CircuiTree - March 2009 - (Page 36) IPC Standardizes Electrical Testing Requirements of Unpopulated Printed Boards John Perry and less expensive flying probe testers on the market, many of the expensive bed-of-nails test fixtures were quickly eliminated. I estimate that by 2003, 30 to 50 percent of bare boards were tested with probe technology and methods, which were not documented in any IPC standard,” says Hill. According to Green, “While IPC-9252 was a guideline on how to test boards, it did not provide a requirement for what you should test them to. The revised version defines things much more clearly, so revision A can be used as a requirements document—it tells how to do the tests as well as the requirements for test.” Another major reason for the revision that was discussed, debated, and analyzed at length had to do with adjacency—a subject not addressed in the original standard. Electrical test says that if there are three points ittle did Michael Green know that sending in a comment regarding IPC-9252 would set off a complete revision of the standard. As production design engineer for Lockheed Martin, Green recommended that Level C Continuity in Table 4-1 should be changed to 10 Ω from 50 Ω. He believed that 50 Ω is excessive resistance for a Class 3 trace, as anything greater than 10 Ω should be considered an open. He wrote, “My improvement satisfies the intent of the specification requirements listed below and our drawing requirements since 2000.” The result is IPC9252A, Requirements for Electrical Testing of Unpopulated Printed Boards, released in November 2008. IPC-9252A defines levels of appropriate testing and assists in the selection of the test analyzer, test parameters, test data, and fixturing required to perform electrical test(s) on unpopulated printed boards and inner layers. Realizing that the original concept document was “from the last millennia” and didn’t fit the different types of board testers on the market today, Mike Hill, quality manager for Colonial Circuits and chair of the IPC 7-32c Electrical Continuity Task Group, and Clifford Maddox, engineering specialist, Material and Processes for the Boeing Company, reconvened the task group to begin the revision process. The standard needed to support all phases of electrical testing of unpopulated 36 March 2009 • circuitree.com L boards, not just one type of testing. Assumptions used in the original document were now different. As a result, almost every paragraph of the standard has been changed. As Maddox explains it, “IPC-9252A has been extensively revised with new coverage of bare-board test methods not previously addressed by the original 2001 release. While the original document included requirements for test methods most commonly employed on fixed-grid [bed-of-nails] systems, there was insufficient coverage for test methods available on many moving-probe and/or moving-grid test systems.” Adds Hill, “The three major drivers for test during that time were increased high density interconnect designs, test equipment improvements, and innovations in test software [for example, adjacency methods]. The high-density designs couldn’t be tested with many of the bed-of-nails test fixtures because they couldn’t touch the pads with tight spacing. The 2001 version of IPC9252, associated IPC documents, military specifications, and customer’s prints were written entirely around bed-of-nails testing technology.” To test such dense products, printed board fabricators and assembly houses purchased flying probe testers. In a relatively short time frame, the flying probe tester equipment suppliers increased test speeds by an order of magnitude with mechanical and software innovations. “With these improved Figure 1 Adjacency Distance Example Figure 2 Line of Sight Adjacency http://www.circuitree.com
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