Geometric Proportion
Dimensional relationships between the depth and the diameter of a drilled hole define the limits of the fabrication process. A high aspect ratio indicates a deep, narrow channel that is difficult to plate, which is typical of thick multilayer boards with microvias. This characteristic is calculated by dividing the board thickness by the drill diameter.
Plating Limitation
Chemical transfer and fluid flow during electroplating are restricted when the via channel is long and narrow. Inadequate fluid exchange within the hole prevents the uniform deposition of copper, which leads to thin knee plating or completely unplated voids. When a high aspect ratio is present, fabricators must use specialized agitation, vibration, or vacuum-assisted chemistry to ensure copper reaches the center of the barrel.
Assembly Challenge
Reliability risks arise when the copper in the barrel is too thin to withstand thermal stresses. Thin copper plating in the center of the via cannot withstand the thermal expansion of the surrounding laminate during solder reflow, which often leads to barrel cracking and open circuits. High reliability industries typically restrict high aspect ratio designs to a safe threshold to ensure that plated copper survives multiple thermal cycles.
Advanced processing techniques like pulse-reverse electroplating are required to achieve uniform coverage without over-plating the surface pads, which increases fabrication costs and cycle times.
