SMT007 Magazine

SMT-Dec2016

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76 SMT Magazine • December 2016 as possible due to costs, and where testing can- not be avoided, it would be beneficial to at least agree on one primary method. Several fundamental factors play a role in answering this question. These are discussed in the following. A structure test can prove that the integrity of the circuit is present on an electronic physi- cal level. The tests are typically of a mainly stat- ic nature, partially at-speed. It can also provide precise fault diagnostics in this context. It does not, however, test any functions and is therefore not able to detect a malfunction of the circuit in operating mode. This limits the fault coverage, when measured against the entire range of pos- sible faults. Thus, structure tests are ideal as in- direct process sensors, but by no means replace the functional test. The functional test on the other hand pro- vides high fault coverage in principle, and can also detect anomalies in operating mode, pro- vided it is carried out with the corresponding dynamic. However, the test patterns used also exert a very strong influence on the outcome, as faults must be stimulated in order to be detect- ed. The test can also detect structural faults if they effect the function, but the fault diagnoses are very unclear. If structural faults are not stim- ulated by functions, this generally leads to fault slippage with potentially serious consequenc- es at a later stage. This also affects parametric faults on individual components, open pins with floating signal level, missing pull resistors or incorrectly mounted decoupling capacitors, to list but a few of the problems. For this rea- son, functional tests in production are particu- larly suitable for providing basic proof that the selected operation modes are fulfilled, such as those carried out in the end-of-line (EoL) test. However, they are by no means a replacement for the structure test. A representation of the trends of the basic factors, as shown in Figure 1, provides a tangible display of the strengths and weaknesses of both BOUNDARY SCAN MEETS FUNCTIONAL TEST Table 1: Characterization of fundamental test principles. Figure 1: Performance potential of selected structural test and function test properties.

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