PCB Drill Bit Wear and Its Effect on Plated Hole Quality
A drill bit is a cutting tool that spends its life doing the hardest work in the fabrication shop. It wears progressively, and the wear shows up first in the quality of the hole wall rather than in the diameter of the hole. By the time the drill is visibly dull, the plating process has already been fighting a surface it cannot plate well. This article explains how bits wear, what that does to the hole, how drill life is managed, and how drilling limits end up visible in the plated barrel.
How a Drill Bit Wears
Wear appears in three places. The cutting edge itself rounds and loses its sharpness, which raises the cutting force and generates more heat. The margin, the narrow land that guides the bit against the hole wall, wears and changes the hole size and the surface finish. The coating, usually a hard film applied to extend life, is abraded away and exposes the softer carbide underneath.
Wear is not linear with hit count. A bit cuts reasonably consistently for a period and then degrades quickly, which is why a drill life figure is a limit rather than a schedule. The rate also depends on what is being drilled, because a bit running through a stack of filled laminate wears far faster than one running through a standard material. A bit that has been resharpened several times also loses flute length, which changes chip evacuation and raises the temperature inside the hole.
What Wear Does to the Hole Wall
A sharp bit shears the glass fibres and leaves a relatively clean wall. A dull bit generates heat, and heat softens the resin, so the resin smears over the fibres and the wall becomes a mixture of exposed glass and resin smear. That smear is exactly what the desmear process is designed to remove, and a heavy smear load can exhaust it.
The wall also becomes rougher and less uniform along the hole. Plating covers a rough surface less evenly, and the copper thickness varies from one side of the barrel to the other. The result is a hole that meets the average thickness figure while having thin spots, which is why barrel inspection looks at the minimum rather than the mean.
Nail Heading and Its Causes
Nail heading is the name for the tapered or flared shape a hole takes when the laminate is deformed during drilling. The layers above and below the hole are pushed inward, and the wall angle becomes non-uniform along the barrel instead of straight. In section, the barrel looks like a nail head.
The usual causes are a dull bit, excessive feed, insufficient support behind the stack, or a combination of all three. Because the deformation happens in the laminate rather than in the copper, it also stresses the layer-to-layer registration, and a hole that is nail headed is often also a hole with a marginal annular ring.
Drill Life and Hit Counting
Drill life is expressed in hits, or the number of holes a bit is expected to drill before it is resharpened or retired. The figure depends on the material, the bit diameter, the hole tolerance and the coating, and it should come from the bit supplier with the process conditions that were used to derive it.
The counting has to be reliable. Every stack, every panel and every hole counts, including the tooling holes and the holes drilled for a coupon. A system that counts only the production holes underestimates the wear and lets the bit past its limit, while one that over-counts retires good bits and drives up cost and handling.
Parameters: Speed, Feed and Retract
Spindle speed and feed rate together set the chip load, which is the amount of material each cutting edge removes per revolution. A low chip load causes rubbing and heat, while an excessive chip load overloads the edge and breaks it. The right combination is narrow and it changes with the hole diameter, because a small bit cannot remove material at the rate a large one can.
Retract rate affects the wall as the bit leaves the hole, and a fast retract on a small hole can lift material and damage the entry. Entry and exit quality is often the first visible sign that a parameter set is drifting away from the optimum. The parameter set should be recorded with the bit type and the laminate so that a change in hole quality can be traced back to the conditions that produced it.
Entry and Backup Materials
An entry material sits on top of the stack and stops the drill from burring the copper foil as it enters. A backup material sits underneath and supports the laminate as the bit exits, which reduces exit burrs and prevents the last layers from being pushed out of shape. Both are cheap compared with the cost of a rejected panel.
Entry and exit quality is not cosmetic. A burr on the copper at the entry forms a raised ring that interferes with plating and with the subsequent lamination, and a roughened exit leaves a surface that traps chemistry and produces voids. Our hole types guide describes the different holes a fabrication process has to produce.
Inspection for Wear-Related Defects
In-process inspection for wear looks at hole wall roughness, entry and exit condition, and the presence of smear on a microsection. The best measure is the drum of a sample panel: a rough wall with a high smear load points at a dull bit before the diameter moves out of tolerance.
Hole diameter is a lagging indicator. By the time the hole is out of tolerance, the walls have been poor for a considerable time, and lots produced in between carry a risk that has to be assessed rather than ignored.
When to Resharpen or Replace
Resharpening restores the cutting geometry but shortens the bit, and a bit can only be resharpened a limited number of times before the flute length is no longer adequate for the stack depth. A coated bit usually loses its coating at the first sharpening, so its life after resharpening should be set lower than its original life.
The decision to resharpen should be driven by the measured hole quality, not by a fixed count. Keeping a small number of bits with known histories and inspecting them at intervals gives a far better picture than a single figure applied to every bit in the crib.
Linking Drilling to Plating Results
The plating process can only work with the wall it is given. A rough wall with heavy smear produces a barrel with thin spots, voids and poor adhesion, and no amount of plating time corrects a surface that was never prepared. This is why a plating defect investigation should start with the drilling record for the lot.
Our hole copper notes explain how the copper thickness in the barrel is specified and measured, and our aspect ratio guide explains how the depth to diameter relationship limits the throwing power. Our plating thickness guide covers the measurements that confirm the result, and our quality guide describes how a barrel defect is classified at gopcb.
FAQ
Is drill life a fixed number? No. It depends on the material, the diameter, the parameters and the coating, and it should be derived from the supplier data plus the measured hole quality on the actual stack. The figure is a starting point rather than a law.
Does a bigger hole tolerance allow a longer drill life? To some extent, because the bit can be used after more margin wear before the diameter leaves tolerance. The wall quality usually degrades before the diameter does, so the tolerance is rarely the real limit.
Can nail heading be corrected by plating? No. The deformation is in the laminate, and plating follows the wall it finds. It has to be corrected by changing the bit, the feed or the support behind the stack.
