BGA/PGA Pin numbering

[akshayadiga]

Hi,
i am a beginner in PCB design. for a BGA package, the pins are numbered as A1,A2...BA42 . So, I found that not all the letters are used in numbering the rows . Letters like I,O,Q,U,X and Z were not used . Could somebody tell me what is the logic behind it ?

Thanks

 

[RCinFLA]

Those letters may be misinterpreted. Like "I" and "1", or "O" and zero.