what is metal migration in layout

Metal migration is related to the level of dc current, and this information is
used to specify current limits in an IC. To explain metal migration, consider
that normally the diffusion process is random, but with dc current, metal atoms
are bombarded more from one side than from the other. This causes the
movement of metal atoms, which is referred to as metal migration. Sufficient
movement in the metal can result in gaps or open circuits appearing in metal
and subsequent circuit failure. Any defects or grain boundaries can make the
problems worse.
The maximum allowable current in a metal line also depends on the
material. For example, aluminum, though lower in resistance, is worse than
tungsten for metal migration, due to its much lower melting temperature. Thus,
even though there is less energy dissipated per unit length in aluminum, it is
less able to handle that energy dissipation.