The problem you are encountering is entrapment of air in the small vias.
These appear to be filled after screening, but upon drying the air is
released due to initial lowering of viscosity which allows the entrapped air
to escape and also sealing the via with a thin film , holding the air until
it expands and then blows out. The only way to possibly solve the problem is
to place each board in a vacuum prior to drying and evacuating to at least
30” of Hg. This will draw all the air out of the vias and then the mask will
flow in to the vias.
I know this is not the normal set up for masking boards but it is the only
way to do it effectively.  Thinning out the mask will only cause other
problems that will show up later down the line.

-----Original Message-----
From: Chi-man Chan <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, September 24, 1998 6:00 AM
Subject: [TN] Help! Plugging vias


>Currently we have a big problem on plugging vias with solder resist. Our
>customer requires that vias must be completely filled with solder
>resist. (i.e. light cannot go through). The via size is 0.3-0.5 mm. We
>use liquid imageable solder resist. I have tried to print with 43T then
>32T open screen wet on wet on both side of the board. It does not work.
>The vias seems to be blocked with resist after screen-printing. However,
>some vias open again after track dry and developing. I have reduced the
>thinner from 5% to 2%. It does not work and causes a lot of bubbles
>among tracks. In order to completely plug these vias, we now print these
>boards with dot pattern before solder resist  printing and after hot air
>solder leveling. Some vias are very close to SMD. It is a very hard and
>time-consuming job. I think there should be an easier way to do this
>job.
>
>Any feedback is appreciated.
>
>
>Regards,
>
>Chan Chi Man
>[log in to unmask]
>
>################################################################
>TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
>################################################################
>To subscribe/unsubscribe, send a message to [log in to unmask] with following
text in the body:
>To subscribe:   SUBSCRIBE TechNet <your full name>
>To unsubscribe:   SIGNOFF TechNet
>################################################################
>Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section
for additional information.
>For technical support contact Hugo Scaramuzza at [log in to unmask] or
847-509-9700 ext.312
>################################################################
>
>

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################