We have experienced serious bubble problems on at least three occasions.

In one case, we had applied polyurethane conformal coating to assemblies   
that had residual moisture on them.  Moisture under the coating can cause   
problems with any chemistry but it was particularly troublesome with this   
polyurethane because it reacts with moisture to generate carbon dioxide   
gas.  The coating bubbled like 7-Up.  Prior to conformal coat, the   
assemblies had been washed in an aqueous process and then oven dried but   
the bake cycle was not adequate.  The corrective action was to increase   
the pre-coat bake cycle. I'm not sure if your chemistry is sensitive to   
moisture.

On another occasion, we had lots of bubbles on an assembly that had been   
cleaned with isopropyl alcohol and then coated shortly after cleaning.   
 The assembly retained solvent and the solvent apparently volatilized   
during the elevated temperature cure and caused bubbles in the coating.   
 The corrective action was to implement a bake cycle between alcohol   
cleaning and conformal coat.

We use solvent based, heat cure, polyurethane and epoxy conformal   
coating.  We have found that we can cause bubbles in the coating by not   
allowing enough time for the solvent in the coating to flash off before   
we start the elevated temperature cure cycle.

I hope this information is helpful.


Mary Davis
Sr. Material & Process Engineer
Alliant Techsystems
206-356-3311
[log in to unmask]

 ----------
From:  David.Bruni[SMTP:[log in to unmask]]
Sent:  Thursday, November 07, 1996 4:57 PM
To:  technet
Subject:  Bubbles in Conformal Coating

     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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