Finished Slot Size Tolerances
A slot is basically an elongated hole and are usually used for the fixation of components on a PCB.
Slots can be Plated or Non-Plated and are usually defined in the drill layers.
The Finished Slot Size tolerances are based upon the following factors:
- 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 plating process.
The amount by which a plated slot is enlarged is determined by the plated copper thickness and the known accumulated manufacturing tolerances.
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 talking 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 used for creating slots have normal tolerance of +0.01/-0.00mm or +0.00/-0.01mm.
This may change depending upon the size and function of the tool 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.