PCB Dimensioning and Drill Notes: Fabrication Documentation Guide

A fabrication data package is only as good as its least ambiguous instruction. Gerber layers define copper, mask, and legend geometry, but they say nothing about units, tolerances, material expectations, surface finish, impedance targets, or special drill treatments. Those requirements live in two places: the dimension drawing and the drill notes table. When they are incomplete, the fabricator fills the gap with a default assumption, and assumptions surface later as scrapped panels, mismatched impedance, or connectors that will not seat.

This article covers how to annotate dimensions on a PCB drawing and how to build the accompanying drill notes table. It addresses unit and precision conventions, the parameter settings used in the CAD tool, the specific features that must be dimensioned, the layer on which annotation belongs, and each field of the notes table from board identity through impedance and special drill handling. The goal is a package that a fabricator can execute without asking a single clarifying question.

Highlights:

Mils serve as the primary dimension unit with millimeters as secondary; if the design is authored in millimeters, millimeter-only annotation is acceptable. Mil dimensions carry no decimal places, while millimeter dimensions use two.
Dimension only what the fabricator cannot infer: locating pins of fixed components (press-fit parts without exception), mounting and tooling holes, and board outline geometry including chamfers, arcs, overall length and width, and any open windows.
The drill notes table must capture board name, layer count, thickness tolerance, panelization mode, surface finish, dielectric constant, impedance targets, and special drill conditions such as BGA via plugging, tolerances on 2mm press-fit holes, and irregular slot counts and sizes.

Units, Precision, and Drawing Setup

Unit Convention

Mils are the primary unit of annotation and millimeters the secondary unit. When a design has been created in millimeters, annotating in millimeters alone is acceptable. Presenting both units on the same drawing is common practice and reduces the chance of a unit misread at the fabricator, since many shops in different regions default to one or the other.

Precision

Dimensions given in mils require no decimal precision. Dimensions given in millimeters use two decimal places. Mixing a mil value with a decimal fraction invites rounding errors that are meaningless at fabrication scale, and it makes the drawing harder to scan.

Parameter Settings and Text Placement

Configure the dimension parameters in the CAD tool as shown below; all remaining parameters can stay at their system defaults.

Figure 70 | Standard dimension parameter settings [[IMG_2]]

Dimensions are placed using the Datum Dim command or by clicking the dimension icon. The permitted text presentation styles are illustrated below.

Figure 71 | Example of dimension annotation 3) Overlapping annotation text is not allowed because it affects dimension viewing.

Annotation text must never overlap. Overlapping leader text obscures the value being communicated and forces the fabricator to guess which feature a dimension belongs to, which defeats the purpose of the drawing.

Which Features Require Dimensioning

Not every feature on the board belongs on the dimension drawing. Dimensioning everything produces a cluttered sheet in which the critical values are lost. Three categories cover the requirements.

Locating pins of fixed components. If the count is large, the key components may be marked in simplified form, but press-fit parts must always be dimensioned. A press-fit connector that is even slightly mispositioned will not seat, and the error is only discovered at assembly.
Mounting holes and locating holes across the board. These determine mechanical fit and must be unambiguous.
Board outline dimensions. This includes chamfers, arcs, overall length and width, and open windows. Any region where solder mask is opened must additionally carry the words "open window" so the intent is explicit rather than inferred from the artwork.

Dimension Layer

All annotation text is placed on the Board Geometry/Dimension layer. Keeping every dimension on a single dedicated layer makes the drawing easy to review, easy to suppress during fabrication output, and impossible to confuse with copper or legend data.

Structure of the Drill Notes Table

The notes table consolidates manufacturing information that has no natural home in the artwork. It is typically laid out in a compact block at the edge of the fabrication drawing.

Board Name, Layer Count, and Thickness

BOARD NAME	LAYERS	BOARD THICKNESS mm +/- 10%
UNITS: MILS	OPEN WINDOWS	SPELL MODE

Board Name holds the name of the individual board. If the full name is too long for the field, an abbreviation is acceptable.
Layers records the layer count of the board.
Board Thickness records board thickness in millimeters.
Open Windows records the number of mask openings on the board. If there are none, the field is left blank.
Spell Mode records the panelization scheme. A value of 2x3 means two boards along the horizontal (x) direction and three along the vertical (y) direction. If the board is not panelized, the field is left blank.

Fabrication Process Fields

Figure 73 | Fabrication process in the Notes table

Solder Mask specifies the solder mask standard; the default is green.
Silkscreen Color specifies the legend color; the default is white.
Dielectric Material specifies the base material; the default is FR-4.
Er @ 1 GHz to 1 MHz specifies the dielectric constant of the material over that frequency range; the default is 4.2 to 4.5.
Plated Technics selects the surface finish. Hot air solder leveling (HASL) is the common choice. For boards with gold fingers, the required entry is "gold finger, others sprayed with tin solder." Other available finishes include electroless nickel immersion gold (ENIG), immersion silver (ImAg), immersion tin (ImSn), and organic solderability preservative (OSP).

Impedance Specifications

Impedance requirements are described in three forms:

Single-ended impedance: the controlled impedance value required for a given trace width on a given layer.
Differential pairs: the controlled impedance value required for a differential pair of a given trace width and gap on a given layer.
Separate entries: traces with different widths, gaps, or impedance values must each be described individually. The trace width used for differential pairs must be distinguishable from that used for single-ended traces, so the fabricator can tell the two apart without reference to the artwork.

Figure 74 | Impedance description in the Notes table

Additional Drill and Quality Notes

The QA field records the employee number of the responsible QA personnel.
All drill sizes are stated as finished sizes after plating, as shown below.

Figure 75 | Drill hole diameter description in Notes2

All drills within BGA areas are to be plugged with green solder mask, as shown below.

Figure 76 | BGA area via plugging description in Notes2

For drills used with 2mm press-fit parts, the drill tolerance must be stated, as shown below.

Figure 77 | Press-fit component hole diameter tolerance description in Notes2

If irregular holes are present, state the quantity and size of each specification. Where there are many types, describe them on separate lines, as shown below.

Figure 78 | Irregular hole description in Notes2

All notes text is placed on the Board Geometry/Dimension layer. Any note that does not apply must be deleted rather than left in place. If the board has no BGA, remove the BGA via plugging item; if there are no 2mm press-fit parts, remove the drill tolerance item. A leftover note that contradicts the design is worse than a missing one, because the fabricator has no way to tell which instruction is current.

Building the Package

The dimension drawing and the drill notes table work together. The drawing answers "where and how big," the notes answer "made of what, finished how, and to what tolerance." Both depend on a consistent unit convention and a single annotation layer so that nothing is lost when the fabrication outputs are generated. Reviewing the notes table line by line against the actual design, and deleting every inapplicable entry, is a short step that prevents most of the questions a fabricator would otherwise raise during pre-production review.

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