>
>
>To Anyone Interested,		
>	We have found the described problem with our acrylic conformal coatings to
>be associated with a skin that forms on the outer surfaces the coating as the
>coating goes through it's normal curing. Any coating process which deposits a
>relatively thick (both in dimension and viscosity) coating skins over and
>inhibits the evaporation of thinners and or other volatiles from the
>underlying material. If the CCA's are then introduced into an environment of
>elevated temperature (higher than processing or initial curing temperature)
>the trapped volatiles expand and form the bubbles.
>	We were never successful in thinning the coating sufficiently to eliminate
>the bubbles. If a thinner material, e.g. lower viscosity, was used then the
>edge definition from the Select Coating System was not acceptable. We did not
>consider masking the CCA's as this would defeat the labor savings of a Select
>Coat System.  In general the thickness which gave us acceptable
>coating/noncoating edge definition would always bubble to some degree if
>exposed to temperature.
>	We were successful in eliminating the bubbles by allowing the CCA's to air
>cure at room temperature before subjecting to elevated temperature or test.
>Most probable reason is the low room temperature outgassing pressure can't
>create visible bubbles, something about pv=nrt. Minimum cure time for the
>thickness and coating we were using was on the order of 10 days. Obviously
>this is a significant problem for Demand Flow/ JIT assembly lines or those
>concerned with WIP inventory and cycle time. 
>
>Semper Fi
>Tom   
>
>----------
>From: 	Phil Bavaro[SMTP:[log in to unmask]]
>Sent: 	Monday, April 14, 1997 12:36PM
>To: 	Paul Stolar; [log in to unmask]
>Subject: 	Re: Bubbles in Conformal Coating
>
>Is there any chance that the boards have moisture in them which is then
>reacting with the coating?
>
>We used to get very pretty acrylic coated CCAs by prebaking the masked,
>clean assy.
>
>Phil
>
>
>
>At 7:56 AM -0500 11/8/96, Paul Stolar wrote:
>>     Sometimes the cause is the CC is too thick and does not allow the
>>     gaseous products to get out. Try thinning the CC.
>>
>>
>>______________________________ Reply Separator
>>_________________________________
>>Subject: Bubbles in Conformal Coating
>>Author:  [log in to unmask] at Internet
>>Date:    11/7/96 5:06 PM
>>
>>
>>     I threw this question out once before but I still cannot determine the
>>     source of bubbles in our conformal coating process.  We are using an
>>     acrylic coating and a Nordson Select Coat System.   The bubbles come
>>     and go from day to day.
>>
>>     We have looked at viscosity, temperature, humidity, pressure,
>>     z-height, contamination, micro-adjust, and a few other variables. Has
>>     anyone else seen this phenomenon or do you have any insight on it?
>>
>>     Any help is very much appreciated.
>>
>>     Thank you.
>>
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>
>
>Regards,
>
>
>Phillip A. Bavaro
>Senior Manufacturing Engineer
>[log in to unmask]
>(619) 658-2542
>
>
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