Types of PCB Holes

PCB Holes: Types and Functions by Rush PCB UK

PCBs made by Rush PCB UK may have many types of holes in them to meet different requirements. The PCB manufacturing process includes several processes such as cutting, grinding, filming, exposing, etching, developing, electroplating, laminating, drilling, surface treatments, solder masking, potting, etc. For a good quality PCB, the manufacturer maintains strict control of each of these processes. The drilling process makes many types of holes in a PCB, such as:

  • Through Holes
    • Plated Through Holes
    • Non-Plated Tool Holes
    • Plated Slotted Holes
    • Non-Plated Slotted Holes
  • Vias
    • Blind Vias
    • Buried Vias
  • Special Holes
    • Side-Stepped Holes
    • Countersunk Holes
    • Crimp Holes
    • Hole on Pads

Types of PCB Holes

Through Holes

Most printed circuit boards have through holes. These are holes passing through the entire thickness of the board. PCBs with more than two layers usually have a few plated through holes, some non-plated tool holes, and some plated and/or non-plated slotted holes. An electroplating process provides a thin conductive barrel from one end of the hole to the other, through the PCB.

Tool holes are mounting holes for aligning the board to a text fixture or a machine in its operating position. In general, a tool hole does not have any plating, which makes it non-conducting any electrical component or trace on the PCB.

The manufacturer provides tool holes on the periphery of the board, along a major axis. The second hole is typically a slotted hole, also non-plated. The board may have other slotted or through holes, plated and/or non-plated, for other mechanical/electrical requirements.

Apart from tool holes, a PCB may have many plated through holes for mounting through-hole components. The diameter of these plated-through holes is significantly smaller than tool holes.

Vias

Vias are holes necessary in the PCB for establishing electrical connections between PCB layers. They may be of two types–blind and buried.

Blind vias start from one of the two top surfaces of the board and end at an inner layer. They may electrically connect to more internal layers in between, depending on the design.

Buried vias start from one of the inner layers in the PCB, and electrically connect to one or more internal layers. They are not visible from outside the PCB.

As the function of vias is only to provide electrical connection between layers, their diameter is significantly smaller than those of tool holes and plated through holes. Additionally, vias are always plated through to provide electrical contact between the layers they interconnect. While a fabricator may drill a plated-through hole after assembling all the layers, they must drill via holes individually in the respective layer before assembling them.

Multilayer PCBs mostly use blind and buried vias, as they provide the maximum routing density. In most cases, where plated through holes cannot offer performance and manufacturing requirements, blind and buried vias help to break the density limitations in special designs such as those involving BGAs and fine-pitch components. Using blind and buried vias can help to reduce the size and number of layers required in a PCB. For very high density such as in HDI boards, fabricators may use lasers to drill tiny holes for vias.

To meet performance requirements, fabricators may cover the via opening with a solder mask or fill it with a conducting or non-conducting material. Tenting is the process of covering the via opening, and plugging is the process of filling the via hole.

Special Holes

These are holes for meeting various mechanical requirements of the PCB.

Conclusion

Older printed circuit boards had more plated through holes for mounting through-hole components. As there is greater demand for higher density boards, manufacturers have moved towards using surface mount components that do not require plated through holes. Moreover, higher density has led to the use of miniature vias drilled with lasers.