IPC-2221 5.4.3 Datum Features

1. IPC-2221 5.4.3 Datum Features Datum features indicate the origin of a dimensional relationship between a toleranced feature and a designated feature or features on the printed board. Datum features are chosen to position the printed board in relation to a set of three mutually perpendicular planes. There are some cases where a single datum reference is sufficient, for example a printed board profile or a datum hole position.

2. IPC-2221 5.4.3 Datum Features Typically, printed board drawings are oriented with the primary (component) side or the designated Layer 1 facing up. This orientation establishes the backside of the printed board as the first (primary ("A")) of the three required datum planes. The other two datum planes (secondary ("B") and tertiary ("C")) are typically established at minimum material conditions using holes and/or etched features on the printed board.

3. IPC-2221 5.4.3 Datum Features The choice of features to be used for datum’s depends on what design elements are intended to be controlled. Board edge or feature edge datums should not be used primarily because their use may not represent the function of the printed board. They also pose producibility problems to the fabricator. Coordinate zero should be located at the secondary datum feature.

4. IPC-2221 5.4.3 Datum Features Datum features shall be specified on the master drawing by means of datum symbols per IPC-2615. The use of implied datums is not allowed. Datum features shall be functional features of the printed board, and should relate to mating parts such as mounting holes, connectors, or component leads or terminations. All datum features should be located on grid or establish the grid criterion, and they shall be located within the printed board profile.

5. IPC-2221 5.4.3 Datum Features To maximize the total available tolerance, a good practice is to separately locate and tolerance as patterns those printed board features that are produced in separate fabrication operations. Applicable patterns are as follows:

6. IPC-2221 5.4.3 Datum Features A. Plated Through-Hole Patterns The plated through-hole pattern (see Figure 5-5A) is generally the first drilling operation and is the first operation that defines the printed board. It is dimensioned as a basic grid with each hole toleranced to a basic grid intersection. The hole location tolerance is specified either in the hole list or by drawing note. A minimum of two holes on the grid shall be dimensioned and toleranced and shall establish the primary datum reference frame.

7. IPC-2221 5.4.3 Datum Features B. Unplated Through Hole Patterns Unplated through-hole patterns, especially tooling and mounting holes (see Figure 5-5B), are generally drilled in a separate drilling operation as one of the last fabrication operations. They shall be explicitly dimensioned and toleranced, even if they occur on grid. Two of these holes are typically established as secondary datum features because of their function, in spite of their place in the fabrication sequence.

8. IPC-2221 5.4.3 Datum Features C. Conductor Patterns The conductor pattern does not need a separate datum reference provided a minimum annular ring is specified. Minimum annular ring is a common way to tolerance the conductor pattern location with respect to the plated through-hole pattern. For some designs, particularly where automated assembly is used, this method may allow too much tolerance. In these cases, a feature location tolerance may be required and shall appear on the master drawing.

9. IPC-2221 5.4.3 Datum Features Alternately, fiducials may be required to locate and tolerance the conductor pattern with respect to the assembly tooling holes (see Figure 5-5C). The fiducial size, shape and quantity may depend on the type of equipment used in the assembly process and lead pitch and lead count. Figure 5-6 shows the Surface Mount Equipment Manufacturers Association (SMEMA) recommended fiducial design.

10. IPC-2221 5.4.3 Datum Features Another method to locate and tolerance the conductor pattern is by dimensioning to the centerline of a conductor. A critical area is edge board connector contacts; these should be dimensional as shown in Figure 5-7. This figure establishes a second X datum from which the tang edge is established as well as any keying slot. Tolerances used for edgeboard and keying slots shall be such that keying slots do not cut into or damage the contact finger. Dimensioning to the edge of a conductor is not allowed.

11. IPC-2221 5.4.3 Datum Features D. Printed Board Profile The printed board profile, including cutouts and notches (see Figures 5-5D and 5-7), requires a minimum of one datum reference. The use of three datum references and maximum material condition modifiers, as shown in Figure 5-5D, maximizes allowable tolerances and allows the use of hard tool gauging, which is particularly useful in high volume production situations.

12. IPC-2221 5.4.3 Datum Features E. Solder Resist Coatings The solder resist coating pattern may be located by specifying a minimum land clearance, or targets may be provided which serve the same function as fiducials for conductor patterns (see Figure 5-6). A minimum land clearance serves the same purpose as a minimum annular ring requirement in that it tolerances the solder resist pattern location with respect to the conductor pattern.