Backdrilling is a technique used to remove unused via barrel segments (via stubs) after the PCB has been drilled and plated. This process involves drilling from the back side of the board to a controlled depth, eliminating the stub while preserving the functional portion of the via.
How backdrilling works
After standard through-hole drilling and copper plating, a secondary drill bit (typically 5-10 mil larger than the original via diameter) is used to remove the unwanted stub portion. The process is controlled to stop just before (5-10 mil) the last signal layer (as shown in the image below).
Maintain a clearance of at least 10 mil between the backdrilled hole wall and adjacent copper features in all penetrated layers to prevent breakout or shorting.
When to backdrill your vias
Use backdrilling when via stub length exceeds λ/10 (one-tenth of the operating wavelength) for optimal signal integrity. For example, at 1 GHz, any unused via segment exceeding 570 mil in length should be backdrilled to prevent signal reflections.
The critical limit is λ/4, this is the lumped distance threshold. When stubs approach or exceed λ/4, they behave as resonant transmission line elements, causing signal reflections and degradation.
In some designs, backdrilling can also be used to remove connections to unused layers, making it a low-cost alternative to blind vias.
Things to include in your fab notes on backdrilling:
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Vias or layers that require backdrilling.
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Backdrill start and stop layers.
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Drill size, symbols (BD1, BD2), tolerance, and maximum allowable stub length.