Working with the Max Via Stub Length (Back Drilling) Design Rule on a PCB in Altium Designer

This document is no longer available beyond version 21. Information can now be found here: Max Via Stub Length (Back Drilling) Rule for version 24


Rule category: High Speed

Rule classification: Unary


A stub is the length of via or pad barrel that protrudes beyond the last-used signal layer that the via/pad connects on. Back drilling will be applied to suitable vias/pads in nets targeted by this rule, in accordance with the enabled side-of-board Layer checkboxes, and the back drill pairs defined on the Back Drills tab of the Layer Stack Manager.

As well as being used to define which vias/pads are to be considered for back drilling, during design rule checking this rule tests for via and pad stubs longer than the specified Max Stub Length, for all nets targeted by this rule (regardless of whether back drilling has been applied to that via or pad). This rule also specifies how much larger the drill size must be for vias and pads that are back drilled.

Use appropriate rule scoping to target vias/pads in a specific net (or collection of nets), using query expression keywords such as InNet, InNetClass, or In AnyNet.
All design rules are created and managed within the PCB Rules and Constraints Editor dialog. For a high-level view of working with the design rules system, see Defining, Scoping & Managing PCB Design Rules.


Default constraints for the Max Stub Length rule.Default constraints for the Max Stub Length rule.

  • Max Stub Length - the maximum length of stub allowed. This setting is used to test for remaining stubs of this length or longer, it does not define the drill depth used for back drilling.
    • Top Layer - back drill vias/pads, in targeted nets, from the top side of the board.
    • Bottom Layer - back drill vias/pads, in targeted nets, from the bottom side of the board.
  • Back Drill Oversize - the radial increase in size of the drill used for back drilling, over the original via/pad hole size.
    • Tolerance - allowable +ve and -ve tolerance allowed on this drill size, enter both as a positive number.
The drill depth is defined by: the sum of the layer thicknesses of all layers from the first layer (included) through to the last layer (not included), defined in the Layer Stack Manager.

How Duplicate Rule Contentions are Resolved

All rules are resolved by the priority setting. The system goes through the rules from highest to lowest priority and picks the first one whose scope expression matches the object(s) being checked.

Rule Application

Batch DRC.

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