Tom
12-05-2002, 05:30 PM
IPC Advanced Study Guide Page Reference: Page 270, Section 5.1
Historically, printed board designs have used bilateral tolerances for size and position, which is acceptable with some restriction in regard to datum references; however, the use of geometric dimensioning and tolerancing (GDT) has many advantages over size and bilateral tolerancing.
Geometric tolerancing allows at least 57% more tolerance area with true position than with bilateral tolerancing. This is especially true for the location of holes and the lands circumscribing the hole. The technique provides for maximum producibility while assuring the mechanical function of the printed board. It also allows a ‘‘bonus’’ or extra tolerances when the maximum/least material concept is used.
The application of tolerance principles to different features helps in establishing which methodology to use in getting the most advantage of any of the dimensioning systems. The principles can be divided into two specific groups; location of features related to a true position and edge features of the printed board. The industry has for many years used the principle identified in ANSI Y14.5. That standard has been revised by the American Society of Mechanical Engineers (ASME) has been renamed ASME Y14.5M.
The document has served the industry well for many years, however the examples and explanations have been focused on the mechanical characterizations of the physical products. This has been a disservice to the electronics industry, in that it is difficult to explain the tolerancing of features to a new printed board designer when the standard contains no specifics on electronic components, the printed board, nor the assembly.
As complexity of the mechanical features of the electronic products increased a better focus needed to be established. The Joint Electron Device Engineering Council developed JES 95, a handbook that provides the mechanical outlines for semiconductor packages. The IPC developed IPC-2615 which takes the principles of ASME Y14.5M and captures those that pertain primarily to the features of the printed board.
As an example only three feature control symbols are identified to help control the form, fit, and function as well as the manufacturing allowable variation permitted on any printed board. The three conditions and their purpose are defined as:
Historically, printed board designs have used bilateral tolerances for size and position, which is acceptable with some restriction in regard to datum references; however, the use of geometric dimensioning and tolerancing (GDT) has many advantages over size and bilateral tolerancing.
Geometric tolerancing allows at least 57% more tolerance area with true position than with bilateral tolerancing. This is especially true for the location of holes and the lands circumscribing the hole. The technique provides for maximum producibility while assuring the mechanical function of the printed board. It also allows a ‘‘bonus’’ or extra tolerances when the maximum/least material concept is used.
The application of tolerance principles to different features helps in establishing which methodology to use in getting the most advantage of any of the dimensioning systems. The principles can be divided into two specific groups; location of features related to a true position and edge features of the printed board. The industry has for many years used the principle identified in ANSI Y14.5. That standard has been revised by the American Society of Mechanical Engineers (ASME) has been renamed ASME Y14.5M.
The document has served the industry well for many years, however the examples and explanations have been focused on the mechanical characterizations of the physical products. This has been a disservice to the electronics industry, in that it is difficult to explain the tolerancing of features to a new printed board designer when the standard contains no specifics on electronic components, the printed board, nor the assembly.
As complexity of the mechanical features of the electronic products increased a better focus needed to be established. The Joint Electron Device Engineering Council developed JES 95, a handbook that provides the mechanical outlines for semiconductor packages. The IPC developed IPC-2615 which takes the principles of ASME Y14.5M and captures those that pertain primarily to the features of the printed board.
As an example only three feature control symbols are identified to help control the form, fit, and function as well as the manufacturing allowable variation permitted on any printed board. The three conditions and their purpose are defined as: