From where does Pb comes in IC. Like the are developed on Si wafer with impurities,
then ouer connections & packaging. Which part of it contain Pb. Even ROHs allows some traces of Pb in specs.

 

The "tinned" leads. The leads are really copper with a tin/lead alloy over them. There is no lead intentionally added to the semiconductor / silicon portion of the chip.

 

Really hard to find (or get) and IC's now that aren't ROHS.
The lead is (or was) from the plating on the pins of the IC.