
> <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> [TN] Conformal Coating Issue?
>
>
>
>
>
>
> Hi all,
> We have an issues with one of our customers that we build PCA's for.
> After we complete the build the customer sends it out for conformal
> coating.  It appears the coating or florecent does not like via holes.
> I believe they are still getting good coverage and I don't think this is
> a dewetting problem as there is still coating present in the darker
> locations.  I don't see these boards after they get coating but that's
> what I am tolled.
>
> These boards are double sided, lead free using water soluble paste/flux
> washed in a batch cleaner with a 6% chemical concentration.  We go
> through many rinse cycles and clean to 1.4Meg (resistivity monitoring of
> the rinse water, for what it's worth).
>
> What might the casue of this be?... wash solution in the vias??  Below
> is a link to the picture.
>
> http://ipc-technet.groupsite.com/galleries/photo/117131
>
>
> Rob
>
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