My solution on a recent BGA289 1mm design was to use a -7mil soldermask rule
for the vias under the BGA and used it for both top and bottom. Production
wants to probe on future revs so I will have to combine gerbers from
separate rule applications. For this proto type I made the via size under
the BGA different from the rest of the board so that I could easily select
them. I converted them to pads and then defined a pad class 24dia 10 hole.
My tracks were 5mil and spaces 5mil. This resulted with mask in the holes
only by the amount of the misregistration of the soldermask.  The PCB
engineer at my board house suggests soldermask webs no narrower than 6mil.
This prevents inconsistencies in appearance. I wish for a way to design rule
check the soldermask for narrow webs and solder drains. My thought on the
probe problem is that on BGA designs the probes can be done elsewhere on the
board, there are vias galore.
Dave Eloranta
Locus Inc.
Madison, WI

-----Original Message-----
From:   JaMi Smith [mailto:[EMAIL PROTECTED]]
Sent:   Wednesday, September 18, 2002 9:16 PM
To:     Protel EDA Forum
Cc:     JaMi Smith
Subject:        [PEDA] Selective tenting on one side only

Hi All,

Can I selectively tent only one side of a via, without having to tent the
entire via and then manually go in there and put a little round pad on the
solder mask layer on only one side of the hole, to prevent mask on that
side. (My first mistake may be in assuming that a pad on the soldermask
layer will actually prevent "mask" from being applied there.)

I know that this sometimes yields less than perfect results, since the mask
can simply run down the barrel of the hole, from the inmasked side to the
masked side and gum up the masked side a bit, but I am willing to live with
that.

I have to use tenting on the top side of a board underneath a BGA, simply to
prevent any solder from being wicked into the hole in the via, and causing a
bad BGA "ball".

But at the same time, this is a prototype board, and I have left all of the
unused pads (pins) of the BGA still connected to a via, so that they can be
accessed from the backside of the board if necessary, and it would be nice
if everything was not tented there (at least intentionally and aside from
what may run down the hole), so that people do not have to scrape off the
soldermask with an xacto knife to get to the pad if they want that signal.

JaMi

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