PCB Drilling: Hole Types, Tolerances and Limits

Drilling is the step where a circuit board stops being a flat sheet and starts being a three dimensional structure, and it is the operation with the most potential to damage a design. A hole that is slightly the wrong size, slightly the wrong place or slightly damaged can pass every inspection and still produce an open circuit in the field.

The Hole Types

Five categories cover almost everything.

Through holes are drilled from one face to the other and plated to connect the top and bottom layers and any inner layer they pass through. They are the cheapest and the most common.

Blind vias are drilled to a specific depth and connect an outer layer to an inner one, and buried vias connect inner layers only. Both require a controlled depth or an earlier lamination step.

Microvias are small holes, typically 75 to 150 micrometres, formed by a laser rather than a mechanical drill. They connect adjacent layers in an HDI build-up and are discussed in more detail in our guide to HDI construction.

Mechanical holes also include the non electrical variety: mounting holes, tooling holes, connector bores, slots and cutouts. These may be routed rather than drilled, and their tolerances are set by the mechanical function rather than by plating.

Punched holes exist only on very thin, high volume boards, where a die can punch holes faster than a drill can bore them. Punching suits flexible circuits and thin rigid boards, and it cannot hold the tolerance of a drilled hole on a thick stack.

How the Process Works

A CNC drilling machine holds a stack of panels on a table and moves a spindle head to each programmed position in turn. The number of holes and the number of panels in the stack determine the hit rate, which is the measure the cost is based on in practice.

Entry and exit material are used on the top and bottom of the stack. The entry material supports the copper and stops it from burring as the bit breaks through. The exit material does the same on the other side and also protects the table. Both are consumed per stack, which is another reason the panel count matters more than the board count.

Drill bits are carbide for rigid boards, and they are selected by diameter, by flute geometry and by the material being drilled. A high glass transition or ceramic filled laminate is more abrasive than standard FR-4, so bits wear faster and the cost per hole rises.

Drilled Size Versus Finished Size

One confusion causes more arguments between designers and fabricators than any other single number, and it is this: the hole on the drawing is not the hole the drill makes.

Plating deposits copper on the barrel wall, so the finished hole is smaller than the drilled hole, typically by about 25 micrometres on diameter for a standard plating thickness, and more where heavier copper is required. If the drawing states a finished hole size, the fabricator will drill larger to allow for the plating. If the drawing states a drilled size, the finished hole will be smaller than the designer expected, and a component lead may not fit.

The convention worth adopting is to state the finished hole size, because that is what the component sees. Where a design does not specify, suppliers will follow their own practice, which is a good reason to write the requirement on the fabrication drawing rather than assume it.

Tolerance

Hole position and hole size are separate tolerances, and both are affected by the machine and by the condition of the bit.

Hole diameter is typically held to about plus or minus 0.075 mm on a standard process, and to plus or minus 0.05 mm on a tighter one. That tolerance is on the finished hole, so it covers both the drilling accuracy and the plating variation.

Position is quoted as a registration tolerance relative to the artwork, and it is the number that interacts with the pad. A hole that is at the edge of its positional tolerance eats into the annular ring on one side, and where the ring is small the connection can be lost. Designing a pad with a ring that absorbs the registration tolerance is the standard defence, and it is why minimum annular ring requirements appear in every design rule set.

Aspect ratio is the third limit. It is the ratio of the board thickness to the drilled hole diameter, and it exists because the plating solution has to reach the middle of the barrel. Around 8 to 1 is comfortable for standard processes, 10 to 1 is achievable with good control, and beyond that the process becomes specialised and expensive, with a real risk of a thin or incomplete barrel plating. A 1.6 mm board with a 0.3 mm hole is already at about 5 to 1; a 3 mm board with the same hole is at 10 to 1 and is a different proposition entirely.

Two design habits follow from these limits. Keep the smallest hole as large as the design allows, because every step down in diameter reduces the aspect ratio margin and increases the drill cost. And use as few distinct hole sizes as possible, since each size requires a separate drill bit, a separate setup in the machine program, and a separate inspection.

Bit Wear and Hole Quality

A drill bit is a consumable, and the defects that appear in a hole are usually a sign that the bit has been used too long.

Oversize holes are the first symptom. As the cutting edges wear, the bit cuts less efficiently and the hole grows. A worn bit on a 0.3 mm hole can drill 0.34 mm, which is outside the tolerance and consumes the annular ring.

