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Or is there just a typical value? Specifically, I'm using a 0.65mm-pitch PBGA with a recommended 0.5mm pad diameter.

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    \$\begingroup\$ Please indicate how the ball diameter, PBGA pad size, and PBGA pad pitch relate have anything to do with drilling a hole. The only thing that would even come close here would be the via PADs that one would branch off the PBGA pads for access to inner layers of the PC board. For these vias the drill size will be primarily defined by two factors; 1) The actual size of the via pad that fits in between the PBGA pads, and 2) The minimum allowable drill size that your PC board fabrication shop can work with. \$\endgroup\$ Jul 2, 2013 at 1:43
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    \$\begingroup\$ Are you asking about the homebrew technique of using a via under each pin so solder can be applied from the bottom pin by pin? I don't know much about that technique but clarifying would make the question clearer, production BGA doesn't use a drill hole. \$\endgroup\$
    – PeterJ
    Jul 2, 2013 at 11:46
  • \$\begingroup\$ Thank you all for the answers. To be honest, I was using a PCB component-making tool and one of the parameters for a BGA component was a "drill diameter". I was not sure what exactly that referred to. \$\endgroup\$
    – Crunchy
    Jul 2, 2013 at 20:14

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You have a major misconception. You don't drill holes for BGA pins. You put small pads on the board for the BGA pins to sit on. In the manufacturing process, solder paste is put on these pads, then the whole thing heated. This results in the solder making both a electrical and mechanical bond between the pads and the pins. There are no holes in the board envolved in this process.

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    \$\begingroup\$ +1 quite right, although some time ago I remember reading about a homebrew technique of using a via under each pin so solder could be applied from the bottom pin by pin. Possibly that's what the OP was referring to but if so they should have mentioned it. \$\endgroup\$
    – PeterJ
    Jul 2, 2013 at 11:22
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The hole size isn't important. What matters is that it can be built; if it is too large or small, shorts and/or opens will happen.

If I had to pick a number I would start with 0.25 mm for a 0.5 mm pad.

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