Through holes on a PCB refer to a hole that is drilled from the top and through to the bottom of a PCB, these can be plated (PTH) or non-plated (NPTH).

Through-hole technology, also spelled “thru-hole”, refers to the mounting scheme used for electronic components that involves the use of leads on the components that are inserted into holes drilled in PCBs and soldered to pads on the opposite side either by manual assembly (hand placement) or by the use of automated insertion mount machines.

Through-hole technology almost completely replaced earlier electronics assembly techniques such as wire wrapping. From the second generation of computers in the 1950s until surface-mount-technology (SMT) became popular in the late 1980s, every component on a typical PCB was a through-hole component.

PCBs initially had tracks printed on one side only, later both sides, then multi-layer boards were in use. Through holes became plated-through holes (PTH) in order for the components to make contact with the required conductive layers. Plated-through holes are no longer required with SMT boards for making the component connections, but are still used for making interconnections between the layers and in this role are more usually called vias. ≤4.00mm the tolerance on the diameter is +/-0.05mm
>4.00mm the tolerance on the diameter is +/-0.10mm Average Cu plating thickness is min 20µm.
Any individual hole should have a min of 18µm Cu.
Any localised part of a hole wall should have 15µm Cu as a min.
The tolerance on the diameter please see Tolerances on a PCB. If you remember back to your electronics classes, you’ll likely remember the integrated circuits you used with dual-inline package (DIP) or plastic DIP (PDIP). These components are normally seen as being mounted on breadboards for proof-of-concept development, but they are commonly used in real PCBs. The DIP package is common for active through-hole components, such as op-amp packages, low-power voltage regulators, and many other common components. Other components like transistors, higher power voltage regulators, quartz resonators, higher power LEDs, and many others may come in a zig-zag in-line package (ZIP) or transistor outline (TO) package. Just like axial or radial passive through-hole technology, these other packages mount to a PCB in the same way.

Through-hole components came about at a time when designers were more concerned with making electronic systems mechanically stable and were less concerned about aesthetics and signal integrity. There was less of a focus on reducing space taken up by components, and signal integrity problems were not a concern. Later, as power consumption, signal integrity, and board space requirements began to take center stage, designers needed to use components that provide the same electrical functionality in a smaller package. This is where surface-mount components come in.