FPC manufacturer tells you What is microvia aspect ratio and why is it important for FPC design

The aspect ratio of a through hole is the ratio between the depth of the hole and the diameter of the hole (hole depth to hole diameter). For example, a standard circuit board at 0.062" thick (with 0.020" vias in it) will have an aspect ratio of 3 to 1. This ratio is used as a guide to ensure that the PCB manufacturer does not exceed the manufacturer's capabilities. their equipment while drilling. For standard drilling, the aspect ratio should generally not exceed 10:1, which will allow a 0.062" board to drill a 0.006" (0.15mm) hole through it.

When using microwells, the aspect ratio varies greatly due to their size and depth. Plating smaller holes can be difficult, and plating a small hole on 10 layers of a board can cause a lot of problems for the FPC manufacturer. However, if the hole spans only two of these layers, plating becomes much easier. IPC used to define micropores based on their size, which was equal to or less than 0.006 inches (0.15 mm). Over time, this size became common and the IPC decided to change its definition to avoid constantly updating its specifications as technology changed. The IPC now defines a microhole as a hole with a 1:1 aspect ratio, as long as the hole does not exceed 0.010 inches or 0.25 mm in depth.