Subdivision of the common usage of through holes in PCB design

PCB is a board that connects electronic components. They are part of the electronic products we use in our daily lives in all walks of life. They are made of non-conductive materials and have wires, pads and other functional parts etched from copper sheets, which can connect the electronic components in the product. Components such as capacitors and resistors are also soldered on some PCBs.

Type of assembly process
There are two main methods used for PCB applications: through-hole mounting and surface mounting.
1. Through hole installation
During the through-hole installation process, the assembler puts the component leads into the holes drilled into the bare PCB. This technology is the original technology used for PCBs. Through-hole technology produces stronger connections than SMT PCB, so it can be used in applications that require high reliability. This is because the leads run through the entire circuit board, instead of being fixed with solder as in surface mounting. Through-hole technology is also useful for testing and prototyping that require manual adjustment of components.
2. Surface mount
During the surface mounting process, the components are directly mounted on the PCB surface using solder. It is the most commonly used component installation method. Using surface mounting allows the assembler to fix the components on both sides of the circuit board. Surface-mounted components can also be smaller, so that more parts can be mounted on a single board. This reduces costs and has made electronic devices smaller and smaller over the years. Compared with through-hole mounting, surface mounting can also be completed faster and involves fewer processes, thereby further reducing costs.
Subdivision of the common usage of through holes in PCB design
Using vias in PCB design allows designers to shorten the distance that traces must be routed to complete their connections. Just like using a washing tank, the through hole will lower the trace to another layer with a clear path instead of wandering over a long distance. It is important to understand what the different via types and applications are in order to effectively use them for manufacturing and signal integrity. Let's take a look at the different vias available to PCB designers and how to use them.
Through holes are metal-lined holes connected to the metal circuit of the PCB. These holes conduct electrical signals between different layers of the board. Although the size of the via, the shape of the pad and the diameter of the hole may be different, there are only a few different via types or structures:
Through hole: This is the most commonly used type of through hole on a circuit board. Drill holes in the entire board with a mechanical drill bit, the size can be reduced to 6 mils.

Buried hole:
 This hole only connects the inner layer of the board, which is useful for PCBs with very dense wiring.A blind Via connects exactly one outer layer with one or more inner layers.A buried via is a via between at least two inner layers, which is not visible from the outer layers. This technology allows more functionality in less board space (packing density).

Blind hole: The hole starts from the top or bottom of the board, but does not run through it all the time.A blind Via connects exactly one outer layer with one or more inner layers.The holes for each connection level must be defined as a separate drill file.The ratio of drill diameter to hole depth (aspect ratio) must be 1:1 or larger. The smallest hole determines the depth and thus the max. distance between the outer layer and the corresponding inner layers.
Micro-holes: For holes less than 6 mils, use laser-drilled micro-holes. These vias only connect two adjacent layers of the board and can be on the surface or buried in the stack of board layers. Micro vias have many uses, and can be stacked together or on top of buried vias, but the manufacturing cost is relatively high.
In-pad vias: These vias can be standard vias or micro vias, but their location in the surface mount pad makes it unique. If a standard mechanical drill is used, the via will require additional manufacturing steps to prevent the solder on the pad from flowing down through the hole. On the other hand, micro-vias do not have this problem, but they may be more difficult to manufacture due to the smaller wiring and space tolerances in high-density designs.
blind vias and buried vias are increasingly used in high-density circuit boards (HDI PCB).
Buried via: The holes for each connection level must be defined as a separate drill file.
The ratio of drill diameter to hole depth (aspect ratio) must be 1:12 or larger. 
The smallest hole determines the depth and thus the max. distance between the respective inner layers.

Ultimately, which through hole to use depends on the technology of the printed circuit board, the circuit requirements, and the target cost of PCB manufacturing. For example, due to its small size, it is highly desirable to use micropores, but this does not necessarily make it the best choice. The manufacture of micro-holes involves more steps, so they are more expensive than mechanically drilled through-holes. However, if you are designing high-density interconnect boards, micro-vias will become a better choice.