Hello Dick,

The best way is the best that works.  If you we're doing all .050 inch pitch
BGAs I think just about anything works and at that point it is a question of
thru-put.  When you are "floating" the micro BGAs (<.86mm) I find the
neighborhood follows.  I would not find it to hard to believe the .050 pitch
has 300 plus pin QFP and a gaggle of 1206 passives while the .5mm would not
be to surprising to find 201 packages.  It has been my experience that BGAs
are the nice guys in the group.  My experience is all stencil and usually
flatness of PCB counts so I like Silver (or Tin).  It "converts" to solder
joint quickly compared to ENIG (without fear of Nickel saga) and has the
required flatness (etc, and down the finish road we go).  I think in terms of
substrate (assuming you have appropriate state of the art equipment, etc),
ensure to the extent you can that surface topology is flat solder masked vias
(nice term) are properly thought out, and if using PCB material that requires
prebake it is done properly (big subject).  The stencil I would use is 5 mil
laser with instructions for ease of release, teaming with paste supplier.  I
like BGAs, it's the none BGAs that gave me lots of conversation.

Brad

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