Martin,

        In my experience the problem with the solder paste building up on
the underside of the stencil is initially caused by poor gasket of stencil
to board. This can either be due to poorly finished PCB's or oversize or
poorly defined apertures.

        With regard to your underside cleaning issues, sometines the solvent
used in the  underside cleaning unit can have an adverse effect in drying
out the small quantity of solder paste on the underside of the stencil  very
quickly. This accompanied by poor undersceen cleaning performance can result
in a very quick build up of solder paste.

        The best advice I can offer is to look at the optimisation of the
underscreen cleaning unit including the paper being used and the solvent in
the tank. The other area to look at would be the gasketing of the board
against the PCB. Look at the aperture size and finish in the stencil. Have
the apertures been reduced correctly in line with the PCB pads?

I hope this is of some help.

If I can be of any further assistance please do not hesitate to contact me.

Doug Black
Technical Manager
Dek Precision Screens Division
Dek Printign Machines Ltd.
e-mail: [log in to unmask]
 ----------
From: Martin, Jesse
To: [log in to unmask]
Subject: [TN] Solder paste stencils
Date: 16 August 1999 20:08

Has anyone had a problem with solder paste stencils that have a build up of
solder paste on the bottom (PCB) side of the stencil that won't come off in
the stencil cleaner? I think what happens is the vacuum/paper wiper leaves
some on the bottom after each wipe and the more it wipes the harder it is to
get off. I can pop it off with an Xacto knife or the like but is very time
consuming. Only other thing that works is using a Scotchbrite pad but that
leaves a lot of scratches (which probably causes faster build up). Have also
tried stainless steel cleaner with no luck. We use an old Austin American
batch aqueous cleaner with saponifier at 120 degrees F. Any and all ideas
welcome.

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