# PCB Glossary: Backdrilling

**URL:** <https://sierraconnect.protoexpress.com/t/pcb-glossary-backdrilling/3682>\
**Category:** Manufacturing\
**Tags:** drilling\
**Created:** [November 7, 2025, 7:05am UTC](https://sierraconnect.protoexpress.com/t/pcb-glossary-backdrilling/3682 "2025-11-07T07:05:31Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![Pooja\_Mitra](https://avatars.discourse-cdn.com/v4/letter/p/ce73a5/32.png) [@Pooja\_Mitra](https://sierraconnect.protoexpress.com/u/Pooja_Mitra)\
**Post date:** [November 7, 2025, 7:05am UTC](https://sierraconnect.protoexpress.com/t/pcb-glossary-backdrilling/3682/1 "2025-11-07T07:05:31Z")

</div>

[Backdrilling](https://www.protoexpress.com/blog/back-drilling-pcb-design-and-manufacturing/?utm_source=sierraconnect&utm_medium=display&utm_campaign=glossary%252Bbackdrilling%252Bthread%252Bnov%252B2025) 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).

 ![Backdrilling-to-remove-via-stubs](https://global.discourse-cdn.com/sierraconnect/original/1X/8a7c4dddac56efc9b906062ea2198992cb35804e.jpeg)

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:**

- Vias or layers that require backdrilling.

- Backdrill start and stop layers.

- Drill size, symbols (BD1, BD2), tolerance, and maximum allowable stub length.