I have a design that will use a 400-ball PBGA. It has a ball to ball
pitch of 50mil and ball diameter of 35mil/30mil/24mil which I don't
understand why there are 3 readings given on the technical data. I also
noticed that the corner ball is 8mil shorter than the rest of the ball
with respective from the seating plane. My questions are :

1. What pad size should I use for the ball?
2. I have heard of "dog-bone". Must all BGA of such pitch used this
"dog-bone"? What should the recomended dog-bone dimension be (via size,
hole diameter & track width)?
3. Can I use via-on-pad instead using the same recommended via in (2)?
If no, why it cannot be so?
4. Why is the corner ball offset by 8mil as compare with the rest?

I am a why? why? person so please pardon me if there are too many
questions asked. However, I would deeply appreciate anyone of you out
there for your time in answering my questions.

Thanking you in advance.
Regards.