Back Drilling Design Guide

Back drilling is the process that removes the unused part of a via barrel after plating. That unused part is the stub, and on a thick board it behaves as an open ended transmission line that resonates inside the band of a high speed link. The process is simple to describe and difficult to control, because the depth of the cut has to be held to a fraction of a millimetre across a panel that is not perfectly flat.

What Back Drilling Does

A through hole via is plated from the top to the bottom of the board, and the signal usually uses only part of that barrel. The rest is a stub, connected at one end and open at the other.

Back drilling uses a slightly larger drill to remove the unwanted part of the barrel from one side, leaving a controlled stub at the layer where the signal leaves. The drill does not cut the copper that the signal uses.

The result is a via that behaves as a short transition rather than a resonator, and the improvement in a time domain measurement is immediately visible as the disappearance of a dip in the impedance profile.

The depth of the counterbore is measured from the surface of the board, and the residual stub is what remains between the bottom of the counterbore and the layer that carries the signal. The two numbers are related but they are not the same, and a drawing that quotes only one of them leaves the shop to work out the other.

The process is applied to the whole board, so every via in the drill program is counterbored to the same depth unless the program is split.

Stub Length and Resonance

A stub resonates when its length is a quarter of a wavelength at the frequency of interest, because the open end reflects the signal back in phase with the incident wave.

The relation is the reason a thick board is worse than a thin one. A stub of five millimetres resonates near seven gigahertz in a typical laminate, which is inside the band of a fast link.

The remedy is to shorten the stub, and the required length comes from the highest frequency that the link uses rather than from a rule of thumb. A common target is a stub of a few tenths of a millimetre.

The same reasoning explains why a blind via is better than a back drilled through via: the blind via has no stub at all, and the construction does the work that the drill would otherwise do.

Depth Control and Tolerance

The drill has to stop at a defined distance from the layer that the signal uses, and that distance is the residual stub. The tolerance on the depth is the parameter that decides whether the process is usable.

The board thickness varies across the panel, so a drill that is set from the surface has a depth error that follows the thickness variation. The error is often larger than the residual stub that the design allows.

The solution is to control the depth from the layer of interest rather than from the surface, which requires the drilling machine to know the position of that layer. A depth controlled machine uses the stackup data and an electrical or a mechanical sensing method.

Where the tolerance cannot be held, the design has to accept a longer residual stub or to use a different via construction. The fabrication notes should state the residual stub and the method of controlling it.

Drill Size and Registration

The back drill is larger than the original via, because it has to clear the plating without touching the barrel that the signal uses. The difference is typically a few tenths of a millimetre.

The larger drill removes copper from the surrounding area, which means the antipad and the neighbouring traces have to provide the clearance. A dense area may not have room for the larger tool.

Registration is the second variable. The back drill has to be aligned with the via, and an error that is large enough to cut into the signal barrel creates a defect that is invisible from the surface.

The clearance between the back drilled hole and the nearest copper is a design rule that has to be added to the layout. The via construction options give the context for choosing between back drilling and an alternative.

When to Use It

Back drilling is used where the board is thick, the data rate is high and the residual stub would otherwise resonate inside the band. A thin board seldom needs it.

The second application is a board that has to keep a through hole construction for cost or for mechanical reasons and that cannot use blind vias. Back drilling gives most of the benefit of a blind via at a lower cost.

The third application is a board with a mixed stackup, where only some of the vias are critical. The drill program can be split so that only the critical vias are counterbored, which saves machine time.

The decision should be made with the loss budget, since a link that is comfortable without back drilling does not need the extra process step and its cost.

Process Risks

The main risk is a depth error that leaves a stub longer than the design assumed, which leaves a resonance in the band and a link that fails intermittently.

The second risk is damage to the signal barrel, either by cutting into it or by leaving a burr that later breaks loose inside the hole. A burr is a conductive particle that can move and short a net.

The third risk is drill wear, which changes the diameter and the depth over a production run. The tool life has to be controlled and the first and last panels of a batch should be checked.

The aspect ratio of the original via affects the process, because a deep small hole is harder to counterbore accurately. The aspect ratio guide gives the limits that the shop can hold.

Practical Rules

Decide the residual stub from the highest frequency in the link, and write it on the drawing as a number with a tolerance.

Check that the layout has room for the larger drill, including the clearance to the neighbouring copper and the antipad.

Ask the shop how the depth is controlled and what the depth tolerance is, before the design depends on a number that cannot be held.

Verify the result on a cross section coupon, and keep the section with the panel documentation. The high speed design rules should be applied together with the process capability.

FAQ

How short should the residual stub be? A few tenths of a millimetre for a fast link, and the value comes from the highest frequency of interest. A stub of a quarter wavelength at that frequency is the limit to avoid.

Does back drilling damage the via? It can if the depth or the alignment is wrong. A controlled process removes only the unused barrel and leaves the signal path intact.

Is back drilling cheaper than a blind via? Usually yes, because it uses a through hole process with an extra step rather than a sequential lamination. For a very high layer count the comparison changes.