Toby,

For HDI designs (via in pad) I pour ground underneath BGAs and ensure that
any connections to the BGA ball are thermals. This eliminates soldermask
defined land areas which are problematic during manufacture.

For most non-HDI designs your observation is correct - there's not likely a
lot of pour under the BGA anyways, so not much point in having it there, and
I'll normally block out the area if pouring on the surface. I do normally
employ a stackup where the adjacent layer (t-1, b-1) is a plane in these
situations. The one reason I would consider keeping the pour under the BGA
is if it is large enough (and required) to provide some thermal benefit.

Hth,
Dave

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