Peck Drilling Deep Holes on High Aspect Ratio Boards

Peck drilling is a drilling cycle in which the bit advances, retracts partly or fully, and advances again until the hole is complete. It is used for deep holes on thick boards, where a single continuous stroke would leave chips packed at the bottom of the hole, generate heat along a long flute and produce a wall that cannot be plated cleanly. The cycle is slower than a straight stroke and it is chosen only where the geometry demands it.

Why Deep Holes Need a Different Cycle

In a shallow hole, chips leave through the flutes almost as fast as they are cut, and the coolant reaches the tip without difficulty. As depth increases, the flutes fill, the chip has to travel further and the coolant struggles to reach the cutting edge, so the same parameters that work on a thin panel fail on a thick one.

<img src="https://www.gopcba.com/wp-content/uploads/2025/05/员工风采.jpg" alt="Drill bit retracting during a peck drilling cycle” />

The failure is not gradual. Beyond a certain depth the chip load rises suddenly, the bit rubs instead of cutting, and the wall is smeared with resin rather than cut cleanly. That transition is what makes depth, rather than hole diameter, the variable that decides whether a peck cycle is needed.

Chip Removal and Heat in a Deep Hole

Chip removal is the purpose of the retract. Pulling the bit clear lets the flutes empty and lets the coolant or the air blast carry the debris away, so the next advance starts from a clean bottom rather than from a compressed plug of resin and copper.

Heat follows the same path. Friction at the cutting edge raises the temperature of the bit and of the wall, and a retract gives both time to cool. A deep hole drilled in one stroke builds temperature continuously, and the temperature is what drives resin smear onto the copper of the inner layers.

What a Peck Cycle Looks Like

A typical cycle advances in steps of one to three times the diameter, retracts between 0.2 and 1 millimetre for a partial peck or fully out of the hole for a full retract, and repeats until the target depth is reached. The retract distance and the number of cycles are set in the drilling programme.

The differences between machines matter less than the rule behind the settings: each advance has to be short enough that the flutes do not fill, and each retract has to be long enough that they empty. Increasing the number of cycles while keeping the same feed makes the process slower without improving the wall, which is a common way a new programme is over-engineered.

Aspect Ratio and When to Switch

The aspect ratio, the board thickness divided by the hole diameter, is the figure that predicts the difficulty of the job. A ratio below about 5 to 1 is normally drilled in one pass, ratios between 5 and 10 are usually pecked, and above 10 to 1 the cycle becomes mandatory on most materials.

The material shifts those thresholds. A hard, highly filled laminate produces more frictional heat and needs pecking earlier than a standard FR-4, while a soft material may tolerate a deeper single stroke. The threshold for each stack-up should be established from sectioned holes rather than from a general table.

Entry and Backup Material

The entry material on top of the stack and the backup board beneath it do more for a deep hole than any change to the feed rate. Entry material holds the drill on centre at the start of the hole, and backup material supports the exit side where the last layers of copper are the least supported.

For deep holes the backup material also has to clear the flutes. A soft, resin-coated backup holds a slug of debris against the exit and pushes it back into the hole, which is why backup hardness and thickness change with depth even when the board material does not.

Smear, Nailheading and Wall Quality

Smear is resin dragged across the copper of an inner layer by a hot, dull or overloaded bit, and it appears on the hole wall as a dark, glassy band that plating cannot bond to. A peck cycle reduces it by controlling heat and by clearing the chip before it re-cuts, but it cannot compensate for a worn bit.

Nailheading is the deformation of an inner layer where the drill pushed the copper ahead of the cutting edge instead of shearing it, and it shows as a taper at the corner of the layer. Both defects are visible only in a section, which is why sectioned holes, rather than visual inspection, are the evidence used to release a deep drilling programme.

Drill Wear and Hit Count

Deep holes consume bit life faster than shallow ones because each hole involves more contact time and more heat. A hit count set for a standard panel will be too high for a thick one, and the first sign is usually smear rather than a change in hole diameter.

A practical rule is to reduce the hit count limit in proportion to the depth beyond the reference stack-up, and to reset it entirely after each regrind. The regrind itself changes the geometry of the cutting edge, so a reground bit and a new bit should not share a limit.

Verification and Records

Verification is a sectioned hole taken from a production panel at the start of a run and at intervals through it, examined for smear, nailheading and wall roughness. The section is the only method that sees the full depth of the hole rather than its entrance.

Records should carry the drill diameter and part number, the spindle speed and feed, the peck step and retract, the number of cycles, the entry and backup material and the hit count at the time of sectioning. Together they describe the cutting condition, and that is what has to be compared when a desmear step later fails to rescue a wall that was damaged at the drill.

The Cost of a Peck Cycle

Peck drilling is slower per hole, and the cost is real. On a thick panel the retracts can add 30 to 60 percent to the drilling time, and the extra spindle time is what makes a deep stack expensive rather than the material itself. The trade is justified by the yield it protects.

A wall that is smeared usually reaches the plating line before anyone knows it, and the plating either fails to bond or bonds well enough to pass test and then cracks in the field. Comparing the added cycle time against the cost of a scrapped panel, or of a field return, is the calculation that decides how conservative a peck programme should be.

FAQ

At what aspect ratio is peck drilling needed? Many shops switch between 5 to 1 and 10 to 1 and treat pecking as mandatory above 10 to 1, but the threshold depends on the laminate and should be confirmed by sectioning the holes.

Does peck drilling slow production? Yes. The retracts add cycle time, which is the price paid for a hole wall that can be plated reliably and for a bit that is not destroyed by heat.

Can peck drilling cure smear on its own? No. It limits the heat that causes smear, but a worn bit, a soft backup or a heavy copper layer can still produce it, so the section remains the acceptance test.