The pad size is the easy one!

If it's a dense 0.5mm pitch, that's equivalent to 19.7 mils.  If you
need to route between pads, then you will need to use 3x3 mil
linexspace.  That leaves 10.7mils for the pad.  This is about 0.26mm,
and going less than that can be pretty problematic:  It's hard to get
solder paste to print a dot even that small.

What that doesn't tell you is:
-For many etching technologies, the smaller the radius of curvature
(smaller round pad), the more over-etched they end up.  So your 10.7 mil
pad will probably over-etch unless the pcb fab makes a special effort to
add extra "swell" to them.  I have tried some flip chips with 8mil
(0.2mm) pads and seen them so over-etched they were useless.
-You need to figure out how to get a useable solder mask put on the
board.  If you don't have routes between the pads, then this isn't a big
problem, but if you do, then you will need to work very carefully with
the pcb fab and you need to be prepared to reject the boards based on
poor mask alignment.  You can also enter the great debate on whether to
use mask defined pads!

Wayne Thayer

>>> [log in to unmask]  >>>
Hi

I am working on a new board with a 0.5mm pitch BGA and I am wonder if
someone can advise me on the footprint.

1) the datasheet says the solder ball is 0.3mm +/-0.05mm
2) IPC recommends to use 80% of ball diameter for the pad which would be
0.24mm
3) Would it be OK to use a 0.2mm pad? I am hesitant to go against
recommended 
size without some corroborating documentation. The reason I would want
to
use 0.2mm pad is to avoid using via-in-pad.

I am curious if IPC defines footprints based on what is good for
manufacturing or is it simply theoretical?

Thanks
Bob 

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