Drill Chart: 5 Checks Before the Drill Runs

A drill chart is the document that lists every hole in a design together with its diameter, its plating requirement and the tolerance that applies. It is the bridge between the design file and the drilling machine, and it is also the reference used later by the plating line and by inspection.

A chart that is incomplete or out of date produces a batch that cannot be salvaged, because the holes are formed at the very start of the process. There is no way to add material back into a hole that was drilled at the wrong size, and the cost of the error is the panel.

What a Drill Chart Has to State

The minimum content is a table of tool numbers with their diameters, the hole count for each tool, and a note on whether the hole is plated or not. Plated and unplated holes usually have different finished sizes, so the distinction has to be explicit rather than assumed.

Beyond the table, the chart should state the tolerance on the finished bore, the plating thickness required and any hole with a special requirement. A design with a press fit connector or a shielded can opening depends on a tight finished size, and that requirement belongs on the chart rather than in a note on the order.

Hole Size, Plating Thickness and the Finished Bore

The drill diameter is not the finished size. Plating adds copper to the wall, so a hole intended to finish at one millimetre is drilled slightly larger, with the allowance taken from the specified copper thickness. A chart that lists only the finished size leaves the drill operator to guess the allowance.

The allowance also interacts with the plating process, because a high aspect ratio hole plates more slowly at the centre than at the surface. Where the plating thickness is tight, the drill size and the plating parameters have to be set together, and both belong to the same order record.

Tolerance, Registration and the Drill Program

The tolerance on a hole position is set by the machine, the fixture and the material, and it has to fit inside the annular ring that the design allows. Where the chart asks for a tolerance tighter than the machine can hold, the result is a batch that fails inspection for a reason that was visible on paper.

Registration between layers adds to the same budget. A hole that is correct on its own can still break out on an inner layer if the layer to layer registration is loose, so the tolerance on the chart should be written with that stack up in mind. Our annular ring notes set out the arithmetic.

Slot, Counterbore and Special Holes

Not every feature is a round hole. Slots are routed or drilled in a series of overlapping hits, counterbores are formed in two operations, and a hole with a tight depth tolerance needs a controlled depth program rather than a through hit.

Each of these should appear on the chart with its own tool and its own tolerance, because the operator cannot infer them from the artwork. Where a slot is used for a connector key, the width and the position both matter, and a small error there becomes an assembly problem that is difficult to correct.

Reading the Chart Against the Stackup

The chart should be compared with the stackup before the program is released. Hole depths, the number of layers a via passes through and the presence of a buried feature all depend on the layer order, and a chart written for a previous revision will not match a changed stackup.

The comparison is quick work with the drawing open beside the data, and it is the cheapest possible check. A mismatch found here costs a few minutes, while the same mismatch found after lamination costs the batch and the schedule.

Via Types and Their Diameters

A through via, a blind via and a microvia have very different diameters and very different process routes. The chart should mark each type clearly, because a microvia drilled with the through via tool will either be too large for the pad or too small for the plating to fill reliably.

The count per type also matters for cost and for the program itself. A panel with many small tools needs more tool changes, which affects the cycle time and the tool wear, and both are visible on the chart if the counts are stated rather than summarised.

Tooling and Locating Holes

Tooling holes locate the panel on the drill, on the imaging machine and on the router, so their size and position are process decisions rather than product ones. They should be listed on the chart with their tolerance, because a worn tooling pin produces a gradual registration error across the whole panel.

The number of tooling holes and their symmetry matter as well. Three holes in a triangle locate a panel better than two, and a panel with holes only on one edge can rotate slightly during handling. Our drill accuracy notes describe how that error is measured.

First Panel Checks That Prove the Chart

The first panel should be measured for hole size, position and count, and the measurements compared with the chart rather than with the design file alone. A tool that is missing from the program produces a hole that is simply absent, and the count check finds it immediately.

Plugged or blocked holes should be recorded at the same time, because they are a drilling or entry material problem rather than a plating one. Our backup material notes explain how the entry and backup stack affects hole quality.

Documentation for the Plating Line

The plating line needs to know which holes are plated, the required thickness and the aspect ratio it will be working with. That information comes from the same chart, which is why the document should be issued to both areas rather than kept with the drill program.

Where the plating thickness is critical, the chart should state whether a coupon is available and where it sits on the panel. A coupon drilled with the same tools as the product is the evidence that the plating meets the specification, and it is worth the space it takes on the panel.

Revision Control and Repeat Orders

A repeat order should be drilled from the same chart and the same program unless the design has changed. When a chart is edited without a revision note, the next batch can differ from the previous one in a way that nobody notices until an assembly line finds that a connector no longer fits.

Charts should therefore carry the design revision, the date and the author, and be stored with the order package. The IPC documentation standards describe the data that should travel with a fabrication drawing, and the drill chart belongs in that set.

FAQ

Can the drill size be adjusted by the shop? Yes, within the allowance needed for plating, and the adjustment should be reported rather than silent. Where the change would alter the finished size beyond the stated tolerance, the designer should be asked before the panel is drilled.

Who owns the tolerance on a hole position? The design sets the requirement through the annular ring it allows, and the shop sets what the machine can hold. The two figures should be compared at the CAM stage, and where the requirement is tighter, the design is the place to change.

Is a drill chart needed for a simple two layer board? It is, because the plating allowance, the tooling holes and the finished sizes still apply. A short chart is fine, but a board built with no chart at all leaves the plating thickness and the finished bore to be decided by whoever is running the machine.