When you design a PCB you will need to place holes for different reasons as described below.
When you design two layer or multi-layer circuits the holes are typically used to connect ground planes of the top layer to the general ground plane. This is required so all the components connected to the ground are connected to the same plane.
When you design a multilayer PCB the holes are used to interconnectdifferent paths of different layers to redirect DC voltage or RF signals from one layer to another.
Via holes are also used to design an electromagnetic barrier to conduct the RF signal through the RF lines and avoid the dispersion of the signal to other regions of the PCB. This is also very important to avoid coupling between different RF lines. A typical rule of design is to place the holes separated in around lambda/4, calculating lambda from the highest frequency of work.
Holes are also placed below ground planes of high power consumption devices in order to dissipate better the temperature generated by these components.
The holes can also be used to mount through holes components.
The holes can be either metallized or non-metallized. You will need they are metallized when they are used to interconnect layers or paths. You will need they are not metallized when you want to place through hole components and you do not want they can be short-circuited with other layers.
When you design a multilayer PCB (more than two layers) the holes can be performed to the final stackup, obtaining a through hole for all the layers. Or you can drill each couple of layers, very used when you need to implement blind via holes or buried via holes. This is required when you do not need a connection between all the layers, only some of them.
Depending on the manufacturer you will need to follow the rules of design they are able to manufacture regarding the drill sizes. You will need to take into account that, when it is possible, it is better to select the higher hole size in order to save money. The reason is that if you select the minimum hole size, the drill tools required for this task are more expensive, usually takes longer time since the velocity of drilling is lower, and also they are more fragile and after some work they can be broken or even they need to be replaced to obtain a good finish. In pcbGO the minimum hole size is 0.2 mm and you also have the possibility to select 0.25 and 0.3 mm.
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