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occurs in service in a known way, i.e., that the data can be used to predict the life distribution in the in-service PCA environment. This assumption makes sense only if the same failure modes occur in the test as in real life. It cannot be overemphasized that the accelerated tests must be designed around the real failure modes. Probable failure modes may be identified from past service experience, the literature, or preliminary testing or analysis. 4. For each failure mode, construct an acceleration model. An acceleration model that allows test data to be interpreted in terms of the expected service environment is crucial to life distribution estimation. It is also extremely helpful in designing good tests, so ideally the acceleration model should be developed before the accelerated reliability tests are carried out. Equation 57.2 for solder joint reliability plus Eq. 57.1 for strain in solder joints to rigid components is an example of an acceleration model. It predicts that increasing strain will decrease the number of cycles to failure in a specific way. Within a certain temperature range, increasing the temperature cycling range is a way of increasing the strain. In general, the acceleration model should be based on the rate-controlling step in the failure process. In some cases, the rate will be determined by an Arrhenius type equation; for example, if diffusion is the rate-controlling process: D = Doexp t2 = where D Do Ea k = = = = Ea kT and x Dt (57.3)
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1 D1 Ea 1 t1 = t1exp D2 T2 k T1
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diffusion rate diffusion constant activation energy for the process Boltzmann constant
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and T1 and T2 and t1 and t2 are two temperatures and corresponding equivalent diffusion times Note that even when temperature is an important factor, an Arrhenius relationship may not exist; in the preceding thermal cycling example, the failure rate is roughly proportional to ( T)2. Some acceleration models will be explored in the following sections. The limits of applicability of an acceleration model are as important as the model itself. Increasing or decreasing the temperature too much may promote new failure modes that would not occur in service or invalidate the quantitative acceleration relationship. For example, if the temperature is elevated above the Tg of the board, the z-axis CTE increases sharply and the modulus decreases, which may actually lessen the strains imposed on solder joints, but may also promote PTH failures. Finite element modeling (FEM) can be invaluable in developing and/or applying acceleration models for thermal and mechanical tests. Two-dimensional nonlinear modeling capability will usually be required in order to get meaningful results. Models can be constructed to estimate the stresses and strains in the material (e.g., the Cu in a PTH barrel or the solder in a surface-mount or through-hole joint) under operating conditions as well as under test conditions. These estimates will be far more accurate than the simple models provided in this overview because they can account for the interactions between materials in a complex structure and both elastic and plastic deformation. 5. Design tests based on the acceleration models and accepted sampling procedures. Using the acceleration model and the service environment and life, select test conditions and test times that simulate the life of the product in a much shorter period of time. The sample size must be large enough that it is possible to determine whether the reliability goal (acceptable number of failures over the service life) has been met.41 Ideally, the life distribution in the accelerated test should be determined, even when the test period must be extended to do so.
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6. Analyze failures to confirm failure mode predictions. Since an accelerated test is based on the assumption that a particular failure mode in the accelerated test is the same one that occurs in service, it is important to confirm by failure analysis that this assumption is valid. If the failure mode in the accelerated test is different from the one expected, several possibilities should be considered. (1) The accelerated test is introducing a new failure mode different from the one that will occur in service. Usually this means that the acceleration of one parameter (e.g., frequency, temperature, humidity) was too severe. (2) The initial determination of the dominant failure mode was incorrect. In this case, to understand the significance of the test results, a new acceleration model must be developed for this failure mode. The new failure mode may be promoted more or less effectively by the test conditions than the mode originally assumed. (3) There may be several failure modes. In this case, the two failure distributions should be considered separately, so that life predictions will be meaningful. The difficulty in determining which of the above scenarios holds is that for genuinely new technologies or service environments, the failure mode in service may not be known. In these situations, it is desirable to conduct a parallel test with less aggressive acceleration for comparison. 7. Determine life distribution from accelerated life distribution. The accelerated life distribution should be determined by fitting the data with the appropriate statistical distribution, such as the Weibull or log-normal distribution. The life distribution in service can be determined by transforming the time axis of the life distribution using the acceleration model. This predicted life distribution in service can then be used to estimate the number of failures in the specified service life. The following discussion of testing for some specific failures will provide examples of this methodology. 57.6.2 Printed Circuit Board Reliability Tests Thermal. PTH failures are the predominant source of PCB failures in service and predicting them is the primary goal of PCB testing at elevated temperatures. PTH reliability testing should simulate the thermal excursions of a PTH throughout its life. Generally, the most severe thermal cycles are experienced during assembly and rework. Two basic types of tests are conducted: thermal stress or solder float tests, and thermal cycling tests. Both of these tests are intended to be accelerated tests for the PTH, not for the laminate; the thermal stress test, in particular, is expected to severely degrade the laminate. The delamination test is similar to a solder float test, but is conducted at a lower temperature specified by the laminate manufacturer; typically, a different fluid is required. The most commonly accepted thermal stress test is MIL-P-55110 (also found in IPC-TM650). Following baking at 120 to 150 C (250 to 300 F), the specimens are immersed in an RMA flux and floated in a eutectic (or near-eutectic) Sn-Pb solder bath at 288 C (550 F) for 10 s. Other investigators use a bath at 260 C. Following the test, the samples are crosssectioned and the PTHs are examined for cracks. This is a severe test that ensures that the sample will survive a single wave-soldering or solder pot rework cycle. Most thermal cycling tests for PCBs cycle the PCB repeatedly over a wide temperature range; many are actually thermal shock tests using liquid-liquid cycling. The results of five accelerated tests with different temperature extremes, ramp rates, and dwell times have been compared by the IPC, which also provides a simplified analytical model to estimate PTH life.3 The results of all tests suggest the same approaches for maximizing PTH reliability, but they do not all correlate well quantitatively. Two of the most common tests are (1) oven cycling from 65 to +125 C, and (2) thermal shock cycling between oil or fluidized sand baths at +25 to 260 C. Figure 57.23 shows a suitable test coupon that contains 3000 PTHs interconnected in series, several PTH sizes, and varying annular ring sizes. The PTHs can be monitored during the testing. Figure 57.18b shows the type of data that can be collected in this type of test.
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