Hi Glenn! IMHO I would avoid mixing SMD and NSMD pad methodologies on the
same component. My experience has been that the NSMD pads have better
solder joint thermal cycle fatigue life and have fewer board fabrication
issues (e.g. misregistered soldermask, pad contamination, etc.). The
difference on standoff height between the SMD and NSMD is negligible
depending on your pad design/overall wettable area - depending on the
weight of the BGA, you will have a sphere shape instead of a columnar shape
of solder joint form. There was also a fun discussion last week on TechNet
(check the archive) on the interaction of the solder joint and intrusion of
soldermask on the pad you should take a look at for information. Good Luck.

Dave Hillman
Rockwell Collins
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Glenn Pelkey <[log in to unmask]>@ipc.org> on 10/29/2002 12:48:18 PM

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Subject:    [TN] SMD and NSMD pads on same BGA


Technetters,

        There is a proposed design here that incorporates Solder Mask
Defined (SMD) and Non Solder Mask Defined (NSMD) BGA pads on the same part.
Solderable area on the pad is the same for both as is the solder ball
diameter to place.  I'm concerned about coplanarity, but I don't have any
data to back it up.  I know the SMD balls will be higher off the substrate
than the NSMD balls, but by how much?  The reason for this design is
electrical performance.

        What do you think?  Is the amount of variation in height too small
to be concerned about?

Thanks,

Glenn

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