Johannes,
     I have read the other responses you have received on this, and agree
with them.  I will just give you my input from my experience.  I studied
this issue when I was setting up the BGA process.  Having attended a BGA
symposium, SMI, etc. and talking with experts at IBM and Motorola I choose
to go with non solder mask defined pads and to print 1:1 with the pad.  We
were also placing PBGA.  With PBGA the ball will melt so you do not need an
excessive amount of paste so 1:1 is fine.  This may be different with
Ceramic.  There are pitfalls to both solder mask defined and non-soldermask
defined, but the non-solder mask defined seem to have fewer issues.  This
is the setup I used and had very good yields and the boards held up very
well under temp/humidity cycling and shock/vibration testing.

Good Luck,
Brad Kendall
Hella Electronics
Michigan SMTA President

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