A novel method for fatigue testing of MEMS devices containing movable elements

In this paper we present an electronic circuit for position or capacitance estimation of MEMS electrostatic actuators based on a switched capacitor technique. The circuit uses a capacitive divider configuration composed by a fixed capacitor and the variable capacitance of the electrostatic actuator for generating a signal that is a function of the input voltage and capacitive ratio. The proposed circuit can be used to actuate and to sense position of an electrostatic MEMS actuator without extra sensing elements. This approach is compatible with the requirements of most analog feedback systems and the circuit topology of pulsed digital oscillators (PDO).Comment: Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/EDA-Publishing

Vibration Induced Fatigue Analysis of [0 n /90 n ] s Simply Supported Composite Plate Under Central Patch Impulse Loading

Fatigue analysis of a simply supported composite plate with laminate configuration of [0 n /90 n ] s under central patch impulse loading is presented using an analytical method. The method mainly consists of two steps, one, evaluation of vibration induced stresses for the given central patch impulse loading using modal analysis, and two, fatigue analysis using S-N curve approach, residual strength approach as well as failure function approach. The stress state in the plate was evaluated using viscous damping model as a function of time. The stress-time history was converted into block loading consisting of many sub-blocks. In the present study, the block loading consisted of four sub-blocks and a total of 175 numbers of cycles. The block loading was repeated after every 5 s. Next, fatigue analysis was carried out based on the block loading condition evaluated. Number of loading blocks for fatigue failure initiation and the location of failure were obtained. Studies were also carried out using two-dimensional (2D) finite element analysis (FEA). Number of loading blocks required to cause fatigue failure initiation under central patch impulse loading was found to be 3120 and 3170 using the analytical method and 2D FEA, respectively

HARMONIC AND RANDOM VIBRATION DURABILITY INVESTIGATION FOR SAC305 (Sn3.0Ag0.5Cu) SOLDER JOINT

ABSTRACT Title of Dissertation: HARMONIC AND RANDOM VIBRATION DURABILITY INVESTIGATION FOR SAC305 (Sn3.0Ag0.5Cu) SOLDER INTERCONNECTS Yuxun Zhou, Doctor of Philosophy, 2008 Dissertation directed by: Professor Abhijit Dasgupta Department of Mechanical Engineering 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

Structural Health Monitoring of Nonlinear Beam under Combined Translational and Rotational Vibration

This study presents a nonlinear dynamic methodology for detecting fatigue damage precursor in an isotropic metallic cantilever beam exposed to harmonic transverse, rotation or combined ¬– transverse and rotation – base excitations. The methodology accounts for important dynamic nonlinearities due to the complex loading generated by uniaxial and multiaxial nonlinear oscillations. These nonlinearities include: 1) structural stiffening due to gyroscopic motion and high-response amplitude at the structure fundamental mode, 2) structural softening due to inertial forces and gyroscopic loads, and localized evolution in the material microstructure due to fatigue damage and 3) cross-axis coupling due to multiaxial loading. The loading intensity and number of vibration cycles intensified these nonlinearities. The damage precursor feature is acquired by quantifying the reduction in the nonlinear stiffness term in the equation of motion due to localized evolution in the material micromechanical properties at high stress concentration regions. Nanoindentation studies near high stress concentration sites confirmed the evolution in the local micromechanical properties, as a function of loading cycles. The nonlinear analytical approach tracks the degradation in the structural stiffness as a function of the nonlinear dynamic response for the uniaxial transverse or rotation base excitation. The change in the dynamic response due to damage precursor is captured experimentally. The nonlinear stiffness terms are found to be sensitive to fatigue damage precursor for translational or rotational excitation. Therefore, the nonlinear stiffness sensitivity to fatigue damage precursor appeared to be a promising metric for structural health monitoring applications. This method is applicable to a cantilever beam only. Additional investigations will be required to extend its applicability to more complex structures. For the combined transverse and rotation base excitation, the experimental and analytic results demonstrated the importance of cross-axis coupling. The Experiments are performed using a unique multiaxial electrodynamic shaker with high controllability of phase and base excitation frequencies. The analytical model captures the modulation in the nonlinear dynamic response behavior seen in the experiments as a function of cross-axis coupling and the phase relation between the axes. Although the model is successful in capturing these general trends, it does not agree with the beam deflection absolute values obtained from the experiments. The discrepancy is due to fatigue damage accumulation during the experiments, which is manifested by a shift in the resonance frequency and an increase in the response amplitude

End-of-Life and Constant Rate Reliability Modeling for Semiconductor Packages Using Knowledge-Based Test Approaches

