Registration Accuracy
Laser drilling processes must place tiny blind vias precisely over the centers of the landing pads on the target layer to ensure reliable electrical connection. Proper microvia alignment prevents the drill from hitting the edge of the pad or missing it entirely, which would result in an open circuit or a weak joint. This precision is measured by the offset between the actual via center and the theoretical pad center across the entire panel.
Stackup Yield
Dimensional stability of the substrate material influences the success of the drilling operation. During the lamination of multi-layer boards, the resin can flow and the core can shrink or expand slightly, causing the pads to shift from their original positions. Advanced imaging systems on laser drills compensate for this movement by scaling the drill pattern to match the actual layout of the inner layers.
If the microvia alignment is poor, the resulting interconnect may have a reduced contact area, leading to higher resistance and potential failure under thermal stress. High density designs often use teardrop shapes on the pads to provide more area for the via to land safely. The inspection of these features typically involves cross-sectioning or automated optical inspection to verify that the annular ring requirements are met.
Failure Mode
Breakout occurs when the via is so far off center that it penetrates the side of the pad. This condition creates a reliability risk where the plating may be thin or the connection could be severed by subsequent processing. Microvia alignment determines the minimum pad size that a fabricator can reliably produce without sacrificing yield.
