One thing not to be overlooked is when the design calls for tenting on one
side, but not the other, of a via.  If both sides of the board are coated
with soldermask, UV energy from the tented side may go through the vias and
unintentionally expose soldermask on the other side.  It might not be enough
to tent the front side, but I've seen instances where enough mask will be
left in the "untented" side to keep electrical test pins from hitting metal,
resulting in "opens".  Obviously, if caught up-front, solutions are
relatively easy (coat and expose each side with a separate pass or pick
different test points).  If not caught, nothing is worse than getting a call
on Saturday night from the test folks that the hot boards you are working on
have "tons of opens".

        Mike McMaster
        RF Product Engineer
        Merix Corporation
        503-992-4263


        ----------
        From:  Timothy Reeves [SMTP:[log in to unmask]]
        Sent:  Friday, March 03, 2000 3:27 PM
        To:  [log in to unmask]
        Subject:  [TN] Mask-tented vias

        Technetters:
        What is your opinion on tenting of vias with LPI soldermask? If a
design
        gives leaves the mask film open over vias, the mask will be
UV-exposed,
        though not to the degree of the mask on the surface, resulting in
some mask
        coverage of copper inside the vias, but not as durable/thick as that
on the
        surface. Some of it will remain, some may end up HASL'ed. What is
the
        reliability of the "in-between" surfaces that don't coat with
solder, but
        also don't appear to be covered with mask? The "old specs" used to
allow
        some "haloing" around pads where thermal cured mask tapered off and
left a
        coppery ring around the solder-coated pad. Is this the same thing,
and is it
        a concern?
        Designers/end users: If your design allows the vias to be covered
with mask,
        but the vias don't fill with mask, what problems do you foresee?
        Thanks for any and all replies.
        G'day
        Timothy Reeves
        ECD Circuit Board Division
        13626 South Freeman Road
        Mulino, OR 97042
        [log in to unmask]
        (503) 829-9108 (800) 228-8198  FAX (503) 829-5482

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