We had seen this on our Nordson back when we still used VOC based
coatings. If you need to prebake the panels, you have a poor cure oven
setup.
Try one of these ideas-
Adjust the atomization pressure, nozzle size, temperature or
viscosity of the coating (thin or thicken it). If any of these are out
of the process window, you can get bubbles. Try a controlled
experiment and you should find the exact window.
Another cause may be your cure cycle. If the surface is allowed to
skin over prior to removing solvent from underneath, you will have
many bubble issues. We essentially tacked the panels and then elevated
the temperature above Tg, in less than 1 minute. No bubbles, no
problems, dry to the touch.
______________________________ Reply Separator _________________________________
Subject: RE: Bubbles in Conformal Coating
Author: [log in to unmask] at internet
Date: 11/8/96 1:40 AM
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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