Drill Bit Life and Regrind Control in PCB Drilling

Every hole in a circuit board is cut by a carbide tool that is wearing from the moment it touches the stack. The wear shows up as a change in hole diameter, a rougher wall, and eventually as a smear or a nailhead. Managing drill bit life is therefore a production control problem rather than a tooling curiosity, and the hit count alone is not enough to describe the state of a bit.

How a Carbide Bit Wears

Two mechanisms dominate. Abrasive wear occurs where the cutting edge meets glass fibre, which is harder than the cobalt binder that holds the tungsten carbide grains together. The edge rounds, the cutting action becomes more of a rubbing action, and the temperature in the hole rises. Adhesive wear occurs when laminate material welds to the edge and then tears away, taking a small amount of carbide with it.

The consequences appear in this order: hole diameter drifts, wall roughness increases, drill smear becomes more likely, and eventually the bit breaks. Because the progression is gradual, a drilling line that measures hole quality at intervals can schedule a bit change before the defects appear, while a line that relies on the hit count alone is always working with an unknown margin.

What Determines Bit Life

Hit count is the usual metric, but the number of holes a bit can drill depends on several variables. The most important are the resin and glass system, the number of laminate plies in the stack, the presence of copper on the entry and exit sides, and whether the bit is coated. A stack with more plies consumes a bit faster because the tool passes through more glass interfaces in the same hit.

Rotation speed and chip load also matter. A spindle that runs too slowly generates more heat and wears the edge faster, while one that runs too fast can chip the cutting edge. The feed rate controls the chip load per revolution, and a chip load that is too small causes the edge to rub rather than cut, which is a common cause of premature wear that is not visible on the hit counter.

Why Regrind Is Necessary and Risky

Regrinding restores the cutting edge geometry and extends the life of the tool, which is both an economic and an environmental benefit. A properly reground bit can deliver performance close to a new one, provided the geometry, the surface finish, and the concentricity are controlled. The regrind process removes material from the tip, so the flute length shortens and the maximum drilling depth for that bit is reduced.

The risk is that a poorly reground bit is worse than a worn one. An incorrect point angle changes the cutting forces, a rough edge leaves a poor wall finish, and a bit with excessive runout drills an oversized hole. This is why regrind should be bought to a drawing and verified on receipt rather than treated as a commodity service with no measurable specification.

Setting a Hit Count Policy

A practical policy sets the hit count for each bit type from a measurement of hole quality rather than from the tool supplier’s maximum. The approach is to drill a controlled stack until the hole diameter or the wall roughness moves outside its limit, then set the production count at a fraction of that figure, typically 60 to 70 percent. The margin covers variation between bits and between stacks.

Different hole sizes get different counts. A large bit removes more material per hit but has a stronger edge, while a small bit is mechanically weaker and wears faster relative to its size. The policy should be stored with the drilling program so that an operator cannot accidentally apply a small bit’s count to a large tool, and the counts should be reviewed whenever the laminate or the stack changes.

Measurement and Verification

Hole diameter is the most accessible measurement and the most useful. It can be checked with a pin gauge or with an optical measurement on a section, and the trend across the hit count shows when the bit is approaching its limit. Wall roughness requires a microsection, but it explains the defects that a diameter measurement misses, particularly smear and resin recession.

Nailhead and smear should be monitored directly on a sample from each spindle. Nailhead occurs when the exit copper is pushed away from the hole rather than cleanly cut, and it is a strong indicator that the bit is dull. Smear occurs when the resin melts rather than being cut, and it points to excessive heat, which may come from wear, from the feed rate, or from inadequate backup material.

Stack Design and Backup Material

The stack itself influences bit life. Entry and exit materials protect the board from burrs and from the initial impact, and they also affect the heat generated at the cutting edge. A hard entry material that requires more pressure wears the edge faster, while a soft one allows the bit to wander and produces positional error. Aluminum entry foil and phenolic backup are common compromises.

Panel stacking height is the other design variable. A taller stack drills more boards per hit but requires a longer bit, which is less rigid and more prone to wander and breakage. Where positional accuracy is critical, a shorter stack with a shorter bit gives better results even though the throughput is lower, and the trade-off should be evaluated for the specific product rather than assumed.

Records and Traceability

Bit life data belongs with the drilling program. Recording the bit type, the hit count at which it was changed, and the measured hole quality at that point builds a history that shows whether the policy is working. A sudden change in the data usually points to a change in the laminate lot, the entry material, or the spindle condition rather than to the bits themselves.

The same records support costing. Knowing how many hits a bit delivers before it must be reground makes the tooling cost per hole a measurable figure, which can be compared between suppliers and between regrind services. That comparison is what turns bit management from a fixed overhead into a variable that can be improved. The fabrication process as a whole benefits from the same discipline.

Process Control and Verification

On a design of this kind, hit count 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.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

How many hits should a drill bit deliver? It depends on the laminate, the stack, and the bit size, and the right figure comes from measuring hole quality rather than from a generic table. Setting the production count at about two thirds of the measured limit is a practical starting point.

How many times can a bit be reground? Typically three to five times before the flute length becomes too short for the drilling depth. The limit should be stated on the tool drawing and enforced through incoming inspection.

What causes nailhead on the exit side? A dull cutting edge, an inadequate backup material, or excessive spindle runout. Nailhead is one of the earliest visible signs that a bit should be replaced.