PCB Drill Aspect Ratio And Hole Quality Limits

Every plated hole in a board begins as a drilled hole, and the drilling process imposes limits that the design has to respect. The most important of those limits is the ratio of the hole depth to its diameter, because it governs whether the plating solution can reach the middle of the barrel, whether the drill can stay straight, and whether the hole can be cleaned before plating.

This article explains the ratio, the way it constrains the process, and how a stack up can be designed to stay inside the limits rather than to test them.

What The Aspect Ratio Measures

The aspect ratio is the depth of the hole divided by its diameter. For a through hole in a board of a given thickness, a smaller diameter produces a higher ratio, so a fine pitch design with small vias in a thick board is the case where the ratio becomes large. The ratio is quoted for the drilled diameter rather than for the finished diameter, because the drilling is where the limit applies.

The depth is the thickness of the material the drill passes through, which for a through hole is the board thickness and for a blind hole is the depth of the layer stack above the target. A blind hole therefore has a lower ratio than a through hole of the same diameter, which is one reason a high density design moves to blind and buried vias as the board gets thicker.

Limits Of The Drilling Process

A drill bit is a slender tool that spins at high speed and is pushed through a stack of glass reinforced laminate. As the ratio rises the bit becomes more flexible, so it wanders from the intended path and the hole is no longer perpendicular to the surface. The wander shows up as a misregistration at the far side of the board, which reduces the remaining copper between the hole and the pad or the plane.

The bit also wears. A worn bit cuts a rougher wall and generates more heat, and the heat can smear the resin over the copper of the inner layers rather than cutting it cleanly. That smear has to be removed by a desmear step before plating, and a hole with a large ratio is harder to desmear because the chemistry has to reach the middle of the barrel. The practical limits quoted by fabricators, typically in the range of ten to twenty to one depending on the process, come from these effects rather than from a single rule.

Plating Thickness In A Deep Hole

The plating solution has to flow into the hole and the current has to reach the barrel, and both become harder as the ratio rises. The result is a plating thickness that is greatest at the surface and least in the middle, and the minimum thickness in the middle is what the specification applies to. A requirement for twenty five micrometres of copper in the barrel therefore has to be met at the centre of the hole, not at the surface.

To meet that requirement at a high ratio the fabricator has to increase the plating time, which raises the thickness at the surface and closes the hole diameter. A design that specifies a small finished diameter and a thick barrel plating is asking for a process window that may not exist, and the requirement should be checked against the fabricator capability before the pad geometry is fixed.

Registration And Drill Wander

Registration is the alignment of the drilled hole with the pad and with the inner layer features. It is affected by the accuracy of the drilling machine, by the movement of the material during lamination, and by the wander of the bit itself. A high ratio makes the wander worse, so the registration allowance has to be larger, which means a larger pad or a larger clearance to the plane.

The interaction with the design is direct. A hole that is small relative to the pad thickness leaves little margin, and the annular ring that remains after the worst case misregistration is what determines the reliability. Where the stack up allows, keeping the ratio moderate and the annular ring generous is a more robust choice than a tight design that depends on the machine holding its best case.

Back Drilling And Its Constraints

Back drilling removes the unused portion of a plated through hole barrel from the far side of the board, which reduces the stub that reflects a high speed signal. The operation is a second drilling step with a slightly larger bit, and it is controlled by depth rather than by breakthrough.

The depth control has to leave enough of the barrel to make the connection and enough clearance to avoid damaging the last layer that is in use. A high aspect ratio makes the depth control harder, because the drill has to travel further and the tolerance on the depth is larger. The design has to leave room on the far side for the back drill, which is why the technique belongs in the stack up discussion rather than being added after the layout. The routing context is described for microstrip and stripline and in the general fabrication process.

Designing Within The Limits

The first decision is the board thickness, and it should follow from the mechanical requirement rather than being increased for convenience. The second is the smallest hole that the design needs, which usually comes from the via count and the routing density rather than from the electrical requirement. The ratio is then the quotient of the two, and it should be compared with the fabricator capability before the layout is committed.

Where the ratio is too high, the options are a thinner board, a larger via, a blind via that reaches only the layers in use, or a change of material. Each has a consequence: a larger via reduces the routing density, a blind via adds process steps, and a thinner board may not meet the mechanical requirement. Choosing among them is a design decision, and it is easier to make at the stack up stage than after the artwork is finished. The layered options are discussed in the selection of blind and buried vias and the prototype sequence in multilayer prototype requirements.

Process Control and Verification

On a design of this kind, plating thickness is the item that decides how the rest of the board is arranged. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

FAQ

What aspect ratio should a design target? There is no single number, because the limit depends on the fabricator and on the plating specification. A ratio well inside the quoted limit leaves margin for the registration and the annular ring.

Does a blind via have a lower aspect ratio than a through hole? Usually yes, because the depth is only the part of the stack above the target layer. That is one of the reasons a thick high density board uses blind and buried vias.

Why is the barrel plating thinner in the middle? Because the solution and the current have more difficulty reaching the centre of a deep hole. The specification applies to the thinnest point, which is normally the middle of the barrel.