Drill Map Verification and Tool Assignment Control

The drill map is the document that connects the design to the drilling machine, and it is the point at which a design error becomes a physical one. A hole assigned to the wrong tool, a diameter entered in the wrong unit or a plated hole missed from the program will all produce a panel that looks correct at the drilling stage and fails later.

Verification is therefore a data exercise before it is a measurement exercise. The program is compared against the design, the tool assignment is checked against the finished hole sizes, and a first article confirms that the machine executed what the program intended.

What the Drill Map Contains

A drill map lists every hole by coordinate and diameter, groups them into tools, and states the finished hole size and its tolerance. The map also carries the panel identification, the stack height, the entry and backup materials, and the expected hit count per tool.

Every one of those fields has to agree with the fabrication drawing. Where the drawing and the map disagree, the drilling stage will follow the map, so the discrepancy should be resolved on paper rather than discovered on a panel, which is also the concern behind drill registration reviews. Where the drawing and the map disagree, the drawing governs and the map is corrected, because the drawing is the document the customer approves.

Tool Assignment by Diameter

Finished hole size is not the drill diameter, because plating reduces the hole during the plating process. The map usually specifies the drilled size and notes the expected finished size after plating, and the two have to be checked against the drawing requirement.

A common error is to assign one tool to two tolerances, which forces the process to hold the tighter of the two across every hole of that size. Tool assignment should follow the tolerance groups, so that a tight tolerance applies only to the holes that need it, and the annular ring and drill tolerance calculation should be repeated for the smallest ring in the design.

Reviewing the Program Against the Design

Review compares hole count, coordinates and diameters between the design data and the program. The check is best done as a comparison rather than as a reading exercise, since a single missing hole in a thousand is invisible to the eye but obvious to a count.

Layer-specific requirements also belong in the review: a hole that must not be plated, a hole that is depth controlled or a via that is back drilled. Each of those needs a separate tool or a separate machine instruction, and the check should confirm that the program carries it. A hole that must stay unplated, a hole that must be countersunk and a hole that must be back-drilled are all easy to lose in a map that carries only coordinates and diameters.

First Article Measurement

The first article confirms that the machine executed the program correctly. A sample of holes is measured for diameter and position, and the results are compared with the map rather than with the drawing, since the map is the instruction the machine was given.

Measurement is normally made with a pin gauge, a vision system or a microsection for the hole wall. Position is checked against the datum used in the program, and the result recorded with the entry and stack conditions that applied to the panel.

Hole Size Tolerance and Drill Wear

A drill wears as it cuts, so the hole it produces changes over the life of the tool. The change is usually small within the specified hit count, but it becomes significant when the count is extended, and it appears as a drift in the finished hole size rather than as an obvious defect.

The wear rate depends on the tool diameter, the stack height and the laminate, which is why the drill regrind and bit life limits belong in the same document as the drill map. Where a tool is run beyond its count, the finished hole size may still pass, while the wall quality fails later in plating. Stack height matters as much as the tool, because a taller stack gives the drill more chance to wander and shortens the interval at which the tool has to be changed.

Change Control for Drill Programs

A drill program is a controlled document and should carry a revision that can be traced to the design revision it implements. A program edit made at the machine without a corresponding record is the most common reason for two panels built to the same drawing being different.

Changes should be released through the same route as the drawing, with the first article repeated after any change that affects a tool assignment or a coordinate. Where the change is only a comment or a note in the program header, the repeat measurement is not necessary, and the two cases should be distinguished in the instruction.

Common Errors and Their Symptoms

A missed hole appears as an unplated void in the finished board and is usually found at final inspection or by electrical test. A wrong diameter shows up as a hole that is tight for the component lead, while a coordinate error appears as a ring that is thin on one side.

Unit errors produce the most dramatic results, because a hole specified in millimetres and entered in inches is either ten times too large or far too small. The check that prevents it is a units field in the program header and a first article measurement, not an operator memory.

Acceptance Criteria

Acceptance should state the finished hole size with its tolerance, the position tolerance against the datum, and the hole wall quality that plating requires. The three belong together because a hole can be correctly sized, correctly placed and still unusable if the wall is torn.

The criteria should also state the sample plan for the first article, since measuring two holes proves very little. A practical minimum is the smallest hole, the largest hole and one hole in each quadrant of the panel.

Records and Verification Cycle

Records should carry the program revision, the map revision, the tool assignment used, the first article results and the hit count at the time of measurement. With those fields, a change in hole quality can be traced to a specific program or a specific tool lot.

The verification cycle should be tied to the program revision rather than to a calendar. A program that has not changed does not need a fresh first article, while a program that has been edited twice needs a measurement after each edit, even within the same shift.

FAQ

What is the difference between the drilled size and the finished hole size? Plating reduces the hole, so the drill map specifies a larger drilled diameter that is expected to finish at the drawing dimension within tolerance after plating.

How many holes should a first article measure? At least the smallest and largest holes plus one in each quadrant, with the position checked against the program datum rather than against the drawing alone.

Why do holes drift out of size during a run? Tool wear changes the cutting diameter as the hit count increases, so the finished size drifts within the specified count and beyond it if the limit is extended.