In days gone by, I worked on just such animals. Matte solder masks were
developed specifically for reduction in solder balls and all our testing
indicated it was the way to go. This was due to the no clean fluxes being
used which did not eliminate solder balls from the gloss masks because they
were "no longer cleaned".  The fluxes tended to run off (dewet) from the
gloss masks as the solder wave passed over the board thus leaving a dry
surface for the trailing end of the wave to adhere to, in part due to the
low Tg of the masks, and no cooling effect from the evaporating flux which
wasn't there any more. The matte mask surface is like the rocky mountains
and will trap most liquids into puddles due to this very irregular surface.
This helped in retaining flux and even though the surface area is increased,
the peaks and valleys were close together so no solder ball could get
trapped and the evaporating flux blew off any that did get stuck. (in theory
only).  The problem of high ionic contamination is due to the OA flux which
is highly activated and wets most surface due to the wetting agents used.
Highly activated fluxes tended to show poorer ionics because of the surface
characteristics and very dependent on cure. Even though with the proper cure
and the right flux, we were able to achieve acceptable ionics.
Undercured mask would be a disaster. One way to check this out is to have
some Meseran tests done at different controlled conditions to see what your
window really is with the current masks.  The whole thing becomes even more
complicated with PISM due to the large amount of processing and control
required.
I hope this has given you some ideas as to where you can improve your
process.

Have a great day.
Fred Shubert

----- Original Message -----
From: "Michael Forrester" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, January 13, 2000 4:40 PM
Subject: [TN] Soldermask and Ionic Contamintation


> We recently changed some of our PWB's soldermask from a glossy to matte
finish.
> Since then, we have had problems with contamination
> on the surface of the PWB's due to poor cleaning.  To fix the problem the
finish
> was changed back to a glossy.  There is an effort to change all
> of our boards to a glossy soldermask.  I believe it is not an issue of
matte vs
> glossy but rather the porosity of the specific soldermask being used.
> Our boards are high density surfacemount with frequencies above 1 GHz
running
> through the circuitry.  I prefer the matte finish for ease of inspection
> and reduction in solder balling.  We use an OA flux in manufacturing the
PCA.
> Can someone give me the Pros and Cons of matte and Glossy soldermasks?
> Also, am I correct in stating that whether the finish is matte or glossy
has
> nothing to do with whether or not we will have ionic contamination after
> cleaning?
> Thank you.
>
> Best Regards,
>
> Mike
>
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