Larry,

The mask is being drawn into the vias causing the material to thin out.

There a few things you can do:

1. Change to dry mask if you are using LPI.

2. Decrease your via size.

3. Add "via lids" to the legend silkscreen to reinforce the mask over
vias (if you have a silkscreen).

4. You can leave the vias uncovered by mask and plate them during the
HASL process (if uncovered vias are an option)

Are you calling out a min. mask thickness on your fab dwg. or referencing
IPC-600?

Scott
 ----------
From:  DesignerCouncil Mail Forum.
Sent:  Wednesday, September 24, 1997 2:53 PM
To:  DesignerCouncil
Subject:  [DESIGNERCOUNCIL] Solder Mask Problems?

I have switched to SMOBC (solder mask over bare copper) and have noticed
that
some of the via's on the board are well covered while others seem to have
only a light coating - I can see the copper hue of the traces - in some
via's
there is a slight splash of solder from the plating process. I don't
require
plugs for vac. fixtures etc.

My main concern is these boards are subject to harsh conditions (coal
mines)
and I'm worried about corrosion at these points.

Has anyone else noticed this with SMOBC?   The board house suggested
going to
a smaller drill size for via's (015). Don't suggest a new house - these
guys
are great and have been working on it.

What is the IPC spec for min coverage for class 2 boards regarding SMOBC?
 Should I be concerned?

Larry Machak (designer, not cad operator!)
Saminco Inc.

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__________________________________
Scott Buscomb, Senior CAD Designer
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voice 770 338-6170
MTCC/Panasonic, Pager Development
1225 Northbrook Pkwy, Ste 2-358
Suwanee, GA USA 30024
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