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February 1998

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Subject:
From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 13 Feb 1998 18:03:21 EST
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Hi Kathy,

     I can give you the spec's on the stencil and solderpaste that we use to
build a little "Keychain" board when we're exhibiting at trade shows.

     The keychain board is panelized 5-up and contains; a Tessera TV46 Micro
BGA, a 128-pin 15.7-mil pitch TQFP, a pair of mini-melf's, a pair of SOT23's,
a pair of 1206's, a half-dozen 0603's, and thirty-six 0402's.

     We print using a MPM Ultraprint 2000 decked out with metal blades, and
Alpha's LR735 at 400 mesh. The stencil we use is 4-mils thick and the
apertures for the Micro BGA are 1 to 1 with the pads. The stencil we use is
from BeamOn Technology, and something I think is worth mentioning, chemically
etched.

      It's not been trapezoidally etched, laser cut, or electropolished
either. What they do is something called "Band-width etching", or "Band-Etch".
What this is basically, is the way that they generate the artwork or phototool
used to etch the stencil. The way that it works is instead of etching the
whole aperture completely from the foil, they just etch a very thin line
around the perimeter of each aperture, it's much easier to control that way.
You don't get the kind of "hour-glass" cross section that you get with
standard chemical etching that requires electropolishing to remove.

       There was a study done at the University of Mass. in 1996 by a class of
engineering students to objectively evaluate the effects that different
stencil technologies had on printing solderpaste. One of the conclusions that
I found very interesting was that Band-Etched stencils performed just about as
good a laser cut stencil at a fraction of the cost...not everybody can afford
the $800-1200 price of laser cut stencils...

                                                     -Steve Gregory-

P.S. Just another note, what a lot of people did with the "keychains" that we
       passed out at Surface Mount International is to walk over to the
Nicholet
       Booth and ask them to x-ray the solder joints...the guys at Nicholet
were
       sick of seeing our keychains (GRIN). We didn't hear anything negative
from
       anybody during the entire show....('course I did the same thing with a
few of
       the first panels after we had set up the line and before we started
passing
       them out...I'm no fool! hehehe)

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