Back Drilling For Stub Removal In High Speed Boards

A plated through hole that connects an inner layer to the surface continues all the way to the opposite side of the board, because the hole is drilled through the whole stack. The portion of the barrel below the layer that the signal uses is not connected to anything, and at high frequency that unused length behaves as a branch of the transmission line rather than as a harmless piece of copper. Removing it is the purpose of back drilling.

This article explains why a stub degrades a signal, how the back drilling process works, what tolerances it holds, and when the technique is worth its cost.

Why The Stub Matters

A stub is an open circuited transmission line attached to the signal path at the point where the signal leaves the via. Energy that travels down the stub reflects at the open end and returns, and the reflection either adds to or subtracts from the signal depending on the frequency. At the frequency where the stub is a quarter of a wavelength long, the returning energy cancels the forward signal and produces a deep notch in the insertion loss, which appears in a digital channel as a collapse of the eye diagram at that data rate.

The resonant frequency falls as the stub gets longer, and it also falls as the dielectric constant rises. A stub of one and a half millimetres in a conventional laminate resonates at around twenty five gigahertz, while a stub of three millimetres, which is common on a thick backplane, resonates near twelve gigahertz. Since a digital signal occupies a band around its fundamental, the stub starts to matter when its resonance approaches the upper part of that band, which is why back drilling appears on high speed channels and not on a general purpose board.

The effect is easier to see in the frequency domain than in a schematic. A plot of insertion loss against frequency shows a series of dips, and the first one is the stub resonance. Below that frequency the channel performs close to its design, and above it the loss rises steeply, so a link that works at one data rate fails at the next without any change to the layout. An eye diagram measured at the receiver shows the same information as a closed eye at the affected frequency.

Not every via needs treatment. The vias that matter are the ones in the signal path of a high speed channel: the connector breakout, the transition between a surface layer and an inner layer, and the via fields under a ball grid array. A via that carries a low speed control signal has a stub that is electrically irrelevant, and drilling it would add cost and risk for no benefit. The selection is therefore a channel by channel decision rather than a board level rule.

How Back Drilling Works

The process is a second drilling operation, performed from the side of the board opposite to the layer that the signal uses. A drill of a larger diameter than the original via is advanced to a controlled depth that stops just below the layer to be connected, which removes the unused copper barrel and a small amount of the surrounding material. The result is a counterbore that is wider than the via and that ends a short distance below the last connected layer.

The operation is usually placed after plating and before the outer layers are finished, so that the plating in the remaining barrel is not disturbed. The drilling depth is the critical parameter, and it is controlled either by the machine depth or by a conductive sensing technique that detects the moment the drill reaches the target layer. The repeatability of that control is what sets the residual stub length, which is typically between two tenths and half a millimetre.

The Tolerances And The Risks

The residual stub is the difference between the drilling depth and the depth at which the connected layer sits, and it therefore contains the tolerance of the board thickness, of the layer positions, and of the drilling machine. A design that needs a very short stub has to allow for all of them, and the fabricator should be asked what residual length can be guaranteed rather than what the machine can achieve on a good day.

The two failure directions are different. Under drilling leaves a longer stub than intended, which degrades the electrical performance without damaging anything, and it is invisible unless the boards are sectioned or measured. Over drilling removes copper from the connected layer or leaves the connection with an inadequate barrel, which is a reliability risk that may pass continuity test and fail in the field. The safe process therefore has a bias towards leaving a small stub rather than towards removing the last fraction of a millimetre.

Design Considerations

Back drilling applies to through vias, since a blind via already stops at its target layer and has no stub to remove. The layers that carry the signal should not be close to the far surface of the board, because the process needs distance to drill into, and the antipads around the via have to be large enough that the larger drill does not clip a plane that is not part of the connection. Where a via passes through several planes, the back drilled counterbore will cross them, and the design has to allow that.

The remaining barrel carries the same current as before, because the connected portion is untouched, but the counterbore removes material around the via, which can affect the local mechanical strength and the planarity of the surface where it breaks out. Where a back drilled via sits under a pad or a ball, the counterbore opening has to be filled or plugged so that solder does not wick into it, and the surface has to be flat for the component to seat on.

Verification And Cost

The electrical effect is verified with a coupon. A time domain reflectometer shows the impedance profile along the via, and a vector network analyzer measures the insertion loss and the return loss, which reveals the notch and confirms that it has moved above the band of interest. A cross section of a sample shows the residual stub length and the condition of the counterbore, and it is the only direct check on the mechanical result.

The cost is a second drilling operation, an additional inspection, and the risk of a scrap part if the depth control fails, so the technique is reserved for the channels that need it. Where the same performance can be obtained with a thinner board, with a different stack that places the signal layers closer together, or with blind vias that never had a stub, those options are usually cheaper than back drilling every through via on the panel. The transmission line context for those decisions is described under microstrip and stripline routing, the differential pair geometry under differential trace routing, and the layer strategy under multilayer advantages at high speed.

FAQ

How much stub is acceptable? It depends on the data rate and on the loss budget. A shorter stub pushes the resonance higher, and the practical answer is to keep the resonance above the highest frequency that carries significant energy in the channel.

Does back drilling damage the via? Not when the depth is controlled and the process leaves a residual stub. Over drilling removes copper from the connected layer and is the failure mode to avoid.

Can a blind via replace back drilling? Where the stack allows, yes. A blind via stops at its target layer and has no unused barrel, so the stub problem never arises, at the cost of a more complex lamination sequence.