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TABLE 55.1 Examples of Manufacturing Defects Seen at the Board Level, Sources of the Problems and Causes Defect Shorts between solder pins Solder open Missing component Wrong component Misoriented component Dead component Source(s) Wave/reflow soldering Solder application, wave/ reflow soldering Placement, soldering Placement setup, inventory, handling Placement setup Placement, soldering Cause(s) Too much solder, solder screen defect, pin misregistration, bent pins Too little solder, solder screen defects, tombstoning, bent pins Shock, abrasion, too little glue Handling error, mismarked packages, operator error, wrong specifications Handling error, operator error Dead on arrival, handling damage, electrostatic damage
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Specification Failures Specification failures are similar to performance faults. Performance specifications are checked against requirements for the full range of operating conditions expected, such as temperature, humidity, vibration, electronic noise, etc. Specification test is often a regulatory or contractual requirement. One may argue that these specifications are largely unnecessary because, if a circuit design is robust, makes use quality components, is accurately assembled, and is thoroughly tested for manufacturing defects, one does not have to make it perform all of its functions to know that it works. This is the proof by construction argument, used by successful manufacturers of kitchen matches and bomb detonators. Full specification test may also be impossible in practical terms because there may be too many combinations of circuit functions versus operating ranges. If only a subset of combinations is to be checked, this begs the question of which subset. If specification test is required nonetheless, such testing is usually carried out with customtailored test equipment that can simulate the range of operational environments of interest. Such testing may be quite time consuming, and the cost of the supporting test equipment may range from trivial to hyperexpensive. At one extreme, a manufacturer of I/O cards for personal computers may simply plug each one into a computer and see if it performs a simple loopback test. At the other extreme, a manufacturer of guidance computers for missiles may require a missile shot on a test range, full telemetry, and support from the Air Force and Navy. The bottom line is that you should seriously question the motivations for and expectations of testing for specification faults.
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We have seen a spectrum of faults, performance faults, manufacturing defects, and specification faults. Each has evolved a test technology.
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55.4.1 Testing Boards for Performance Faults Performance testers are often known as edge connector functional testers. They connect to the edge connectors of the board under test with mating connectors that are then adapted to the tester resources by a customized fixture called an adapter. In most cases, no other connections are made by the test fixturing to any internal nodes of the circuit. In essence, the board is tested in an environment that resembles its application environment to some degree.
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Some performance testers have guided probe capability.1,2 A guided probe is a manually positioned test probe with a measurement (and sometimes stimulus) capability and supporting software. It is used to temporarily gain access to internal nodes of a circuit, one at a time, where observations of internal circuit behavior during a test can be made and processed by software to provide enhanced fault resolution. How does this software know what the nodes of a circuit are supposed to be doing during a test One could enter all these data by hand if one knew the design of a circuit well, but this is impossible for any circuit that consists of more than a handful of gates. The most popular way to get these data is by logic simulation of the circuit.*2 Good circuit simulation is often used by designers to prove the circuit behaves the way they expect it to under various input conditions (input vectors) they are interested in. Fault simulation is used to study how a circuit behaves when that circuit is perturbed by a modeled fault. An example of a modeled fault is a stuck-at-0 on a gate output, meaning no matter what the gate s inputs are, the output is 0. The single stuck-at model is the most prevalent fault model in existence. It assumes that only one stuck-at (either 0 or 1) may exist in a circuit at any one time. A fault simulator can be used to predict, for a given modeled fault, how the circuit will behave when stimulated with input (test) vectors. If the modeled fault causes one or more observation points (like circuit output pins) to deviate from normal behavior, we can declare the fault detected by that vector. In the common case of sequential circuits, it is more accurate to say all vectors detected the fault up to and including the one that caused the observable deviation. Another technology used by some performance testers is the fault dictionary.2 Fault dictionaries are prepared with fault simulators. A fault dictionary is a three-dimensional data structure of Boolean true/false bits (see Fig. 55.3). The first dimension is an enumeration of all test vectors. The second dimension is a list of modeled faults that are to be simulated. The third dimension is an enumeration of the circuit s output pins. A given bit in the fault dictionary is true if the corresponding output pin fails on the corresponding vector for a given fault.
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* Logic simulation implies the treatment of the digital nature of the circuit, typically omitting any interaction with any analog portions of the circuit. Analog simulators do exist, but are implemented in a very different technology from digital simulators. Mixed-signal simulation is not anywhere near as viable as digital simulation, which presents a problem for test engineers.
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FIGURE 55.3 A fault dictionary. For each page corresponding to a modeled fault, bits set to 1 indicate that a given output pin fails for a given vector number. When testing is performed, the failing output(s) and failing vector number are noted and then each page is searched at those coordinates to look up a matching fault. (Note that more than one fault may produce a match.)
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