Hi Glenn!
Don't have a real concrete answer for you, but as you probably already know, everything I've read recommends NSMD pads over SMD pads. I found figures of ball collapse that range from 20-25%, but those figures were all referencing NSMD pads.
Sorry I couldn't be more help...
But during my search, I found this excellent *.PDF file regarding PBGA's, covers just about everything...
http://e-www.motorola.com/brdata/PDFDB/docs/PBGAPRES.pdf
-Steve Gregory-
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