The electromigration behavior of copper pillars for different current directions and pillar shapes

A significant asymmetry in electromigration behavior was observed for copper pillars depending on the electron current direction; the electromigration performance is very robust for an electron source at the die-side, but vulnerable to the opposite electron flow direction. Through extensive failure analysis, it was observed that die-side electron source leads to a stable layering of intermetallic compounds and no electromigration-induced voiding, while the substrate-side electron source leads to more extensive transformation into intermetallic compounds at the expense of the copper trace as well as electromigration-induced voiding. These phenomena were exacerbated by narrower trace widths but improved by an oblong pillar shape. Further, the presence of a nickel cap between the solder and pillar did not significantly impact electromigration lifetime.