Tony,

I agree that mousebites are caused by precipitation of air from a
supersaturated bath, and that it is not a single aspect problem, both
plating and dry film processes must be examined.  In addition to the
items that others have contributed, there are three additional conditions
that we have observed that may contribute to the problem:

1.  Supersaturation of air in the plating solution can also come from
temperature changes.  As the temperature goes up, the solubility of air
in the bath goes down.  This is similar to the effect seen in a cold can
of beer vs. a warm one, the CO2 is less soluble at the higher temperature
resulting in higher pressure.

The bath becomes saturated with air at the temperature at which it is
running, no problem at this point.  If the bath temperature rises, the
solubility of the air decreases resulting in supersaturation and, if the
conditions are right, mousebites.

Track your bath temperature through the day. If you have a variation of
more than a couple of degrees, this may be a contributor.

2.  A majority of the shops in which we have observed mousebites also
have an "exaggerated resist foot" or residue problem out of dry film. 
The foot is usually greater than 0.5 mil in length and has been observed
to be up to 2 mils long.

The foot can be observed by turning off the plating current completely
for about 30 sec. every 8-10 min. during the plating cycle.  A properly
prepared x-section across narrow traces will show grain boundaries at the
current interruptions revealing that the plating does not start up
against the resist side-wall.  In extreme cases, a normal x-section
across narrow traces (with the resist still on the sample) will show that
the plating never reaches the side-wall.  Most epoxy mounting compounds
will allow mounting of the x-section sample with minimal effects on the
dry film.

This condition would indicate that an examination of the dry film process
is appropriate.

3.  Mousebites tend to show up mostly on one side of the panels.  We have
found that panels that are not hanging vertically in the tank can
exaggerated the problem.  Angled panels also contribute to poor
distribution of the copper deposit.

Good luck, and if you require further discussions you can e-mail me
directly.

Paul Galatis
Electrochemicals

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