Backdrilling: Key Checks Before Release

A plated through hole connects the layers it is meant to connect, and it also creates a length of barrel below the lowest connected layer that serves no purpose electrically. At low frequencies that stub is invisible. At multi gigabit rates it behaves as a resonant transmission line branch, and it produces a notch in the channel response that no amount of equalisation can remove. Backdrilling is the process of removing it.

Why a Stub Matters

The unused portion of the barrel is a copper tube that branches off the signal path at the via. The branch reflects energy at frequencies where its length approaches a quarter wavelength, and the result is a deep notch in the insertion loss together with a peak in the return loss. The higher the data rate, the lower the frequency at which the effect appears.

On a thick backplane the stub can be several millimetres long, which places the notch within the band of a high speed link. Removing the stub moves the resonance far above the band of interest, and it also reduces the capacitance the via adds to the trace.

How Backdrilling Works

A drill bit slightly larger than the finished hole diameter is used to ream out the plated barrel from the side opposite the connected layers. The bit removes the plating and the surrounding copper, leaving a counterbore in the laminate while the upper portion of the barrel and its connections remain intact.

The process is controlled by depth. The critical dimension is the length of barrel that remains between the lowest connected layer and the bottom of the counterbore, which is called the remaining stub. Everything else, including the depth of the counterbore, is a means of achieving that value.

Depth Control and Tolerance

Depth control is where the process is won or lost. The drill must stop close enough to the target layer to leave a short stub, and far enough away not to damage the connection. Board thickness tolerance, layer position tolerance and machine depth accuracy all contribute to the result.

A typical specification calls for a remaining stub of no more than a defined length, often around 0.2 to 0.3 millimetres, with the counterbore diameter a fixed amount larger than the via. The achievable figure depends on the fabricator and on the stackup, and it should be agreed rather than assumed.

Which Vias to Backdrill

Backdrilling every via is unnecessary and expensive. The vias that matter are the ones on the high speed channels, and among those the ones that pass through the most unused layers. A via that terminates on the layer immediately below its connection has almost no stub and needs no treatment.

The selection is therefore a signal integrity exercise. Simulation of the channel with and without the stub identifies the vias that limit the design, and the backdrill list is built from that analysis. Our article on high speed design rules covers how those limits are evaluated.

Design and Stackup Effects

Stackup choices change how much backdrilling is required. Placing high speed routing on layers near the surface keeps stubs short, and routing a channel through fewer layers reduces the number of stubs that need removing. A stackup designed with this in mind can reduce the backdrilled via count substantially.

Via diameter and drill size also interact. A larger via has more capacitance and adds more loss, and the counterbore for a larger via is larger, which removes more laminate. On a dense board the counterbore has to fit between neighbouring features without breaking out, and the annulus around the vias must be adequate. The related limits are described in our article on aspect ratio.

Effects on the Barrel and Reliability

Removing part of the barrel removes plating that was contributing to the mechanical structure of the hole. In most cases this is harmless, because the remaining barrel is still anchored in the connected layers, but the counterbore end creates a step where the plating terminates and where the laminate is exposed.

That step should be inspected on a sample. A clean end with intact laminate and no plating smear indicates a correct process, while a ragged end or one with copper burrs indicates a worn bit or a wrong feed. The inspection method is the same sectioning technique used for any barrel quality check, described in our article on hole copper.

Specifying and Verifying

A backdrill specification should state the vias to be treated, the maximum remaining stub, the counterbore diameter and the acceptance method. It should also state the tolerance on depth, because that is what determines whether the remaining stub is achievable across a panel with thickness variation.

Verification is by cross section on a sample. Because the process is mechanical and depends on tool wear, the sample should be taken from a panel produced at the end of a bit life as well as at the beginning, so that the worst case is represented rather than the best.

Checks Before Release

The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected. Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

FAQ

Does backdrilling change the impedance? It removes some capacitance from the via, which slightly changes the impedance of the transition. The effect is normally an improvement, and it should be confirmed by simulation of the channel.

Can backdrilling damage a via? It can if the depth is set too aggressively and the bit reaches the connected layer. That is why the remaining stub is specified rather than the counterbore depth alone.

Is backdrilling needed below 5 Gbps? Rarely. The stub resonance usually sits well above the band at those rates unless the board is very thick.