Due a big variety of functions and applications of printed circuits the different types of the holes can be present in their structure. They can be classified in a general way – metalized / non-metalized or because of their shape or function etc. 

The classification considering most types of holes types used in printed circuits is presented below.

Basic classification:

• vias/metalized holes
  • standard – diameter from 0.2 mm
  • micro-vias – diameter below 0.2 mm (typical: 0.1 mm; 0.15 mm)
• non-metalized holes: diameter from 0.25 mm
  • without annular ring
  • with the annular ring moved from the hole edge
  • with the annular ring up to the hole edge.

Classification by what layers they pass through / what layers they connect:

• PTH/NPTH – they pass through the entire thickness of the PCB;
• blind/backdrilling – vias/holes drilled to the depth (visible from outside but only on one side of the PCB);
• buried vias – vias connecting the internal layers (not visible from outside).

Classification by a finishing method:

• not covered by a soldermask – without cover or covered with the protection layer e.g. HAL, ENIG etc.
• covered with a soldermask (usually not plugged)
• filled (copper, dielectric)
• filled and covered with a soldermask (standard)
• filled and covered with a soldermask (flat, bigger degree of filling).

Non-typical:

• slotholes:

• special case: slotholes 2D – the slothole length (L) is smaller or equal double it width (2D) – L ≤ 2*D.

  

•  
  • sinkholes (beveled)
  

  Classification by the diameter tolerance (examples):

  •  standard, typically: + 0.1 mm / – 0.05 mm
  •  pressfit: e.g. + 0.05 mm / – 0.05 mm.

  The parameters and application of individual types of holes will be presented in the forthcoming articles.