When to backdrill a via (and when not to)
Backdrilling removes the unused part of a plated through-hole that behaves like an open stub. The calculation is simple: find the quarter-wave null, compare it with the channel's Nyquist frequency, and decide from the margin. The complications are mechanical — drill depth tolerance, antipad clearance and connectors that need the whole barrel.
The mechanism
A stub is a shorted transmission line at one frequency
When a signal changes layers through a plated through-hole, the barrel continues past the destination layer to the far side of the board. That leftover length is an open-circuited transmission line hanging off the signal path. At a quarter wavelength it transforms into a short at the junction and the channel stops passing energy at that frequency.
The null sits at f = c / (4 L sqrt(er)), where L is the stub length and er the effective dielectric constant around the barrel. A 1.0 mm stub in FR-4 resonates near 37 GHz, but the damage starts much lower: return loss degrades from about a fifth of the null frequency, which puts a 1 mm stub inside the band of a 28 GBaud PAM4 channel.
The practical rule is to keep the null at least three times the channel Nyquist frequency. Below that ratio, either backdrill or move the transition.
Run the stub length and stackup through the Via stub resonance calculator to see where the null lands before choosing a drill depth.
Fabrication
Depth control, overdrill and antipad clearance
Backdrilling is a second drilling operation with a bit slightly larger than the original hole, plunged from the opposite face to a controlled depth. Fabricators typically hold depth to plus or minus 0.15 mm and need an overdrill buffer beyond the target layer so the remaining stub is predictable. Ask for the number your fab can hold; a 0.15 mm residual is a common commercial limit and 0.10 mm costs more.
The bit is 0.15 to 0.20 mm larger than the primary drill, which means the plane antipad has to grow with it. If the antipad stays at the original size, the backdrill cuts into the plane and leaves burrs or a short. This is a stackup decision made at design time, not something the fab can fix later.
Exceptions
When not to backdrill
Backdrilling adds a drilling pass, depth calibration and scrap risk. It is not a default setting for every via on the board.
- Press-fit connectors such as AirMax or XCede need the full plated barrel through the connector depth for a gas-tight joint. Backdrilling destroys the retention.
- Through-hole parts that are wave soldered or pin-in-paste need barrel wetting through the full component depth.
- Signals that terminate on an outer or adjacent inner layer have almost no stub; measure before specifying a drill.
- A blind or buried microvia build removes the stub by construction and is often the better answer on a dense board.
- Stubs whose null lands above three times the Nyquist frequency can stay; the loss is not worth the process risk.
FAQ
Common questions
Want this worked through on your board?
Send the stackup and interface list and we will check the via strategy against a real layout — before the quote is fixed, not after.