what is electromigration why metal reliability degrades at narrow lines

ELECTROMIGRATION (EM): when a high current flows through a thin metal wire, the flowing electrons collide with and physically push the metal atoms — the metal slowly migrates in the direction of the current. Over time this creates a VOID (a gap → open circuit) on one side and a HILLOCK (a bump → possible short with the next line) on the other, and the wire eventually FAILS. WHY IT'S WORSE AT NARROW LINES: as lines shrink, current density goes up (same current in a smaller area), and fewer atoms/more grain-boundary paths make migration faster — so smaller nodes = higher EM risk. WHY GPS CARES: electromigration is a classic reliability/field failure that shows up over time, not at first test — exactly the kind of 'it worked then it failed' issue GPS investigates. The one-line frame: 'Electromigration is like a river slowly moving the sand — high current carries the metal away until the wire eventually opens.'