The ratio between a PCB's thickness and the diameter of the smallest hole that may be drilled into the same PCB is known as the PCB Aspect Ratio. The PCB's thickness is its physical size before copper plating, solder, or solder mask. The plating process will be more challenging the larger this aspect ratio is.
At Daleba, we can reach to 10:1 Aspect Ratio as standard, that is if the board thickness is 1.6mm, hole size must be ≥ 0.16mm. Using advanced manufacturing techniques an aspect ratio of 16:1 may be possible.