Drill Chip Load: 6 Rules for Clean PCB Holes

Drill chip load is the thickness of laminate that one cutting edge removes on every revolution of the bit, and it is the number that decides whether a drilled hole comes out clean or torn. Set it too low and the edge rubs, heats the resin and smears the wall; set it too high and the flute overloads, the bit deflects and the wall tears.

Because a drilled hole sets the quality of everything that follows, from plating adhesion to hole wall quality, chip load is a process parameter that has to be calculated for each tool rather than copied from a chart. This guide covers how the number is derived, how it behaves across materials and how to keep it valid as the bit wears.

What Chip Load Means in PCB Drilling

Every drill has two cutting edges, so one full turn removes material twice. Chip load is the amount taken by a single edge in a single pass, usually quoted in micrometres or in thousandths of an inch, and it belongs to the setting rather than to the machine.

It is not the same as penetration rate. The penetration rate is what the operator sees in the program, while chip load is what the cutting edge actually feels, and two programs with the same penetration rate can produce very different chip loads when the spindle speed differs.

How Drill Chip Load Is Calculated

The arithmetic is simple: chip load equals feed rate divided by spindle speed, and then divided again by the number of cutting edges. With a two-flute drill the feed per revolution is halved. Every term has to be stated in consistent units before the result means anything.

Most shops work backwards. They start from the recommended chip load for the laminate and the bit diameter, choose the spindle speed the machine can hold with acceptable runout, and then derive the feed rate from those two values.

Why Chip Load Drives Hole Wall Quality

Drill chip load controls where the cutting energy goes. A correct load lets each edge shear the fibres and the resin cleanly and pushes the debris up the flute. Too little load turns that energy into friction instead of cutting, and the softened resin is dragged across the wall.

The smeared layer that follows is the root cause of many downstream defects. It hides poor plating adhesion, and it is the reason desmear and hole preparation exist at all. Holding the load inside the right window is cheaper than repairing a bad wall afterwards.

The Practical Window for Feed Rate and Spindle Speed

Recommended values for a given material are published by laminate suppliers and by organisations such as IPC, and they are a starting point rather than a rule. The window narrows as the hole gets smaller and as the aspect ratio rises, because a slender bit cannot take a heavy load without bending.

Small holes therefore run at high spindle speed with a modest feed rate, while large holes in thick panels run slower with a heavier load. If the machine cannot reach the required speed, the feed rate has to come down with it, and hole wall quality is the first thing to suffer.

Chip Load and Drill Wear

A sharp edge cuts; a worn edge rubs. As drill wear accumulates the effective chip load falls even though the program has not changed, because the worn edge cannot bite and begins to push material aside instead of lifting it. The result is heat, smear and a rougher wall.

This is why tool life should be counted in hits rather than in hours, and why the re-sharpening interval belongs with the hole quality trend. A wall that roughens before the hit limit is reached is telling you the interval is already too long.

Reading Chips to Judge the Setting

The chips that come out of the hole are the cheapest way to judge drill chip load. A correct setting produces short, curled, uniform chips with light dust; a load that is too high produces long strings, and a load that is too low produces fine powder and heat discolouration on the debris.

Operators should sample the debris at the start of a run and after every tool change. On the machine this takes seconds, and it catches a wrong feed rate long before the electrical test does.

Material, Aspect Ratio and Stack Effects

Material changes the answer. FR-4 cuts differently from a high-Tg laminate, from a filled material or from a polyimide, and high filler content dulls the edge faster than plain resin. Recommended loads drop for brittle and highly filled materials.

Stack height matters as well. In a tall stack the bit has to clear debris from every panel, so the load at the bottom is not the load at the top. Entry and backing materials change how much of that load reaches the wall, and the selection covered in drill backup board selection and drill entry material decides whether the setting survives the stack.

Setup Changes After Re-Sharpening

A re-sharpened bit has a smaller diameter and a different edge geometry from the one that was measured. If the program keeps the original feed rate and spindle speed, the chip load rises, and the hole size drifts. That drift later appears as a registration or plating complaint.

The sensible approach is to group tools by diameter after re-sharpening, give each group its own parameter set, and verify hole size and wall quality on the first panel after the change, the same discipline used for drill bit life and regrind control. Where the plating process is sensitive to the drilled surface, the expectations set out in aspect ratio plating should be checked against the new hole.

Recording Settings and Building a Database

The parameters that matter are the material, the panel thickness, the stack height, the bit diameter and type, the spindle speed, the feed rate, the hit count and the measured result. Recorded together, they turn a machine setting into a process that can be repeated and audited.

The database also shows when a deviation is real. If one spindle produces rougher walls than its neighbours at the same setting, the cause is the machine rather than the recipe, and that is where the work described in spindle runout control belongs. Removing the burr afterwards is a separate step, judged by deburring practice rather than by the drilling recipe.

FAQ

What chip load should a drill run at? It depends on the bit diameter and the laminate. Small tools typically run at a few micrometres per edge and large tools take considerably more. The supplier recommendation is the starting point, and the first panel confirms whether the setting suits the machine.

Does a higher spindle speed always improve hole quality? No. Speed helps only while the feed rate keeps the chip load inside the cutting window. Raising the spindle speed without lowering the feed rate thins the chip until the edge rubs, which generates heat and smears the wall.

How often should drill wear be checked? Check hole size and wall condition at every re-sharpening interval and track the trend rather than a single reading. A wall that roughens before the hit limit is reached means the interval is too long for that material.