The PCB Aspect Ratio is defined as the ratio of the thickness of a PCB compared to the diameter of the smallest hole drilled in the same PCB. The thickness of the PCB is the thickness without copper plating, solder, or solder mask. The larger this aspect ratio is the more difficult the plating process will be.
As the aspect ratio increases more plating will be required and thus there is a proportional increase in the risk of cracking due to expansion during the assembly process. A lower aspect ratio will have the benefit of stronger vias that provide better electrical connections.
At Bittele Electronics our PCB Aspect Ratios can be seen in the below table: