Finished Slot Size Tolerances

There are 2 types of slots on a PCB.

These Plated and Non-Plated both have the same Finished Slot Size tolerances regardless of size or type.

A slot is basically an elongated hole used for the fixation of components on a PCB and are usually defined in the drill layers.

The Finished Slot Size tolerances are based upon the following factors:Type of Slot – Plated, Non-Plated or Via.

  • Nominal Slot Size vs available tool Sizes.
  • Tool Bit Size Tolerance.
  • Tool Bit Wear during use.
  • Slot Cleaning (Desmear).
  • The Plating Process and Copper Balance.
  • The Final Surface Finish of the PCB.

Plated slots like plated holes are created larger than this required finished size, this is to allow for the manufacturing tolerances and downstream processes.

The amount by which the slot is enlarged is determined by the known accumulated manufacturing tolerances applicable to crating slots in a PCB.

There are 2 types of slots on a PCB, Plated and Non-Plated both have the same Finished Slot Size tolerance.

Nominal Slot Size vs available Tool Bit Sizes

The defined tolerances start with the tool bit size, these are available in standard size steps of 0.05mm.

Any slot defined that does not meet this 0.05mm steps will be rounded to the nearest step.

It is possible to have smaller tool bit size steps but the cost far outweighs any reason to do so.

Looking at it from another point of view, we are talk about a rounding with a maximum value of 0.025mm or 25µm.

This should not cause any downstream process issues.

Tool Size Tolerance

The tool bits for creating slots are machined and thus have a tolerance on the size as set by the manufacturer.

Standard Tool bits have normal tolerance of +0.01mm or -0.01mm.

This may change depending upon the size and function of the drill bit.

Tool Bit Wear

When create a slot in a PCB the friction created leads to the wear of the tool bit.

Whilst, this is very small amount (in the microns) the tool bit must be replaced regularly based on this known wear rate.

However, it still must be considered as part of the tolerances accumulated during the manufacture of a PCB.

The Slot Cleaning (Desmear)

After the creation of a slot it must be cleaned.

This is to remove any particles that were left in them by the drilling process.

Cleaning is a chemical process, known as desmear, it also smooths the hole walls ready for the black hole treatment

During the cleaning a very small amount (less than a micron) of the hole wall is removed, thus increasing the slot size.

If the cleaning is not performed correctly then the black hole process may fail and so would the plating process.

The Plating of Slots and the Copper Balance

Our target is to deposit approximately 20-25µm of copper in the plated through slots.

However, the Copper Balance of a PCB greatly influences how evenly the copper is distributed across the whole PCB.

The copper distribution shown below would lead to more copper being deposited in areas with less copper density (red).

Resulting in the plated slots in this area having thicker copper walls and thus a smaller actual finished slot size.

For more information on how to improve the copper balance please see our Tips & Tricks post on How to achieve a more even copper density.

The Final Surface Finish of the PCB

When we talk about the Final Surface Finish we are, referring to the final metallised protective finish for pads and exposed copper on a PCB.

This is usually applied after Soldermask and and can be either a HAL, ENIG, ImAg etc.

Any plated slots will also be plated during the Final Surface Finish process and thus the finished slots size will be reducted.