I have been playing around, blindly, with this topic for a little while.
This is what I have learned, precious little that it is.

The answer depends on the BGA construction.  Many/most BGAs do not actually
have balls on them, they have 'bumps' of 63/37 solder.  These bumps will
completely melt during reflow, allowing the BGA to float on molten solder
during the soldering process.  For these components, you *generally* want
the pad on the board to be the same size as the pad on the component.  With
the pads being the same size, the resulting solder joint will be
symmetrical, top to bottom.  Since the writers of the Specs, (bless them
all), can only guess at what pad size the manufacturers of future BGAs will
use, it becomes difficult to always have the right answer.

Other BGAs have real 90% Pb balls on them.  These do not melt, and the
soldering process for these components create two solder joints.  One from
the Component to the 90% Pb ball, and a second from the ball to the board.
In this case, the board pad size will depend on the ball size.

There are other BGA configurations, such as solder columns, which, again,
you have to look at individually.

As an aside question (or complaint), Have people found that the information
the component manufacturers put in their component spec sheets (pad size and
spacing, and what has to be done to route the pin out....)  Have people
found that these follow any kind of DFM process?  In my experience, when my
Engineers choose the smallest, most dense component packages, we often end
up with a very interesting Fab and Assembly designs.

George Franck

-----Original Message-----
From: Lou Hart [mailto:[log in to unmask]]
Sent: Tuesday, January 08, 2002 5:10 PM
To: [log in to unmask]
Subject: [TN] Use of IPC-7095, -SM-782, and -7525


TechNetters, I have a question regarding consistency of 7095 (Design and
Assembly Process Implementation for BGAs), SM-782 (the Surface Mount pad
document), and 7525 (Stencil Design Guidelines), resulting from discussion
with process engineers and techs earlier today.

We were looking at a BGA, 20x20 package, with 1.27 mm pitch.  The stencil
design guidelines (7525, Table 1 on p. 4), call for an aperture of 0.75 mm.
 That table also refers to a 0.80 mm pad.  SM-782 (section 14) says a 20x20
BGA will have 0.75 mm balls that should go onto 0.60 mm pads.  Do these
documents contradict each other regarding pad size, and is 7525 wrong with
regard to BGA stencils?

Further, the bare board does have 0.60 mm pads for the BGA.  7095, in
6.2.1, p. 33, says "land diameter is usually smaller than the ball
diameter..land size reduction of 20 to 25% has been determined to provide
reliable attachment...".

As always, thanks for any comments.  Lou Hart

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