Hi Paymon,
It is not that the IPC does "not allow" it, but data have shown that the 
stress concentrations in the solder ball geometries resulting from SMD-[soldr mask 
defind] pads is about a factor of 2 to 3 less than well-shaped balls on pads 
not encroached by solder mask. I can show you some beautiful [to an engineer, 
anyway] pictures of the consequences.

Werner



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