Tom
12-05-2002, 05:19 PM
IPC Advanced Study Guide Page Reference: Page 268, Section 5.1
Printed board tolerance conditions can be derived in several different ways. In most instances these techniques are intended to provide information on the form, fit and function of the mechanical characteristics of the printed board and its’ feature. Every element of the board that describes mechanical characteristics should be considered in any analysis of the board properties. Sometimes these relate to the manner in which mating parts go together, other times the tolerances describe the variation that is permitted in the manufacturing process.
The IPC design standards have provided three complexity concepts for describing the physical tolerance variation on many of the features of the printed boards. The following table shows an example of the variation permissible on the periphery of the printed board. These are the recommended tolerances for the location and profile of cutouts and notches, however, the tolerances specified on the printed board drawing need to accommodate the dimensions and tolerances of the mating part.
Level A General Design Complexity—Preferred
Level B Moderate Design Complexity—Standard
Level C High Design Complexity—Reduced
The levels A, B, and C relate to the difficulty that the manufacturer has in producing the board while meeting the expectation of the end product variation. The table provides the allowable variation between the maximum and minimum end product dimensions. These may be referred to as Maximum Material Condition and Least Material Condition. Thus a length dimension for a Level C type product would vary between MMC and LMC no more than 0.15 mm [0.006”].
Printed board tolerance conditions can be derived in several different ways. In most instances these techniques are intended to provide information on the form, fit and function of the mechanical characteristics of the printed board and its’ feature. Every element of the board that describes mechanical characteristics should be considered in any analysis of the board properties. Sometimes these relate to the manner in which mating parts go together, other times the tolerances describe the variation that is permitted in the manufacturing process.
The IPC design standards have provided three complexity concepts for describing the physical tolerance variation on many of the features of the printed boards. The following table shows an example of the variation permissible on the periphery of the printed board. These are the recommended tolerances for the location and profile of cutouts and notches, however, the tolerances specified on the printed board drawing need to accommodate the dimensions and tolerances of the mating part.
Level A General Design Complexity—Preferred
Level B Moderate Design Complexity—Standard
Level C High Design Complexity—Reduced
The levels A, B, and C relate to the difficulty that the manufacturer has in producing the board while meeting the expectation of the end product variation. The table provides the allowable variation between the maximum and minimum end product dimensions. These may be referred to as Maximum Material Condition and Least Material Condition. Thus a length dimension for a Level C type product would vary between MMC and LMC no more than 0.15 mm [0.006”].