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We did a BGA design once but were frightened by the whole layout and pcb building process. Plus you can't scope the pins easily.
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Actually, I prefer BGA designs for probing.
The key is to design the PCB such that all BGA pads go to vias, and then expose the bottoms of the vias. A scope probe then sits nicely in the via on the PCB underside. I add a few ground vias around the outside of the BGA pattern and put a white circle around them on the silk, I can then use those known-grounds as my probe tip ground.
1mm pitch BGAs are also easy to decouple. You have the PCB house expoxy fill the ground and power vias on the underside, and you can place 0402 components directly across the via pairs. If you have a VCC pin without a nearby ground, then you sacrifice an I/O pin, by cutting its BGA pad to via trace, and re-purposing the via as a ground via.
For an example of this, there are PCB design files here:
http://www.ovro.caltech.edu/~dwh/carma_board/index.html (
http://www.ovro.caltech.edu/%7edwh/carma_board/index.html)
Ok, these features are not ideal for the lowest-cost board design, however, if you have to go to a BGA based design, then using this approach makes layout and debugging a lot easier.
Cheers,
Dave
Honored Contributor
