Drill Hit Count Control In PCB Fabrication
A drill bit in a printed circuit board shop does not drill one hole and stop. It drills a set number of holes, is withdrawn, and is replaced or resharpened. That number is the hit count, and it is one of the most consequential process settings in the fabrication of a board, because a worn bit rubs rather than cuts.
This article explains why the count is limited, how wear shows in the hole wall, and how the count is controlled in production.
Why A Bit Wears
The bit is a small piece of tungsten carbide spinning at high speed and cutting a mixture of epoxy resin and glass fibre. The glass is abrasive, and it abrades the cutting edge, particularly at the outer corners where the surface speed is highest. As the edge rounds, the bit stops shearing the material and starts pushing it, which raises the temperature at the cutting zone and deforms the resin instead of removing it.
The consequence is a hole wall that is no longer a clean cut. Resin is smeared across the wall, glass fibres are dragged rather than severed, and the entry and exit of the hole become ragged. Every one of those effects reduces the adhesion of the plating that follows and increases the chance of a barrel defect under thermal load. The mechanisms of the resulting defects are described under copper plating defects prevention.
Setting The Hit Count
The count is set by experiment for each combination of bit diameter, laminate, panel thickness and stack height, and it is the point at which the hole quality begins to degrade rather than the point at which the bit stops cutting. A small diameter bit wears faster than a large one because the cutting edge is shorter and the surface speed for a given spindle speed is lower, which encourages rubbing. A thicker panel or a taller stack means more material per hole and a shorter life.
Typical life figures are a few hundred holes for a very small bit and a few thousand for a larger one, and the figures are conservative because the cost of a poor hole is much higher than the cost of a bit. The count is normally set to about two thirds of the life found in the experiment, so that the deterioration is never reached in production. Where the board is built to a demanding specification, the margin is larger.
How Wear Appears In The Hole Wall
A section of a hole drilled with a fresh bit shows a wall that follows the glass bundles closely, with the resin cut cleanly and the fibre ends flush. A hole drilled near the end of the bit life shows resin smeared over the fibre, a wall that is no longer cylindrical, and in the worst case a hole that is smaller at the exit than at the entry because the bit has been pushed off line.
The electrical consequence appears later. Plating deposited on a smeared wall is mechanically rather than metallically bonded, and the barrel separates under thermal cycling. The failure is not visible at the end of the line, because the barrel is continuous enough to pass a continuity test, and it appears in the field as an open circuit. This is the reason the hit count is treated as a quality parameter rather than as a maintenance item. The prototype stage at which such parameters are established is described under multilayer prototype requirements.
Controlling The Count In Production
The control is a record and a discipline. Each drilling machine tracks the number of hits per bit position, and the machine stops when the count is reached so that the bit can be changed. The count is reset when the bit is changed, and the bit is either discarded or sent for resharpening, where a defined amount of material is ground away and the bit is re-measured.
Two things go wrong with this system. The first is a reset that is done incorrectly, so a bit that has already drilled a full life is given another full life. The second is resharpening that removes too much or re-establishes the geometry incorrectly, so a resharpened bit behaves differently from a new one. Both are prevented by recording the bit identity with the count, so that a bit can be traced through its lives, and by measuring the bit diameter after resharpening rather than assuming it.
Related Parameters
The hit count does not act alone. The spindle speed and the feed per revolution determine the heat generated per hole, so a change to either changes the life. The entry and backing materials lubricate and support the cut, and removing them shortens the life. The stack height changes the temperature at the bottom of the stack, so the bits drilling the lower panels wear faster than those at the top, and a count that is correct for a two panel stack may be too long for a four panel stack.
Because the parameters interact, the count is established for a specific set-up and is not transferable. A shop that moves a product to a machine with a different spindle, or that changes its entry material to save cost, has to re-establish the count rather than carry it across. Treating the figure as a property of the hole size alone is a common source of a sudden change in hole quality.
Verification And Records
Verification of the drilling process is by section, on a sample from each lot or after each significant change, and by the resistance chain on a coupon. The section shows the smear, the wall quality and the plating adhesion; the chain shows the electrical consequence over many holes. Together they detect a change in bit performance before it produces a field failure.
The records that matter are the bit identity, the number of hits, the resharpening history and the drilling parameters for the run. They allow a defect found in a lot to be traced to a specific bit life stage, which is what makes a corrective action possible. Without them, the only available response to a drilling problem is to change everything at once, which does not establish which change helped. The surrounding fabrication controls are set out under design guidelines for manufacturability.
Additional Considerations for This Build
Practical attention to drill wear pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating drill wear explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Deliberate attention to hole wall quality pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating hole wall quality explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, resin smear is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
FAQ
Why not change the bit after every hole? The bit changing time and the cost of the bits would dominate the process. The count is set to the point where quality starts to degrade, which is far short of one hole.
Does a resharpened bit drill as well as a new one? It can, if the grinding restores the geometry and removes the correct amount of material. The diameter has to be measured and the count adjusted, since a smaller bit removed less material and behaves differently.
Is the count the same for every panel in a stack? No. The panels lower in the stack run hotter and wear the bit faster, so the count is conservative for the whole stack. A tall stack may need a shorter count than a single panel of the same thickness.
