Drill Bit Geometry And Hole Quality

A printed circuit board drill is a twist drill made of solid tungsten carbide, with a geometry that has been adapted for cutting a stack of resin and glass rather than metal. The point angle, the helix, the web thickness and the surface treatment all affect how the bit cuts, how it wears and what the hole wall looks like.

This article covers the parts of the bit, how the geometry affects the cut, and how a geometry is selected.

The Parts Of A Drill Bit

The point is the cutting end, formed by two or four flutes ground to an angle. The flutes are the helical grooves that carry the chips away from the cutting edge, and their twist rate determines how quickly the debris leaves the hole. The web is the central column of material between the flutes, and its thickness sets the strength of the bit and the space available for the chips.

The land is the cylindrical surface at the outside of the bit, and the margin is the narrow band on the land that defines the hole diameter. The shank is the part held in the collet. In a board drill the land is often relieved to reduce friction, and the surface may be coated to reduce wear. The way the bit interacts with the laminate is described under PCB design and fabrication.

Point Angle And Its Effect

The point angle is the included angle of the tip, and it sets how the bit enters the material. A sharper point, with a smaller angle, starts the hole more easily and produces less thrust, which reduces the risk of the copper foil lifting at the entry. A blunter point is stronger and resists wear better, and it pushes the material ahead of the edge rather than cutting it.

For board drilling the angle is normally between one hundred and twenty and one hundred and forty degrees, chosen to balance the entry quality against the strength of the tip. Too sharp an angle weakens the tip and it breaks; too blunt an angle increases the thrust and the burr. The angle also affects the shape of the hole bottom and therefore the quality of a controlled depth hole, which is one of the reasons the geometry is specified with the process rather than left to the supplier.

Helix, Web And Chip Removal

Chip removal is the limiting factor in a small hole. The chips are a mixture of resin powder and glass fragments, and they have to travel up the flutes while the bit turns. If they are not removed they are reground by the bit, which raises the temperature and smears the resin on the hole wall. A higher helix angle, with more twist, lifts the chips more quickly and is used for deep holes and for materials that produce fine dust.

The web thickness controls the space between the flutes. A thin web gives more room for the chips and is used on small diameter bits, at the cost of strength. A thick web is stronger and leaves less room, so it is used where the bit diameter allows. The web also has to be thinned at the tip on a small bit, which is a grinding operation that is part of the bit specification. The way a badly cleared hole appears is described under copper plating defects prevention.

Chip Load And Feed

The chip load is the thickness of material removed by each cutting edge per revolution, and it is set by the feed rate divided by the number of flutes and the spindle speed. If the chip load is too small, the edge rubs rather than cuts, which generates heat and wears the bit quickly. If it is too large, the edge is overloaded and the thrust rises, which moves the panel and can break the bit.

There is a minimum chip load below which the cutting action stops working, and it is the reason a very high spindle speed is not automatically better. For a given feed the speed has to be set so that the chip load stays above the minimum, and for a small bit the achievable feed is limited by the machine, which limits the speed. The relationship is what makes the parameter set for a small hole different from that for a large one, and it is normally supplied by the bit manufacturer for each material.

Wear, Resharpening And Geometry

The bit wears at the outer corners of the cutting edges, because that is where the surface speed is highest and where the glass fibre is cut. As the corners round, the bit stops shearing and starts pushing, and the hole diameter becomes slightly smaller at the entry and the wall becomes smeared. Resharpening grinds the point back and restores the geometry, and it reduces the bit diameter slightly, which is why a resharpened bit is measured and its diameter recorded.

Resharpening can also change the geometry if the grinder is not set correctly. A point angle that has drifted, a web that has not been thinned, or an asymmetric grind all change the cut, and an asymmetric point drills a hole that is larger than the bit and wanders. The verification of a resharpened bit is the same as the verification of a new one: the diameter measured, and the hole quality checked on a sample.

Choosing A Geometry

The choice depends on the material, the hole size and the depth. A drill for a standard FR-4 stack at a moderate diameter uses a conventional geometry, while a very small bit needs a thinner web and a sharper point to survive. A drill for a high glass content or a ceramic filled laminate needs a geometry that resists abrasion, which usually means a different carbide grade and a coating.

Because the geometry and the parameters form one system, a change of bit supplier is a process change. A bit that is nominally the same diameter but has a different point angle or helix will drill differently, and the hole quality may change even though the parameter set is unchanged. Requalifying the process, at least by a check on a sample, is the way to make that change safely. The programme that establishes such a change is described under multilayer prototype requirements.

Additional Considerations for This Build

Practical attention to drill bit geometry 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 bit geometry 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, chip load 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

Does a sharper point drill a better hole? It enters more easily with less thrust, which reduces the entry burr. It is also weaker, so a balance is struck with the point angle rather than going as sharp as possible.

Why does a bit stop cutting when it is spun too fast? Because the chip load falls below the minimum needed for the edge to shear material. The edge then rubs, which generates heat instead of removing material.

Are all board drills the same carbide? No. The grade and the coating are chosen for the abrasiveness of the material and for the hole size, and a change of grade changes the wear rate and therefore the hit count.