Definition

Stacked vias are micro-vias drilled directly on top of each other through multiple build-up layers, creating a vertical interconnect tower. Stacking maximizes routing density but requires copper filling of lower vias before stacking subsequent ones, as an unfilled void would cause reliability issues under thermal stress. Stacked via structures are more expensive than staggered vias but necessary for escape routing from ultra-fine-pitch devices. Any-layer HDI typically uses all-stacked construction.