Mechanical Engineering
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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.Item Damage Initiation and Evolution in Voided and Unvoided Lead Free Solder Joints Under Cyclic Thermo-Mechanical Loading(2007-02-05) Jannesari Ladani, Leila; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effect of process-induced voids on the durability of Sn-Pb and Pb-free solder interconnects in electronic products is not clearly understood and researchers have reported conflicting findings. Studies have shown that depending on the size and location, voids are not always detrimental to reliability, and in fact, may sometimes even increase the durability of joints. This debate is more intensified in Pb-free solders; since voids are more common in Pb-free joints. Results of experimental studies are presented in this study to empirically explore the influence of voids on the durability of Ball Grid Array (BGA) Pb-free solder joints. In order to quantify the detailed influence of size, location, and volume fraction of voids, extensive modeling is conducted, using a continuum damage model (Energy Partitioning model), rather than the existing approaches, such as fracture mechanics, reported in the literature. The E-P approach is modified in this study by use of a successive initiation method, since depending on their location and size; voids may influence either the time to initiate cyclic fatigue damage or time to propagate fatigue damage, or both. Modeling results show competing interactions between void size and location, that results in a non-monotonic relationship between void size and durability. It also suggests that voids in general are not detrimental to reliability except when a large portion of the damage propagation path is covered with either a large void or with many small voids. In addition, this dissertation also addresses several fundamental issues in solder fatigue damage modeling. One objective is to use experimental data to identify the correct fatigue constants to be used when explicitly modeling fatigue damage propagation in Pb-free solders. Explicit modeling of damage propagation improves modeling accuracy across solder joints of vastly different architectures, since the joint geometry may have a strong influence on the ratio of initiation-life to propagation-life. Most solder fatigue models in the literature do not provide this capability since they predict failure based only on the damage accumulation rates during the first few cycles in the undamaged joint. Another objective is to incorporate into cyclic damage propagation models, the effect of material softening caused by cyclic micro-structural damage accumulation in Pb-free solder materials. In other words the model constants of the solder viscoplastic constitutive model are continuously updated with the help of experimental data, to include this cyclic softening effect as damage accumulates during the damage-propagation phase. The ability to model this damage evolution process increases the accuracy of durability predictions, and is not available in most current solder fatigue models reported in the literature. This mechanism-based microstructural damage evolution model, called the Energy Partitioning Damage Evolution (EPDE) model is developed and implemented in Finite Element Analysis of solder joints with the successive initiation technique and the results are provided here. Experimental results are used as guidance to calibrate the Energy Partitioning fatigue model constants, for use in successive initiation modeling with and without damage evolution. FEA results show 15% difference between the life predicted by averaging technique and successive initiation. This difference could significantly increase in the case of long joints such as thermal pads or die-attach, hence validating the use of successive initiation in these cases. The difference between using successive initiation with and without damage evolution is about 10%. Considering the small amount of effort that has to be made to update the constitutive properties for progressive degradation, it is recommended that softening be included whenever damage propagation needs to be explicitly modeled. However the damage evolution exponents and the corresponding E-P model constants obtained in this study, using successive initiation with damage evolution, are partially dependent on the specimen geometry. Hence, these constants may have to be re-calibrated for other geometries.