Positional Accuracy
Precise alignment of the hole burned by the laser with the target copper pad on the inner layer determines the continuity of the vertical connection. This laser microvia registration is measured by the offset between the center of the drill hole and the center of the receiving land. Modern systems use high speed optics to scan for fiducial marks on the board before firing.
If the beam is slightly off, the via may clip the edge of the pad or miss it entirely. Success depends on the stability of the board on the table.
Optical Alignment
Cameras detect global and local positions to adjust the firing path in real time for every panel. For laser microvia registration to succeed, the thermal expansion of the board material between processes must be factored into the coordinate system. Boards stretch slightly when heated during previous lamination cycles.
The laser controller corrects for this scale change dynamically. High density interconnects rely on these sub mil adjustments to achieve tiny pad sizes.
Stacking Stability
Reliability of multiple stacked vias depends on how accurately the holes sit on top of each other through subsequent layers. Poor laser microvia registration leads to weak structural joints that can crack during thermal cycling. Acceptance standards usually limit the breakout area where the via partially misses the pad.
If the hole stays inside the copper boundary, the circuit is usually deemed reliable. Consistent alignment is mandatory for high volume yields in mobile device production.
