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Fanout patterns, part 2: through-vias provide the lowest fabrication cost but can limit routing density.

Article Excerpt
THIS IS THE fourth in a series of articles on BGA routing methods. My objective is to reveal the routing problems often associated with large pin-count BGAs, and provide the PCB designer with effective techniques that enable higher route density and reduction of layer count.

Through-Vias

There are not many fanout options, due to the large via pad relative to the 1-mm ball pitch. Either a "Quadrant Dog-Bone" or "Via-in-Pad" method is appropriate.

Theory: Removing Vias

One method proposed to increase route channels on inner layers is not to use fanout vias where possible, for power, ground and unused pins. When using through-vias, there is very little benefit in not adding fanout vias for the 30% to 50% of the BGA pins that will likely be assigned to power, ground or unused. The power and ground pins will be assigned to the center of the...

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