Drill Bit Wear and Hole Quality Management

A drill bit wears with every hole it makes, and the wear shows up in the hole before it shows up on any inspection report. Diameter, wall roughness, burr height and registration all move as the cutting edge dulls, and the shop has to decide where in that drift to change the tool.

What Wear Does to a Hole

The cutting edge of a drill rounds and the margin behind it rubs rather than cuts. The result is a hole that is smaller than the drill by a growing amount, with a wall that is rougher and with more smear than the same bit produced an hour earlier. The change is gradual, which is why it is easy to miss.

Wear also moves the entry and exit. A dull bit pushes the laminate rather than cutting it, so the entry burr grows and the exit burr becomes harder to remove, and both affect how the copper plates onto the wall. Our tooling and registration notes describe where that starts to matter.

Measuring Wear Rather Than Guessing It

Wear is normally tracked as hole count, but the count that matters depends on the material and the stack. A bit drilling a thin two layer board and a bit drilling a thick high layer count board do not reach the same state after the same number of hits, so a single number for the whole shop is not useful.

The practical approach is to measure the hole rather than the tool. Diameter, burr and wall condition are checked on a sample at a stated interval, and the tool is changed when the sample moves outside the band rather than when a counter reaches a figure. Our hole types notes set out what the wall has to look like.

Resharpening and Its Limits

A drill can be resharpened, and each sharpening removes material from the point and changes the geometry. The first sharpening is usually indistinguishable from new, and the later ones are not, because the web thickens and the point angle drifts.

The number of permitted sharpenings belongs in the process instruction along with the inspection that qualifies the result. A resharpen that is accepted on a count alone is a common source of a hole that measures correctly and plates badly.

Burr, Smear and the Plating That Follows

Burr is material pushed out of the hole rather than cut away, and it appears at the entry and the exit. Entry burr affects how the surface is prepared before plating, and exit burr affects how the panel sits on the machine in the next step. Both are removed mechanically or chemically, and both cost a step when they become large.

Smear is resin that the drilling heat has spread across the glass bundles in the wall. It is removed in desmear, and the desmear window has to be wide enough to clear a dulled bit without roughing a wall that was already clean. Working the window around the wear state is cheaper than narrowing the tolerance on everything else.

Stack Height and Hit Rate

The stack height decides how many boards are drilled in one stroke, and it therefore sets the hit rate and the heat in the bit. A taller stack is faster and hotter, and the exit board in a tall stack sees the worst of the swarf and the worst of the heat.

Where entry and exit material is used, it is there to control exactly that. A proper entry sheet lubricates and holds the top of the hole, and a backing board supports the exit, so both extend the life of the bit and reduce the burr that would otherwise have to be removed later.

Registration and the Effect of Drill Drift

Drill wander increases as the point dulls, and wander moves the hole relative to the pad. On a fine pitch design the annular ring is the item that absorbs it, and a ring that was comfortable when the tool was new becomes marginal at the end of the run.

That is why the tool change interval is normally set from the tightest design on the panel rather than from the average. A panel carrying one fine pitch device is drilled as though every hole were part of that device.

Records and Process Control

The record that makes the process controllable is short: drill size, bit identification, hit count at the last change, the sample measurement and the desmear condition. With those five items a drift can be found before it becomes a batch problem.

Keeping a sample from the panel turns a later dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Our board quality notes describe how the finished article is judged against that sample.

Controlling the Cost of Tooling

Tooling cost is a balance between the price of the bit and the cost of the holes it produces when it is worn. Changing early raises the tooling cost and lowers the scrap, and changing late does the reverse, so the optimum sits somewhere that only the data can find.

Where the design is fixed and the volume is high, the interval is measured once and then held. Where the design changes often, the interval is better set conservatively and reviewed against the scrap figure, because an hour of investigation costs more than a few drills.

Design Factors That Decide the Interval

Three design decisions move the interval more than anything the shop does. The smallest finished hole sets the bit size and therefore its strength, the aspect ratio sets how deep it has to cut without clearing, and the annular ring sets how much wander can be tolerated before the ring is marginal.

Stating all three on the fabrication drawing lets the shop set an interval that suits the design instead of an interval that suits the average board. Where a requirement is not written down, the shop supplies its own default and that default is chosen for the process rather than for the design.

Process Control and Verification

On a design of this kind, burr is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

FAQ

Does a smaller drill always wear faster? It is weaker and breaks more easily, and its point geometry changes more per sharpening, so it is usually tracked on a shorter interval.

Can worn holes be rescued by plating longer? No. Plating fills a wall that is already rough rather than smoothing it, and a smeared wall can prevent the plating from adhering at all.

What does gopcb provide for hole quality? We provide drill selection against the finished size and aspect ratio, tool change intervals measured on the hole rather than on opinion, stack and entry material set up for the design, desmear that is matched to the wall, coupons from the same panel, and records for every batch.