> I'm sitting here working on a board, and I can't figure out something-
> it seems like tradition that you don't put your soldermask over a via-
> that's the default in Eagle* and how it has worked out in most layouts
> I've done.

The problem that arises is that the solder resist will flow down the hole i=
n the via, leaving possible thinning of the resist at the hole edge. If the=
 via was not plated as it would be covered by resist then the thinning of t=
he resist may leave exposed unplated copper.

> Is there good reason for this with modern PCB fab equipment? I realize
> that an un-masked via makes for an easy rework/test point.  I can also
> see that I may not want to put mask over a hole (where there isn't
> board to cover), but the silkscreen also seems to be left off where
> there isn't mask, and I want to make sure I don't mess with my
> silkscreen (if I need to get at the via, I'd rather scrape off the
> mask/silkscreen).

One of the tricks for insisting on solder resist over vias is filling vias =
with something before the resist screening process so the resist doesn't fl=
ow down the hole. This becomes another step in the process, adding cost, an=
other inspection step, and may also require polishing to ensure the filling=
 is not proud before the resist is screened on.
=20
> A friend got burned by poor plating a long time ago, and on his protos,
> insists on filling the vias with solder- just in case the plating was
> poor. Valid, but I haven't seen a bad via on my boards in years- if the
> vias are that bad, there probably are other problems.

That would be either a very long time ago while the process was getting wel=
l sorted, or it was a manufacturer just setting up PTH process, and not yet=
 familiar with the process control requirements. If one has a problem with =
via quality these days then the manufacturer should be shouted at very loud=
ly.

> A google search came up with the possibility of trapping chemicals
> under the mask in a drill hole, but that sounds like a more fundamental
> issue with the manufacturing process.

If that is a problem, then it will be just as much a problem if the resist =
doesn't cover the via because it says the board hasn't been washed properly=
 to some stage, leading  to the likelihood of the trapped chemicals eating =
away at the metal anyway.
=20


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