Bga Chip Solder Balls

What are solder balls?
in bga package, it is the small sphere of solder that provides the contact between the bga package and the printed circuit board. Also called a "solder bump." this tiny part could be damaged after fabrication of the bga package, it could produce bad connections due to bad assembly.
What is the role solder balls play in bga reballing?
pcb which fails due to bad bga connections can be repaired either by reflowing, or by removing the chip and cleaning it of solder, replacing and reballing. The most accurate & economic method to restore the full functionality of bga package is reballing. Reballing involves, heating the chip until it can be removed from the board, removing the chip, removing solder remaining on the chip and board, putting new bga solder balls in place and heating the bga package or board to solder it in place.
The new solder balls can be placed via several methods, including:
using a stencil for both spheres and the solder paste or flux
using a bga jig with embedded balls corresponding to the bga package pattern using semi-automated or fully automated machinery.
Solder balls specifications:
packed in plastic jars, each one includes 250'000 balls