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March 2005

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Subject:
From:
Charles Caswell <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Charles Caswell <[log in to unmask]>
Date:
Wed, 30 Mar 2005 12:22:17 -0600
Content-Type:
text/plain
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text/plain (103 lines)
I feel you pain. Just went through a similar execise in Jan. Had 16 and 20
soic-8 op-amps and a high power voltage reg. Had to remove the parts remove
soldermask and the manually apply paste to the pad and reflow. The op-amps
use this as both a ground and heatsink. Really I think you are doing the best
you can, unless somebody comes up with a laser for you. I assume the mask is
the usual green so you will probably need a neon or ruby laser. Several years
ago we used a conformal coat stripper which we found by accident would soften
LPI, I just can't remember the name of it.


-----Original Message-----
From: Stephen Gregory [mailto:[log in to unmask]] 
Sent: Wednesday, 30 March 2005 11:25 AM
To: [log in to unmask]
Subject: [TN] Selective LPI soldermask removal...

Happy "Hump-day"!

I might have asked the list about this over a year ago, I can't remember.
But I have this
assembly that goes into a Radar system. There's a device on the assembly
that has to
be sitting over the metal of the top layer plane, with no solder mask on
the board beneath
the footprint of the device for it to work properly. Unfortunately, the
solder mask file in the
gerbers didn't reflect this. So at the time (over a year ago), I reworked
the boards that we
had to correct the problem...and let me tell you, it's a REAL PIA!

Go to: http://www.stevzeva.homestead.com and look at "Mask Removal", and
"After Tinning"
to see what I'm trying to do.

I did this over a year ago, and we got an order for these assemblies
again...but the gerber never
got corrected, and I get to do this again. WHOOPEE!

I have to remove the solder mask beneath where the device is going to be
installed, and then
I solder coat the area to cover the exposed copper. I'm removing the solder
mask very, very
carefully with a Dremel and a very fine abrasive tip, and I'm using an
exacto knife with various
blades. This is all being done tediously under a microscope to prevent the
copper from being
removed from the plane. We have a micro-blaster here to remove conformal
coating, but it can't
get down to the bare copper. That's why I'm using the Dremel and exacto
knife. This time when
the board house did their micro-etch or pumice scrub just prior to applying
the solder mask, they
really did a good job! That mask is on there, and meant to stay put! I'm
really having a hell of a
time doing this on these latest fabs.

Is there any other way to remove this coating in the area that I need to?
I'm thinking maybe a
laser. Does anybody know where I would go to get this done, and about what
it would cost?
There's only 5-boards that I need this done to, but it takes me a couple of
hours with each board
to complete the job.

Kind regards,

-Steve Gregory-
Senior Process Engineer
LaBarge Incorporated
Tulsa, Oklahoma
(918) 459-2285
(918) 459-2350 FAX
__________________________________________________________________
This message may contain information that is privileged and confidential to
LaBarge, Inc.  It is for use only by the individual or entity named above.
If you are not the intended recipient, you may not copy, use or deliver
this message to anyone.  In such event, you should destroy the message and
kindly notify the sender by reply e-mail.

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