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December 2001

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Subject:
From:
Steve Abrahamson <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 13 Dec 2001 13:44:57 -0700
Content-Type:
text/plain
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text/plain (113 lines)
Hey Stacy,

It sounds as though on paper, your process is OK.  Just to fill you in on
aspect ratio, that is aperture width/stencil thickness.  This is a good rule
of thumb for rectangular apertures such as for fine pitch components.  For
small circular or square apertures most people use the area aspect ratio
which is aperture area/wall area.  The area aspect ratio should be above
66.  Thus on a .005" thick stencil with a .0135" square aperture you are
looking at (.0135*.0135)/((.0135*.005)*4) = .675.
Actually the area aspect ratio for a .0135" round aperture is the same as
that for a square aperture, yet release on the square aperture has generally
been proven to be better (don't forget a .002" or .003" radius on the
corners).

Anyway my point was going to be that I had the exact same apertures ordered
on one particular project, and the apertures all clogged up (luckily we
caught them at the visual step).  It seems the apertures were severely
undersized (more like .0122" x .0122").  So are your apertures really
0135"?

As Kathy mentioned, kneed or condition your paste.  The paste may take a few
prints to fully mix and provide the thixotropic characteristics that you
want.  We use a dummy board for the first print, and then print our first
production board.

Personally I think you could get away with a .015" aperture if you are
dealing with a .0315" pitch product.  Even if they undercut your stencil,
you should be fine.

Good luck....

Steve A


> -----Original Message-----
> From: My Nguyen [SMTP:[log in to unmask]]
> Sent: Tuesday,December 11,2001 11:43 AM
> To:   [log in to unmask]
> Subject:      [TN] Inefficient solder on Toshiba micro BGA
>
> Hello all,
>
> Problem description:
>
> Been happening on Rambus - part number tc59rm81xmb.
> Ball diametter: 0.5 mm; ball pitch: 0.8 mm, ball
> height: 0.4 mm. Pad size: 0.3 mm (11.5 mils)
> 5-10% of the time (we run huge amount of them) failure
> modules came from opening soder join or no solder join
> at all.  As we inspect the screen printing process (we
> use 5 mils stencil - Dek screenprinting, cleaning
> rate: 2 per print, cleaning cycle Wet-Vacuum/Dry), we
> found un-event solder deposit.  Some pad even has very
> little solder or no solder at all.
>
> Stencil is electro-polish;  Diamond shape aperture,
> 13.5 mils open used trapezoid shape.
>
> We afraid that if we open the apperture bigger, then
> we may have bridging or solder ball.  If we reduce the
> stencil to 4 mils to reduce the blockage or
> resistance, then we may have in-efficient solder or
> many other problem relating thin stencil (damage,
> short-life, etc)
>
> What would be your solution?
>
> Thanks,
>
> Stacy
>
> __________________________________________________
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