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November 2005

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Subject:
From:
"Stadem, Richard" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard
Date:
Thu, 10 Nov 2005 14:51:50 -0600
Content-Type:
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Randy,
Been there, same problem with a .125" tooling hole plated through, but
covered with soldermask over the annular grounding "doughnut" on the
bottomside, which was .325" in diameter if I remember correctly. I fixed
these in the following manner.
I set up a frame clamp in a CNC router, a precision router. I used a
cylindrical sanding block which was just a .325" diameter cylinder with
a 6/32 (I think) center hole tapped and countersunk in the bottom of it.
At the top of the cylinder on the side there was a set-screw that
securely held a .250" shaft protruding out of the cylinder top, which
was used to hold the cylinder in the router. I purchased emery pads for
a Dremel tool (rough or medium) and secured them in the tapped and
countersunk hole with a screw that when tightened was recessed below the
surface of the bottom of the cylinder. I then was able to literally sand
away the solder mask so the grounding doughnut was exposed. The drill
needs to be an automatic mill or drill that can come down precisely the
same Z-axis depth every time.
The sander could do about 10-12 pwbs with 4 holes per board before the
emery pad needed to be replaced. When finished we applied solder paste
by adding the doughnut aperture to the bottom-side stencil. It took us
two 8-hour shifts to get the boards ready for regular production, but we
did not lose a single board. It took remarkably little pressure to
obliterate the 5-mil thick solder mask. We just basically came down and
touched each spot for a couple of seconds at the most.
At that time nobody cared about whether or not the solder mask dust was
hazardous, and I don't know if it is, but you probably would want to set
up a vacuum blower to contain it.
The part I do not know is your internal hole size. The retaining screw
head and countersunken area must be smaller than that.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Randy Bock Sr.
Sent: Thursday, November 10, 2005 1:53 PM
To: [log in to unmask]
Subject: [TN] Solder Mask Removal

Hey y'all,

We have several hundred boards that are supposed to have  the solder
mask
removed for around a mounting hole on the back side of the board.   The
fab house missed.  We are faced with a total remake if I cannot find
some way to remove the solder mask relatively easy.  Most are still fab
boards,  but I do have some that are assembled.

Any ideas would be appreciated.  The clear area has to be approx 1/2 "
dia.  (is for grounding)

Randy Bock Sr.
Quality  Engineer
Pitney Bowes
AddressRight Printers
(email)  [log in to unmask]
Newtown Building 2
203 270-4931  Ext
    8-426-4931  In
203 270-4982 Fax

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