Layer Alignment
Positional deviation between the center of a drilled hole and the center of its corresponding copper pad reduces contact area. The target pad misregistration happens when the internal layers of a board shift relative to the drill coordinates. This displacement compromises the connection integrity.
Process Cause
Material dimensional instability and mechanical tolerances during lamination are the main contributors to this alignment error. During the multilayer pressing cycle, the heat and pressure applied to the prepreg can cause the inner-layer copper foils to shift or shrink. Minor drill spindle tolerances and thermal expansion of the board during drilling can further increase the target pad misregistration.
Fabricators track these dimensional shifts using x-ray inspection systems to adjust drill programs and apply scaling factors to compensate for laminate distortion.
Interconnect Risk
Electrical open circuits and reduced circuit reliability are the primary consequences of poor layer alignment. When the drill hole shifts too far from the pad center, it can breakout of the copper border, leading to a reduced annular ring or a complete failure to make electrical contact. This breakout can also allow plating chemicals to penetrate the substrate, causing latent conductive anodic filament growth and short circuits.
To prevent these failures, design rules establish minimum annular ring widths that accommodate the maximum expected target pad misregistration of the production line. High-yield manufacturing relies on maintaining these strict alignment boundaries.
