Back Drilling: Removing Via Stubs for High-Speed Signal Integrity

A via that passes through a thick board and connects only the upper layers leaves a length of plated barrel hanging below the connection, and that unused barrel is an open transmission line hanging off the net. Back drilling removes it, and the drill depth tolerance decides whether the result is a clean stub or a damaged plane.

What a Via Stub Does to a Signal

An unused barrel below the last connected layer behaves as an unterminated transmission line, so energy that reaches it reflects and returns to the net after a delay set by the stub length. The reflection adds a notch in the insertion loss and a dip in the time domain impedance, and the effect grows as the signalling rate rises.

The notch moves to lower frequency as the stub gets longer, so a short stub inside a thin board may be tolerable while the same design in a 3 mm backplane is not. For most designs the practical threshold is a stub under 0.3 mm, which is why signal integrity budgets are written with a stub figure rather than with a drilling instruction.

Controlled Depth Drilling Basics

Back drilling is a second drilling operation that removes the unwanted barrel with a bit slightly larger than the original via, entering from the side opposite the connected layers. The bit is larger so that it clears the plating without touching it, and the diameter step is normally 0.2 mm over the finished via.

Depth is controlled by the machine’s Z axis against a reference surface, which is why a pressure foot and a contact plate are used to establish where the board surface really is. Measuring from the table rather than from the surface introduces the panel thickness tolerance directly into the stub length, and that tolerance is larger than the depth allowance on a thin panel. A contact plate that is worn or a pressure foot that does not seat flat moves the reference surface by more than the whole allowance, so both are checked at the start of every shift.

Depth Tolerance and the Anti-Pad

The useful window is narrow. The drill has to go deep enough to remove most of the stub and shallow enough to leave the connected layer and its anti-pad intact, and a typical target is a remaining stub of 0.15 mm to 0.25 mm with a depth tolerance of plus or minus 0.10 mm.

Two failure modes bracket that window. Drilling too deep nicks the plane on the connected layer and can create a short or a reduced clearance, while drilling too shallow leaves a stub that still resonates. Both are found at microsection rather than in electrical test, so the coupon and the section are the control.

Design Rules for Back Drilled Vias

A via cannot be back drilled unless there is a length of barrel available to remove, so the design has to place the connection well above the bottom of the board and leave the opposite side clear of components and traces. Where a pad or a component sits directly below the via, the drill has no room to enter.

The via pad on the connected layer should be large enough to tolerate the depth tolerance, and the drill programme should name the depth for each via class rather than applying one depth to the whole panel. Our hole copper notes describe how the barrel that remains is specified and measured.

Choosing Which Vias to Back Drill

Back drilling is applied to the vias that carry high-speed signals, typically differential pairs leaving a connector or a large ball grid array, and it is left off power and ground vias because their stubs do not carry signal. Applying it to every via adds drilling time and breakage risk for no benefit.

The selection is made from the stack-up and the routing rather than from the net list alone, because a via with a short stub may need nothing while a long one on the same net does. The list should be recorded per layer pair so that a stack-up change can be assessed against it.

Drill Bits, Wear and Registration

Back drilling uses carbide bits that are wider and shorter lived than the original via drills, because they cut through plated copper and glass rather than through laminate alone. Bit life is counted in holes and the limit is set by the entry and exit quality rather than by a fixed number.

Registration matters as much as depth, because a bit that enters off centre removes the barrel unevenly and can cut into a neighbouring via. Registration is verified on the first panel of a lot with a drilled coupon, and the same coupon is used later for the depth measurement. Bit life is recorded per hole count and per stack-up, because a bit that lasts through a thin panel will not last the same count in a thick one, and the difference shows first as a ragged hole entry.

Where the Operation Sits in the Sequence

Back drilling normally takes place after plating and before solder mask, so that the opening it creates can be covered and no plating is disturbed afterwards. Moving it later exposes the panel to handling after the mask is cured and risks damage that cannot be repaired.

The operation also produces its own debris, which has to be removed before the next step. A hole that is drilled after plating and left uncleaned carries glass and copper dust into the mask process and into the assembly, and the debris is a common cause of a foreign object defect.

Verification by Coupon and Cross-Section

Stub length is verified on a coupon that carries the same stack-up and the same via sizes, sectioned through the via axis and measured at 50 to 200 times. The measurement is taken from the bottom of the connected layer to the end of the remaining barrel, and the figure is compared against the drawing. The section also shows whether the barrel was removed symmetrically, since an off-centre bit leaves a crescent of copper that changes the impedance without changing the measured length by a measurable amount.

Electrical verification uses time domain reflectometry on the same coupon, where a stub shows as a dip in impedance after the via. The two measurements agree only when the section is taken at the same location, so the coupon is designed with a marked via for sectioning, and our plating thickness notes cover the preparation of that specimen.

Cost, Yield and Documentation

Back drilling adds a drilling step, a second registration and a breakage risk, so it is charged per hole and it reduces panel yield more than a normal drill operation. A panel that is drilled too deep is scrap, and the cost of the whole panel is lost at a late stage.

The drawing should state the stub target, the depth tolerance, the vias that are exempt and the reference from which depth is measured. Our fabrication notes set out the way those values are written so that the shop and the designer are measuring the same thing. A depth applied uniformly across a panel with mixed via lengths will leave stubs where the vias are short and cut into planes where they are long.

FAQ

Can a back drilled via be repaired if the depth is wrong? No. Material that has been removed cannot be replaced, and a via drilled into the connected layer is a short or a clearance violation. The panel is scrapped, which is why the depth is proven on a coupon before the lot is drilled.

Is back drilling needed on a thin board? Often it is not, because the stub left by a thin stack is short enough to be tolerable. The figure that matters is the stub length rather than the board thickness, so the decision is made from the stack-up and the signalling rate together.

Does back drilling change the impedance of the via? It removes the capacitance of the unused barrel, so the via impedance rises towards the value the design assumed. That is the point of the operation, and the coupon measurement is what confirms the change happened in the right direction.