We are in the process of designing a board with a large number of both
"MicroStar" 144 ball packages from TI (.8mm ball spacing) and another
part using the Tessera style uBGA (.75mm ball spacing) .

There is a TI application note available which gives a few guidelines.
Tessera has quite a bit more on their web site.

In speaking to a current, high volume user of the TI part he indicated
that it took quite an effort from design thru the test to develop their
processes to accomodate this part.  He mentioned that the part was very
"fragile" and that there were reliability issues to be concerned with.
Unfortunately, most of this info was "proprietary" and he couldn't share
the details.

If any of you have had any experience with either of these packages, or
micro BGA's in general and would be willing to share your knowledge, it
would be greatly appreciated.  We know that it will require micro via
technology in the PCB which will be a first for us.  Our contract
assembly house has not had experience with micro BGA's yet, though they
are willing and anxious to get the experience.  And our PCB design house
is also learning how to design with micro via's.

Thanks.

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