This standard covers dimensioning and tolerancing requirements for rigid and flexible, single, double and multilayered printed boards based on industry capabilities.

Purpose.

The purpose of this standard is to establish rules, principles, and methods of dimensioning and tolerancing used to define the end product requirements of a printed board on a master drawing.

Classification.

This standard provides three classes (A, B, and C) for dimensional features to reflect progressive increases in sophistication of tooling, material and processing. The use of one class for a specific feature does not mean that other features must be of the same class. Selection should be based on the minimum need recognizing that precision, performance and conductive pattern density primarily determines tolerance class. In the event of conflict between the design requirements and the classes defined herein, the former shall take precedence and be reflected on the master drawing.

Design requirements determine class. Class definitions are useful for identifying degrees of precision needed to meet design/performance requirements of the printed board and establish a communication media between design and manufacturing disciplines.

Note: Classification of tolerance requirements should not be confused with the classification of end item use as referenced in other IPC documentation which refer to class 1) consumer products, class 2) general industrial, and class 3) high reliability equipment types. The need to apply certain tolerances should depend on the complexity and precision required to produce a particular printed board, therefore any tolerance class may be applied to any of the end product equipment categories as required.

Presentation.

All dimensions and tolerances are expressed in millimeters (metric) and inches (customary), shown in brackets [ ], and are not direct conversions in order to provide usable numbers. Users are cautioned to employ a single system and not intermix millimeters and inches. Reference information is shown in parentheses ( ).

Dimensioning System Types.

This standard recognizes the existence of two types of dimensioning systems. These systems provide the capability to establish feature size and location and part outline limits for printed boards. In establishing the end product requirements for a particular printed board, one or the other system shall be selected; systems should not be intermixed to define the same characteristic/feature of a printed board. The two systems are:

Type 1 - Nominal dimensioning system (tolerances are applied to dimensions)

Type 2 - Basic dimensioning system (tolerances are applied to features)

Terms and Definitions.

The definition of terms shall be in accordance with IPC-T-50, ANSI Y14.5, and the following:

ACTUAL SIZE: The measured size.

BASIC DIMENSION: A numerical value used to describe the theoretically exact size, profile, orientation, or location of a feature or datum target. It is the basis from which permissible variations are established by tolerances on other dimensions, in notes, or in feature control frames.

CONNECTOR TANG: The portion of the printed board that mates with a card edge connector.

CUMULATIVE TOLERANCES: The summation of all the tolerances permitted between functionally-related features.

(a) Chain Dimensioning. The maximum variation between two features is equal to the sum of the tolerances on the intermediate distances; this results in the greatest tolerance accumulation.

(b) Base Line Dimensioning. The maximum variation between two features is equal to the sum of the tolerances on the two dimensions from their origin to the features; this results in a reduction of the tolerance accumulation.

(c) Direct Dimensioning. The maximum variation between two features is controlled by the tolerance on the dimension between the features; this results in the least tolerance accumulation.

(d) Basic Dimensioning. The maximum variation between two features is controlled by the positional tolerances on the two features; this results in zero tolerance accumulation.

DATUM: A theoretically exact point, axis, or plane derived from the true geometric counterpart of a specified datum feature. A datum is the origin from which the location or geometric characteristics of features of a part are established.

DATUM FEATURE: An actual feature of a part that used to establish a datum.

DATUM, SIMULATED: The surface on a fixture, such as a surface-plate. See Figure 1.

DEPENDENT OF SIZE: The concept that permits tolerances of form or position to vary in proportion to, and is dependent on, a features size. (See 1.5.15 and 1.5.15.)

END PRODUCT (END ITEM): An end product is an individual part or assembly, in its final or completed state.

FEATURE: The general term applied to a physical portion of a part, such as a surface, hole, or slot.

FORM: A features shape.

GEOMETRIC TOLERANCE: The general term applied to the category of tolerances used to control form, profile, orientation, location, and runout.

INDEPENDENT OF SIZE: The concept that requires tolerance of form or position to vary independent of and without regard to feature size. (See 1.5.18.)

LEAST MATERIAL CONDITION (LMC): The condition in which a feature of size contains the least amount of material within the stated limits of size - for example, maximum hole diameter, minimum land, the smallest conductor, etc.

MAXIMUM MATERIAL CONDITION (MMC): The condition in which a feature of size contains the maximum amount of material within the stated limits of size - for example, minimum hole diameter, maximum land, the largest conductor, etc.

POSITIONAL TOLERANCE (PT): The amount that a feature is permitted to vary from the location of true position.

REFERENCE DIMENSION: A dimension used for information only. A dimension that does not govern inspection requirements.

REGARDLESS OF FEATURE SIZE (RFS): The term used to indicate that a geometric tolerance or datum reference applies at any increment of size of the feature within its size tolerance.

LIMITS OF SIZE: The specified maximum and minimum sizes.

SOLDER MASK APERTURE: The opening in a solder mask to permit access for soldering of a printed board land.

TOLERANCE: The total amount by which a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits.

TOLERANCED DIMENSION (TD): It is a dimension with a tolerance as opposed to a basic dimension which does not vary. Example: 1.875 0.010.

TRUE POSITION (TP): The theoretically exact location of a feature established by basic dimensions.

VIRTUAL CONDITION (VC): The boundary generated by the collective effects of the MMC-LMC limits of size of a feature and any applicable geometric tolerances. Examples: (Perfect Form)

a. Male Feature VCs:

MMC + PT = (GO Gauge)

LMC - PT = (NO-GO Gauge)

b. Female Feature VCs:

MMC - PT = GO Gauge

LMC + PT = NO-GO Gauge

Orientation tolerances may be substituted for PT, where applicable.