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Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.
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FIGURE 55.1 A US penny shows the size of some common PC board components, some older, some recent. The small black devices near Lincoln s bow tie are 04-02 (40 20 mils) surface-mount resistors. (Now 02-01 [20 10 mils] devices are being used.) Some 100-, 75-, and 50-mil test probes are shown for comparison. Note the lettering on the penny is done with 10-mil lines and the width of the circular part of the 9 is 35 mils, a common probe target size. (Photograph courtesy of Agilent Technologies.)
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electrical access needed for testing purposes is increasingly hampered. Because access to a circuit is crucial for testing, accessibility difficulties make testing steadily more difficult to accomplish. On top of this, the electronics industry is expected to provide continued improvements in cost, reliability, and quality. Testing plays a crucial role in these improvements, as will be seen.
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In the earliest years of the electronics industry indeed, before anyone may have called it an industry there was no concept of test. A product was put together and shipped, because inherent in the putting together was a basic appreciation for how the product was supposed to look and behave. This crafting quickly gave way to mass production* of somewhat more complicated items by workers who were not themselves experts in the craft. Of course, today, workers with little or no fathoming of what it is they are producing turn out items of unbelievable complexity. If they had to know everything about the process, it would undoubtedly be too expensive to make the product.
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* Perhaps the reader remembers the TV ads for Zenith televisions from about 25 years ago. They showed individual axial lead components being hand-wired onto lug strips and then soldered. The voice-over mentioned hand-crafted quality as if to imply that automated manufacturing (being used by Japanese manufacturers like Sony) was suspect.
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It is also important to appreciate that boards are often themselves components of systems. Boards might have a respectable yield from a board test process of, say, 97 percent, which is to say, 3 of 100 still contain defects.* However, if 20 such boards are used in a system, the probability that the system will turn on is only 54 percent. Because debugging a system is usually far costlier and more difficult than testing a board, there is much interest in higher post-test yields. This is why yields are being pushed into the 99 percent and higher levels. Test has gone through three stages of evolution during this time. It has been used as a sorting process, then as a repair driver, and finally as a process monitor.
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Test as a Sorting Process Test can be used to sort boards into two piles, good and bad. Essentially, such testing provides only one bit of information about a board. This one bit of information provides little clue as to what the problem is or how to repair it. If all we intend to do with a bad board is discard it, then one bit of information may be enough. In some applications, for example making $2 digital watches by the million, it may be economically impossible to attempt to repair bad units. However, as will be seen soon, discarding bad boards may also lead to discarding of valuable information about the health of our manufacturing process.
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Test as a Repair Driver It is very often economically justifiable to repair a bad board, if it can be done quickly and with little skilled analysis needed. This is where modern test systems begin to come into their own. More bits of information are needed per bad board; a diagnostic test is needed that accurately resolves failures into defect reports that can be acted upon to effect repairs. Because a freshly repaired board will need retesting to ensure it is now defect free, one also would like to find as many defects as feasible in each pass across a tester to avoid a glut of work in progress.
Test as a Process Monitor When repairing boards, valuable information about the health of our board manufacturing process is at our fingertips. Indeed, the repair process may have the ultimate defect resolution because faults detected by a tester may still not be perfectly correlated with defects (see Sec. 55.3). If one views test as a process monitor, one can gain valuable insight into what is happening upstream in many of the subprocesses that come together to produce boards. For example, one may see solder opens as a chronic problem. Further examination may show that they are happening in one area of a board more than in any other. That may lead to the examination of the solder application process, which shows that a solder screen squeegee has an uneven amount of solder paste on it. The root cause of this uneven distribution of paste can then be sought and corrected.
* It is important to differentiate test yield from defect coverage of a test. The yield of the manufacturing process before test might be (say) 50 percent, and the yield after test may be 97 percent. This may be accomplished with a test that detects (covers) 90 percent of the important defects. From basic probability theory, (0.97)20 = 0.54 is the chance that 20 independently manufactured boards with a probability of 0.97 of being fault free will all be fault free. Measuring yields in percentages is telling. In the IC industry, yields are measured in parts per million. One ppm corresponds to 99.9999 percent, and 250 ppm, a good IC yield, equals 99.975 percent yield. Will the board industry ever get into these ranges Will it make sense to try
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