Electromigration
Electromigration is the mass transport of metal resulting from the transfer of momentum between conducting electrons and diffusing metal atoms. It occurs in metal wires that conduct high current densities. This problem can be minimized by limiting the current density in metal conductor lines, vias, and contacts.
IR drop
IR drop is a reduction in voltage that occurs on the VDD network of an integrated circuit because of the current flowing through the resistance of the VDD network itself (V = I x R). You can look into foundry docs for sheet resistance to calculate IR drop.
You can find more details and ways to overcome them in some good layout books available.
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