Regarding populating both sides of the board with BGAs, I think Bev Christian
summed it up pretty well.  If you meet the parameters she mentioned, you
should be fine with most PBGAs. The formula I use for calculating what will
stay in place when re-reflowed is the sum of the lead (or collapsed sphere)
surface area (all interconnects on the device) divided by the weight of the
component in grams.  If it comes out to less than 30 grams/ sq in, the
component will stay on as there is ample surface tension.  (Note, most ceramic
BGAs or CGAs exceed this ratio).

You sure don't need "differential heating" and you can do it on ANY reflow
oven (not just an ETS whoopee-flow or whatever) that, as Bev mentioned, has a
smooth running, edge conveyor
HOWEVER........
As Jerry Cupples wisely insinuates - beware when you do populate both sides
with these.  Remember to allow room for removal and replacement (Design for
Repairability 101) !  We don't live in a perfect world, you know.

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