Drill Entry and Exit Quality in PCB Drilling Operations

Every drilled hole has three zones that behave differently: the entry where the bit touches the copper, the barrel where the cutting happens, and the exit where the bit breaks through. The barrel gets most of the attention because it carries the plating, but the entry and the exit are where the visible defects appear and where the causes of a plating failure are often set.

What Happens When the Drill Enters

When the point of the bit touches the surface, the material under it is pushed rather than cut until the edges of the flutes engage and the cut becomes stable. In that first fraction of a millimetre the copper can be lifted, the resin can be pushed aside, and a burr can be raised on the rim of the hole.

The entry is also where the drill is most likely to wander, because at that moment only the point is in contact and the bit has no support from the hole wall. A bit with an offset point, a worn collet or a poorly supported panel will produce a hole that starts off centre and then straightens, giving a tapered or bell mouthed entry.

Entry Material and Its Job

Entry material is placed on top of the stack and has two functions: it holds the copper down to reduce burring, and it lubricates and cools the cutting edge. Aluminium foil backed with a paper or resin layer is the usual choice, and the aluminium face is placed upwards against the pressure foot.

The material also protects the panel surface from scratches and from the pressure foot itself. A missing or reused entry sheet is one of the first things to check when entry burrs appear, because the sheet is cheap and its condition is easy to overlook in the search for a machine fault.

Exit Side Damage and Burrs

At the exit the bit pushes the last fibres and the copper foil ahead of it, and the material has no support on that side unless a backing board is used. The result is a burr around the rim, ragged copper at the edge of a pad, and in a severe case a torn or delaminated area on the exit surface.

Exit damage matters because the exit side is often the side that carries the fine pattern. A burr that is not removed will bridge to a neighbouring feature after plating, and a delaminated exit will trap chemistry that bleeds out later and attacks the plating around the hole.

Spindle Speed, Feed and Chip Load

The chip load, which is the thickness of material removed by each flute per revolution, is the parameter that most directly controls the cutting temperature. Too low a chip load rubs rather than cuts and generates heat, while too high a load overloads the bit and produces a rough wall.

Spindle speed and feed rate are chosen together to reach the recommended chip load for the bit diameter and the material. The values also change with the laminate, since a higher glass transition material with more filler cuts differently from a standard FR4 and generally needs a different set of parameters.

Drill Bit Condition and Regrinding

A bit starts to wear from the moment it enters production. The cutting edge rounds, the point becomes eccentric and the surface finish of the wall deteriorates, and the trend continues gradually until the hole quality crosses a limit.

Regrinding restores the geometry but removes material, so the bit becomes shorter and its diameter may change slightly. The number of regrinds allowed is finite, and each bit should be tracked through its life so that hole quality can be related to the number of hits rather than to the calendar.

Backing Material and Support

A backing board under the stack gives the exit the same support that the entry material gives the top. It is usually a phenolic or a paper based board, and it should be flat and thick enough to absorb the breakthrough without deflecting.

Support also comes from the machine itself. The pressure foot has to press the stack against the table evenly, because a stack that is not flat will produce a different hole in the middle of the panel from the one at the edges, and that difference will follow the panel rather than the drill.

Hole Quality Measurements

Hole quality is measured by several methods. A microsection shows the wall, the entry and any resin smear, and the plated copper it reveals should be compared with the hole copper specification; a hole inspection scope shows the internal finish; and the diameter is checked with a pin gauge or measured on the section.

Burr height is measured on the exit surface with a microscope and a calibrated scale, and the limit is normally set by the smallest gap the pattern requires rather than by a general figure. Measuring on the panel also reveals whether the burrs follow a pattern, which points to a machine axis or to a stack that is not level.

Defects Traced to Entry and Exit

Entry burrs that bridge to a neighbouring pad after plating, exit burrs that survive and cause a short, and wall defects caused by a wandering bit are all traceable to this stage. So is the intermittent open that follows a smear generated by excessive heat, because the heat is often an entry condition that persists down the hole.

The appearance of the defect is the best clue to its origin. A defect that appears only at the edges of a panel is usually a support problem, while one that appears on every hole of the same diameter points at the bit and its parameters. The wider catalogue sits in the guide to board level failures.

Setting Up a Drilling Program

A drilling program is written from the material, the stack thickness, the hole diameter and the aspect ratio, and it should be qualified on a test panel before production. The test panel is used to check the entry and exit condition, the wall quality and the diameter, and it should be drilled with the same bit that production will use.

Records should include the bit identification, the number of hits, the parameters used and the inspection result, because a change in hole quality is hard to interpret without knowing where the bit was in its life. The relationship between hole depth and plating quality is covered in the guide to aspect ratio.

FAQ

What is the purpose of entry material? It holds the surface copper down to limit burring, cools and lubricates the cutting edge, and keeps the pressure foot from marking the panel. Reusing it defeats all three purposes, because a used sheet has already lost its flatness and its lubricant.

Should the backing board be replaced every panel? Its condition matters as much as the entry sheet. A backing board that has been drilled many times no longer supports the exit surface uniformly, which is a common cause of burrs that appear in one region of a panel only.

How often should drill bits be changed? According to the number of hits rather than to elapsed time, with the limit set by hole quality measured on a test panel. The limit also depends on the material, since a filled laminate wears a bit faster than a standard one.