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  Subject: Re: [TN] Stencil aspect ratio
  Sent:    Thu, 10 Feb 2000 12:07:08 -0800

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     I used to print apertures of this size...but you are better off going
     to a 325 to 400 mesh stainless steel screen and emulsion and
     80-durometer rubber squeegees, .030" snap-off (thick film
     microelectronic methods) as opposed to very-thin stencils.
     Just my $.02....
     Jeff Hempton
     UT Electronic Controls


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Subject: Re: [TN] Stencil aspect ratio
Author:  "Stephen R. Gregory" <[log in to unmask]> at Internet
Date:    02/10/2000 2:43 PM


In a message dated 02/10/2000 12:41:18 PM Central Standard Time,
[log in to unmask] writes:

<< Hi all,

 I'm trying to determine the "outer limits" of stencil thickness vs.
aperature diameter at very small diametr (.006" - .010")  Can anyone share
some experience?

 Thanks in advance,

 Ed Popielarski
 QTA Machine
 10 Mc Laren, Ste. D
 Irvine, Ca. 92618

 Ph: 949-581-6601
 Fx: 949-581-2448

 http://www.qta.net

 Check out our BGA Baller and Process Equipment Services! >>

Hey Ed!

.006 -.010" diameters!? Hoo-doggies, that's getting teeny-tiny! Flip chip? I

haven't had any other experience except for when I worked for Zevatech. We
built a little keychain board that had a flip chip on it and if I remember
correctly, they had 6-mil pads. The stencil was like aluminum foil, only 2
or
3 mils thick...you could crease it with your thumb-nail. The normal 1.5
aperture width to thinkness ratio really doesn't apply to circular openings
though...to much sidewall friction to overcome for the paste to release from

the stencil good. That's why I always do square apertures on microbga, the
paste releases better from the stencil.

I just built a prototype board last week that had a 256-ball microbga...I
used a 4-mil stencil...and the pads on the board were giant compared to what

you're talking about...they were 20-mil. I used square openings and it
worked
out fine...

So in my estimation, .006 -.010" diameter openings I don't think you'll get
much thicker than 3-mils and get good paste release.


-Steve Gregory-

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