and why?
I recently saw some printed advertising copy, including this statement
concerning BGA chips
"We safely remove the existing lead-free solder balls and replace them with
SnPb "

cannot find reference to that on their site, just
http://www.retronix.com/Bga_reballing
BGA Reballing (Lead Free / 63-37 / High Melt Ceramic Packages)
$1/2 million investment in a unique technology has allowed Retronix to
reball BGA and uBGA devices without inducing a thermal cycle (mass reflow),
which can damage the integrity of the device. We use a laser energy pulse
which attaches the solder spheres individually to the pad in less than
20m/s. The bonding of the sphere to pad in this short time does not allow
heat dissipation into the silicon die, therefore offering a safe alternative
to mass reflow. The equipment shown is contained within a clean room
environment, supplemented by laminar flow hoods and a nitrogen chamber for
the reballing process. We can process sphere sizes from 760 microns down to
100 microns.