Nailheading is the deformation of the copper foil around the hole entry, where the bit pushes the copper rather than cutting it. It appears as a conical flare and it reduces the effective annular ring at the surface, which is exactly where the pad needs its strength.

Burrs are raised edges of copper left on the entry or exit side. A burr can be pushed into the hole during plating or can short two adjacent pads if it is large enough.

Resin smear is the smearing of softened resin across the inner layer copper during drilling, and it is worse on harder laminates and on thick panels. The resin film insulates the inner layer from the plating, which produces an intermittent or high resistance connection that a functional test may pass at room temperature. Desmear, the chemical removal of the smeared resin, is the standard remedy, and its effectiveness is a process control point rather than a design choice.

Voids in the barrel plating are related to the hole condition. Rough walls, debris and outgassing from the laminate all reduce the chance of a continuous copper layer, and the defect is invisible until a cross section is taken or the board is thermally cycled.

The design response to all of these is the same. Give the hole more annular ring than the process strictly requires, keep the aspect ratio inside the comfortable range, avoid stacking several demanding requirements on the same design, and specify the drilling class on the drawing so that the supplier applies the right bit change frequency and the right inspection level.

Material and Process Variation

The material being drilled changes the process more than most designers expect.

Standard FR-4 with a moderate glass transition temperature drills easily. Higher Tg material is more brittle and more abrasive, so bits wear faster. Ceramic filled and low loss laminates are harder again, and some of them require a different bit geometry and a lower chip load. Metal core boards are a separate case entirely, since the drill has to pass through aluminium or copper, and aluminium chips must be managed to avoid contamination.

Thickness has a similar effect. Drilling a thick stack of thin panels is efficient, while drilling a single thick panel is slow and generates more heat, which increases smear. That is one reason a very thick board costs disproportionately more than a thin one with the same number of holes, and it is worth checking the quoted drilling time rather than only the material price when comparing two thickness options.

Drilling Design Rules

• State finished hole sizes on the fabrication drawing, not drilled sizes.
• Keep the aspect ratio at or below the class the supplier can hold without special measures.
• Use the largest hole the component allows, and as few distinct sizes as possible.
• Size the pad so the annular ring survives the registration tolerance on every layer.
• Keep copper, inner layer clearances and via barrels far enough apart that a drill deviation cannot cause a short.
• Avoid placing a hole where it will cut a plane into a narrow sliver, since a sliver can lift or trap plating chemistry.
• Specify the drilling class explicitly where the design depends on it, including the bit change policy if the product is critical.
• Where a via stub is a signal integrity problem, consider back drilling rather than a smaller hole.

What It Costs

Drilling is priced by the number of hits, the number of distinct tool sizes, the stack height and the class of tolerance. A design with ten hole sizes, small diameters and a tight tolerance can cost several times what a design with three sizes and generous diameters costs for the same number of holes.

The class of tolerance should match what the supplier can hold rather than the tightest figure in a capabilities table, and the number of holes matters as well. Reducing the via count, or consolidating a via fanout where the routing allows, is one of the few design changes that reduces cost without a compromise elsewhere. Where a design has thousands of vias, the drilling step is a significant part of the quotation.

FAQ

• Is the finished hole the same as the drilled hole? No. Plating reduces the diameter, typically by about 25 micrometres on a standard board.
• What is the minimum hole size? It depends on the board thickness and the aspect ratio. A typical class allows around 0.2 mm on a thin board, while a thick board needs a proportionally larger hole.
• Can a hole be drilled after plating? It can, but that is normally a mechanical feature rather than an electrical one, and drilling through a finished board risks damage to the plating.
• Why do laser drilled holes cost more than mechanical ones? The equipment is more specialised, the dielectric has to be controlled closely, and the process is usually part of a build-up sequence with more steps.

Summary

Drilling decides whether a design is manufacturable at a reasonable cost. The important parameters are the finished hole size, the positional and diameter tolerance, the aspect ratio, and the quality of the barrel wall, and every one of them interacts with the pad, the plane clearance and the plating step that follows.

The design levers are simple. Specify finished sizes, keep the aspect ratio inside the comfortable range, use few hole sizes, give the pad enough ring to absorb the registration tolerance, and avoid placing the design at the edge of the process without a reason. Where the density demands more than a standard process can drill, the alternatives are a finer process class, a different via structure or back drilling, and all three are best talked through with the fabricator during the layout stage rather than after the first panels have been drilled.