Mechanical Engineering
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Item DURABILITY DISTRIBUTION ANALYSIS OF LEAD-FREE SOLDER JOINTS FOR PRINTED CIRCUIT BOARD APPLICATIONS(2023) Huang, Chien-Ming; Herrmann, Jeffrey; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fatigue models for predicting the cycles to failure of solder joints under temperature cycling situations have been discussed and developed for decades. However, most models were developed according to specific solder materials, components, and printed circuit boards in each research. There is no study to cluster and compare the fatigue models of solder joints through these different conditions. Therefore, the availability of the durability prediction of solder interconnections by using any of the available fatigue models can be unknown. On the other hand, current energy-based fatigue models for predicting the cycles to failure of the solder joint under thermo-mechanical loadings can only provide point estimates of the characteristic life or mean life. Nevertheless, the prediction of the fatigue life should be distributions with the uncertainties. Unfortunately, no study has been found that propagates the uncertainty of the cycles to failure, especially for the solder joints under temperature cycling. Therefore, the uncertainty propagation analysis of the cycles to failure is necessary to better estimate the distribution of the fatigue life of solder joint.The first part of this dissertation clusters and compares nine existing low-cycle energy-based fatigue models for different solder materials and components, and then analyzes major divergences between these studies. Moreover, the constants of the fatigue models are compared according to the divergences. Finite element simulation tool is applied to demonstrate the contributions of the factors on strain energy density and the variation on the predictions by applying these fatigue models. The results lead to conclusions with the advantages and limitations of using these available fatigue models for durability prediction of solder interconnections. These results provide insights that can help product designers understand and exploit the predictions of fatigue life while designing a printed circuit board and estimating its durability. In the end, precautions that can affect the prediction consistency of fatigue mode are provided for the engineers who will use these selected models or will develop their own models. The second part of this dissertation identifies 11 uncertain input variables, which can propagate the uncertainties, via basic mechanics theory. The eigenvector dimension reduction method and FEA simulation tool are employed to determine the distribution of the system response, which is the strain energy density. Then, the distribution of strain energy density is converted to the distribution of characteristic life (in cycles) by choosing the appropriate fatigue model from the first part of this dissertation. Finally, the distribution of cumulative distribution function of the fatigue life of solder joint is determined by taking the interval of characteristic life and specific shape parameters. In the end, this new uncertainty propagation approach can propagate the uncertainties from the material properties, geometries, and constitutive laws of the solder joint, as well as the uncertainties from strain energy density determination, low-cycle energy-based fatigue model selection, and the shape parameter from the field data in the selected fatigue life models.Item HARMONIC AND RANDOM VIBRATION DURABILITY INVESTIGATION FOR SAC305 (Sn3.0Ag0.5Cu) SOLDER JOINT(2009) Zhou, Yuxun; dasgupta, abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Vibration loading is commonly encountered during the service life of electronic products. However, compared to thermal cycling durability, vibration durability is more complex and has been less investigated. In surface mount technology, solder joints are the primary mechanical, thermal and electrical interconnects between the component and the PWB. So the reliability of solder joints is very crucial for most electronic assemblies. The vibration durability of Pb-free solder joints is the focus of this dissertation. The characteristics of the stress from vibration loading are low amplitude and high frequency, while those from cyclic thermal loading are high amplitude and low frequency. In this study, several exploratory vibration tests were conducted, using both narrow band and broad-band, step-stress excitation at several different isothermal and thermal cycling conditions. The effect of thermal pre-aging on solder joint vibration failures was also investigated. Some of the vibration durability results were analyzed in detail, to obtain quantitative insights into the vibration fatigue behavior of the SAC305 solder material. A time-domain approach was adopted to investigate the durability of solder interconnects under different kinds of vibration and quasi-static mechanical loading. First, the solder interconnects were subjected to narrow-band (harmonic) vibration loading. The test were conducted at the first natural frequency of the test board using constant-amplitude excitation and solder fatigue properties were extracted with the help of a time-domain analysis that is based on quasi-static finite element simulation. Compared to broad-band step-stress vibration durability tests, the advantage of the harmonic constant-amplitude test is less complexity in the model extraction process, hence, less uncertainty in the desired fatigue constants. Generalized strain-based S-N curves have been obtained for both SAC305 and Sn37Pb solder materials. The strain-life model constants show that SAC305 solder material has superior fatigue properties compared to Sn37Pb solder material under low-cycle fatigue loading, while the reverse is true for high-cycle fatigue loading. These results are consistent with test results from other researchers. In actual application, SAC305 assemblies almost always fail before Sn37Pb assemblies under comparable vibration excitation because of (i) higher solder strain at a given excitation level; and (ii) multiple failure modes such as copper trace cracking. Next, durability was investigated under step-stress, broad-band (random) excitation. These test results show that SAC305 interconnects are less durable than Sn37Pb interconnects under the random excitation used in this study, which agrees with the harmonic durability results. The random and harmonic durability results were quantitatively compared with each other in this study. Finite element simulation was used to investigate the stress-strain response in the interconnects. The output of this simulation is the strain transfer function due to the first flexural mode of the PWB. This transfer function is used to obtain the solder strain from the measured board strain. This fatigue assessment method demonstrated that the model constants obtained from the harmonic test overestimate the fatigue life under random excitation by an order of magnitude. The causes for this discrepancy were systematically explored in this study. The effects of cyclic loading and mean stress on the vibration durability were addressed and found to be minimal in this study. The stress-strain curves assumed for the solder material were found to have a very large effect on the durability constants, thus affecting the agreement between harmonic and random durability results. The transient response of the components on the test board under both harmonic and random excitation was also included in the strain transfer function with the help of dynamic implicit simulation, and found to have a much stronger effect on the vibration durability at the high frequencies used in broad-band excitation compared to the low frequency used in narrow-band test. Furthermore, the higher PWB vibration modes may play a strong role and may need to be included in the strain transfer-function. This study clearly reveals that the solder strain analysis for broad-band random excitation cannot be limited to the quasi-static strain transfer-function based on the first PWB flexural mode, that has been used in some earlier studies in the literature. The time-domain approach used in this study provided fundamental and comprehensive insights into the key factors that affect vibration durability under different types of excitation, thus leading to a generalized S-N modeling approach that works for both harmonic and random vibration loading.