Drill Bit Coating Selection: Tool Wear, Drill Life and Hole Quality
A drill bit coating is a thin film deposited on the cutting edges and flutes of a carbide drill to slow the wear that limits its life. The benefit is not the same for every laminate: a coating that helps on a glass-filled high-Tg material may do very little on a soft, unfilled one, and a coating that reduces friction can also change the way chips are evacuated and the temperature the drill reaches. Choosing one means understanding which wear mechanism is actually limiting the tool.
Why Drill Bits Are Coated
Carbide is hard but brittle, and in PCB drilling it wears in three ways: abrasive wear from the glass fibres, adhesive wear where resin sticks to the cutting edge, and chemical wear at the temperatures reached in a long hit run. A coating addresses the latter two directly and the first indirectly, by keeping the edge sharp for longer.
The practical benefit is a slower change in hole quality as the tool ages. Rather than a sudden failure, an uncoated drill widens gradually, the entry burr grows and the hole wall roughens. A coated drill extends the period before those changes reach the specification limit, which is what makes it worth the added cost. The gain is measured in hits per tool and in the width of the window between a fresh edge and a rejected hole, not in the appearance of the first panel.
Coating Types and Properties
Diamond-like carbon films are the most common choice for PCB drilling. They are hard, have a low friction coefficient and can be applied thinly enough that the cutting edge geometry is unchanged. Their weakness is temperature: above roughly 400 C the film begins to break down, which is why the coating and the spindle speed have to be considered together.
Other films used include hard carbon variants with different hydrogen content, and multilayer films designed to resist chipping. The differences show up in the failure mode rather than in the initial performance, so a comparison has to run to the end of the tool life rather than to the first hundred hits.
Tool Wear Mechanisms in Drilling
The cutting edges lose their sharpness first, because that is where the load and the temperature are highest. As the edge rounds, the drill stops shearing and starts rubbing, so the heat input rises, the resin softens, and the wear accelerates. This is the point at which drill life effectively ends, whether or not the tool has broken.
Flute wear is the second mechanism and it matters for chip load rather than for hole size. A worn flute holds less material and releases it less cleanly, so a coated tool that keeps the flutes smooth keeps the chip load the parameters were set for.
Effect on Hole Wall Roughness
Hole wall roughness is the property most obviously improved by a good coating, because roughness is produced by the drill rubbing rather than cutting. A sharp edge produces a smooth wall with the glass fibres cut cleanly; a worn edge pulls the fibres and leaves them protruding into the hole.
The measurement is made on a microsection or by examining the inside of a drilled coupon under magnification, and the comparison should be made at the same hit count. Comparing a new coated drill with a worn uncoated one says nothing about the coating, since the difference is really the wear.
Drill Life and Hit Counts
Drill life, in hits, is set from the point at which hole quality leaves specification, not from the point at which the tool breaks. In a well controlled shop that means drilling a test panel at intervals through the life of a tool and measuring the entry burr, hole size and wall quality each time.
A coating typically extends that limit by a factor that depends on the material. On an unfilled laminate the gain may be small, because abrasive wear is limited; on a glass-filled high-Tg material the gain can be a large fraction of the original life. The regrind and bit life policy should therefore be set per material family rather than globally.
Regrinding Coated Tools
Regrinding removes the coating from the cutting edges, because the film is only a few micrometres thick and the grinding takes off more than that. A reground tool is an uncoated tool unless it is recoated, and recoating requires a clean surface and a process that does not damage the carbide.
Many shops simply reduce the hit count allowance after each regrind and accept the lower performance, which is a reasonable policy provided it is applied deliberately. The mistake is to keep using the hit count that applied when the tool was new, because the hole quality then drifts below specification long before the counter expires. Recording the regrind number against each tool makes the difference visible, and it turns a vague policy into a number that can be checked on the shop floor.
Coating and Entry Material Interaction
Entry material affects the temperature and the friction at the start of the cut. A hard aluminium entry foil generates more heat and more wear on the cutting edge than a soft, lubricated entry sheet, and a coating that survives one may fail with the other. The combination has to be evaluated as a pair.
Some entry materials are supplied with a lubricating layer precisely to reduce this wear, and the benefit is largest on uncoated tools. Where a coated tool is used, the additional gain from a lubricated entry material is smaller, so the two are partly substitutes for each other rather than complementary additions.
Selecting a Coating for a Material
The selection starts from the wear mechanism that dominates. For an unfilled resin, adhesive wear and chip evacuation dominate, so a low-friction film is the priority. For a glass-filled material, abrasive wear dominates, and edge hardness and a film that resists chipping matter more than friction.
Spindle speed is part of the same decision, because the coating has a temperature limit. Running a coated tool at a speed that raises the edge above that limit destroys the film, and the failure can look like a batch of poor drills rather than like a process mistake. The speed range quoted by the coating supplier is worth respecting rather than treating as a starting point.
Verification and Records
A coating is verified by drilling a controlled test panel and comparing hole wall roughness, entry burr and hole size against an uncoated tool of the same geometry, run at the same parameters and to the same hit count. That comparison is the only evidence that the additional cost produces a benefit for the product being made.
The record should carry the coating type, the tool supplier, the hit count policy in use and the results of the last comparison, together with the microsection results that supported it. Where a batch of boards shows a hole wall change, the record is what shows whether the tool or the process moved.
FAQ
Does a coating make the drill last longer? It usually does, but the gain depends on the material. Friction and adhesion-related wear is reduced most, so unfilled and low-glass laminates benefit differently from glass-filled high-Tg materials.
Is a coated drill still coated after regrinding? No. Grinding removes more material than the film thickness, so the cutting edges become uncoated unless the tool is recoated. Hit counts should be reduced to match.
Can a coated drill run at any spindle speed? No. Diamond-like carbon films begin to break down at a few hundred degrees, so the speed has to be kept within the range the coating can survive. Exceeding it destroys the film and the benefit with it.
