It's worse than just using the test equipment manufacturer guidelines. Along with that, you have to deal with each of the test fixture fabricators' preferences as well. What this boils down to is a negotiation with the test engineering group as to what is testable. They are not even slightly embarrassed asking for 100% bottom side only coverage with 0.040" square test pads on 0.100" centers (on a grid, of course) for a board with 6/6 mil line/space, 8/18 mil via hole/pad and 0.8mm BGA. Totally out of touch. Design and board fabricatio technolology moves om, while testing is so 1990s.
For a touch of realism: http://www.datacircuits.com/pages/eng_bga.html
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