Drill Breakage Prevention in PCB Drilling

Drill breakage is the most expensive routine failure in a PCB drilling room, because the machine keeps running after the tool has gone. The hole is not drilled, the panel may still pass the machine count, and the loss is discovered later as an open or a partially drilled hole in a finished board.

Most breakage is caused by a small number of conditions that can be measured: runout at the spindle, a worn collet, the wrong entry or backup material, and a hit count that has been extended past the point where the tool is still sharp. All four are cheaper to control than to recover from.

What Causes a Drill to Break

A drill bit breaks when the bending load on the shank exceeds what the carbide can take. That load comes from lateral force, and lateral force comes from runout, from a stack that is not flat, or from chips that are not evacuated and pack around the flutes.

Small tools break for the same reasons at lower loads, so a condition that is harmless at 1.0 mm will destroy a 0.2 mm tool. The process window therefore has to be written for the smallest tool in the program rather than for the average.

Spindle Runout and Collet Condition

Runout is the lateral movement of the tool at the tip, and it is measured with a dial indicator against a test pin. A spindle that is within a few micrometres produces round holes with even wall quality, while a worn spindle forces the tool to cut on one flute.

The collet and the pressure foot are part of the same measurement. A collet that has lost its grip or that has debris in the bore adds runout of its own, and the drill regrind and bit life record is meaningless if the tool is being held in a worn seat. The runout check should be repeated after any collet change, because a collet that is not seated squarely will show the fault as soon as the first panel is drilled.

Entry and Backup Material Effects

Entry material supports the tool as it enters and stops the top copper from being torn, while backup material supports the exit and limits burring. Both have a specification, and both affect breakage directly through the support they provide to the tool.

A soft or thin entry material allows the tool to deflect at the start of the cut, and a backup board that has been reused has already lost its flatness. Where a stack shows inconsistent breakage across the panel, the entry material and its condition are the first items to check.

Hit Count and Regrind Limits

Every tool has a hit count beyond which the cutting edges have worn and the thrust required rises. The count is set by the tool diameter, the stack height and the material, and it is normally lower for small diameters and for high glass transition laminates.

Regrinding extends the life of a tool but reduces its diameter and changes its geometry, so a regrind count should be limited and recorded. Running a reground tool to the same stack up limits as a new one is a common cause of late-life breakage.

Chip Evacuation and Feed Rate

Drilling produces a chip for every flute, and the chip has to leave the hole. Feed rate and retract speed control how much material is generated per revolution and how quickly it is carried out, and a feed that is too aggressive for the tool diameter packs the flutes instead of clearing them.

The signature of poor evacuation is a rise in drilling temperature, a rougher hole wall and a short tool life. Where the same program produces good holes early in the hit count and poor ones later, the evacuation is marginal and the tool is being lost to heat rather than to wear.

Stack Height and Registration

Panel stack height sets how far the tool has to travel and how much of the shank is unsupported. A taller stack increases the chance of lateral deflection at the bottom of the hole, which is where breakage most often starts.

Registration between the stack and the machine also matters. Where the panel moves slightly during the cycle, the tool cuts on its side rather than its point, and the load rises sharply. The backup material and the clamping system decide how well the stack is held, and the clamp pressure should be verified rather than assumed, and recorded per machine rather than assumed to be equal across the line.

Detecting a Broken Tool

Some machines detect breakage through a contact sensor or a vacuum check on the tool, while others rely on the operator to notice a change in the drilling sound. Where neither is available, the first evidence is usually a hole that the machine counted but did not drill.

Detection can be improved by monitoring the spindle load. A broken tool produces a step change in the load, and a machine that logs the load per hit gives an alarm before the next panel is drilled, which limits the number of boards that have to be inspected. A load alarm also distinguishes a broken tool from a dull one, since the two produce different signatures on the same trace.

Recovery and Re-Drill Rules

When a tool breaks, the panel behind it has to be handled by a rule rather than by judgement. The standard approach is to stop the cycle, identify the last hole that was drilled, and re-drill the remaining holes after replacing the tool and confirming the registration.

Re-drilling a hole that has already been drilled will damage the wall and enlarge the hole, so the recovery instruction should include the hole count and the coordinate where the break occurred. Where the machine cannot supply that information, the panel should be scrapped rather than re-drilled blind.

Records and Tool Room Control

Records should carry the tool diameter, the supplier lot, the regrind count, the hit count at replacement and the reason the tool was withdrawn. A tool that is withdrawn because of breakage is different from one withdrawn at the end of its count, and the two should not be averaged together.

The tool room controls the other half of the process: collet condition, tool storage and the inspection of incoming tools. Where the same breakage recurs on one machine after a tool change, the tool room record usually shows that the collets on that spindle have not been replaced on the same schedule as the rest.

FAQ

What is the most common cause of drill breakage? Excessive runout at the spindle combined with a hit count that has been extended, because the two together raise the bending load on the shank beyond what a small carbide tool can carry.

How is spindle runout checked? With a dial indicator against a test pin held in the collet, measured at the tip, with the result compared against the machine specification and recorded per spindle.

Can a broken tool be detected before the panel is finished? Yes, where the machine monitors spindle load or vacuum, and the alarm should stop the cycle so that the affected holes can be recovered under a documented rule.