STENCIL THICKNESS:
After the squeegee has filled the apertures in the stencil, it gets lifted
up and 'hopefully' all the paste leaves the apertures and stays on the pads
of the board. The battle is between the forces of the 'suction' by the pad
and the friction along the side walls of the aperture on the solder brick.
Experiments have shown that for a typical laser-cut stencil the bottom area
of the aperture has to be greater than 0.7 of the wall area.
After applying a little geometry and math a simple rule of thumb can be
derived:
        for round and square apertures the diameter (or width) has to be greater
than 2.5 times the stencil thickness;
        for rectangles, where the length is several times the width, the width has
to be greater than 1.5 times the stencil thickness.


The above means that for your 5 mil stencil a square aperture should be at
least 12.5 mil wide. So you are working with apertures that are close to the
'rule of thumb'. Given a good laser-cut stencil it is likely that you will
be successful. But a stencil with questionably cut apertures may give you
some trouble. To make life easier, go to larger apertures. You are not yet
close to the other rule-of-thumb that says make the apertures not wider than
50% of the pitch. This means anything under 16 mil is ok, but with BGA-s you
may want to go a little smaller anyway.
To give yourself a little wider process window (fewer print errors) you can
choose a finer mesh solder paste.

Have fun,
Ahne.

-----Original Message-----
From:   TechNet [mailto:[log in to unmask]] On Behalf Of bbarr
Sent:   Wednesday, May 29, 2002 08:40
To:     [log in to unmask]
Subject:        [TN] FPBGA Area Ratio Question

On a fine pitch BGA (.8mm pitch, 384 balls, .5mm ball diameter) I am using a
.014" diameter land per TI's recommendation. I was thinking then of a .013"
square stencil aperture with .003" radiused corners on a 5 mil plate. This
seemed reasonable until I did an area ratio calculation per IPC-7525. The
result was .592, which fails compared to the acceptable value of >.66. The
aspect ratio is a solid 2.6, although IPC says I should not consider aspect
ratio in this case, only area ratio. If I reduce down to a .012" square
aperture I can get to an area ratio of .642. Still fails, but close. But,
reducing the aperture 2 mils compared to the pad seems extreme and violates
the recommendation in para. 3.2.2.4. So my question is, what do you guys and
gals use for an aperture design on a component like this? Do you go by the
IPC guidelines?

Thanks.

Bob

=======================
Robert Barr
Manufacturing Engineering
Formation, Inc.
Voice: 856-234-5020 x3035
Fax: 856-234-6679

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