End-of-life and constant rate reliability modeling for semiconductor packages are the focuses of this dissertation. Knowledge-based testing approaches are applied and the test-to-failure approach is approved to be a reliable approach. First of all, the end-of-life AF models for solder joint reliability are studied. The research results show using one universal AF model for all packages is flawed approach. An assessment matrix is generated to guide the application of AF models. The AF models chosen should be either assessed based on available data or validated through accelerated stress tests. A common model can be applied if the packages have similar structures and materials. The studies show that different AF models will be required for SnPb solder joints and SAC lead-free solder joints. Second, solder bumps under power cycling conditions are found to follow constant rate reliability models due to variations of the operating conditions. Case studies demonstrate that a constant rate reliability model is appropriate to describe non solder joint related semiconductor package failures as well. Third, the dissertation describes the rate models using Chi-square approach cannot correlate well with the expected failure mechanisms in field applications. The estimation of the upper bound using a Chi-square value from zero failure is flawed. The dissertation emphasizes that the failure data is required for the failure rate estimation. A simple but tighter approach is proposed and provides much tighter bounds in comparison of other approaches available. Last, the reliability of solder bumps in flip chip packages under power cycling conditions is studied. The bump materials and underfill materials will significantly influence the reliability of the solder bumps. A set of comparable bump materials and the underfill materials will dramatically improve the end-of-life solder bumps under power cycling loads, and bump materials are one of the most significant factors. Comparing to the field failure data obtained, the end-of-life model does not predict the failures in the field, which is more close to an approximately constant failure rate. In addition, the studies find an improper underfill material could change the failure location from solder bump cracking to ILD cracking or BGA solder joint failures

Through-Life Monitoring of the impact of vibration on the reliability of area array packages using Non- Destructive Testing

In order to keep up with the demands for faster, cheaper and smaller electronics, the packaging industry has evolved tremendously. Area array packages like flip chips and ball grid arrays are therefore widely used in modern day electronics. However, from the reliability standpoint, solder joints in these area array packages are often the weakest link. In case of harsh vibration environments like military and automobile applications, joint failure mainly occurs due to the high stress incurred during extreme environmental conditions that lead to fatigue failures. This thesis aims to study the effects of real time vibration on area array packages (flip chips in particular) using acoustic micro imaging for through life monitoring of the solder joints. Since real time vibration on solder joints have not been studied before, the various steps for successful testing, through life monitoring of the solder joints and data analysis will be investigated and discussed. Based on automobile industry standards, a real time vibration profile was obtained with the help of Delphi experts, who are the industry collaborators of this project. Due to its strong capability to detect discontinuities within materials and interconnections, Acoustic Micro Imaging (AMI) also known as Scanning Acoustic Microscopy (CSAM) has been used to monitor the solder joints. This approach has not previously been used as an effective tool in monitoring solder joints through life performance in vibration testing. The research regime proposed in this thesis was to monitor the health of solder joints through ultrasound images from beginning to failure, and to see how cracks initiate and propagate in them. The effect of the relative position and orientation on the reliability of the solder joints and the flip chips in the PCB was also studied. The data collected was analysed using MATLAB. The results have shown that three types of solder joints- healthy, partially fractured or fractured are formed near the time of complete failure of a flip chip. When about 70- 80% of the flip chips are either partially fractured or fractured a flip chip is expected to fail. The mean pixel intensity and area change in the acoustic image of a partially fractured or fully fractured joint tends to be higher compared to a healthy joint. Crack initiation in a joint occurs at around 35-40% cycling and propagates linearly till 80-85% cycling after which a joint fails. A statistical analysis done on the solder joints showed that the intensity distribution of healthy joints follow a simple Gaussian distribution while that of partially fractured or fractured joint can only be represented by using a mixture of Gaussians. The solder joints near the board edges are the least reliable in a vibration environment. However, solder joints with back to back connections are more reliable than the ones placed in one sided orientation. The most reliable flip chip orientation in a vibration environment is the back to back connection with no offset which was actually found to be the least reliable in the case of thermal cycling. Based on the analysis of the results, a few design guidelines for flip chip layout and orientations in a PCB has also been proposed in this work

Study of photovoltaic (PV) module interconnections failure analysis and reliability

A thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the award of Doctor of Philosophy.Solar Energy is one of the most widely used renewable energy sources, with the solar Photovoltaic (PV) module technologies deployed as one of the primary renewable energy sources to replace fossil fuels. However, the R&D challenge for improving the performance and reliability of PV modules has become an urgent and critical agenda for the energy generation industry sector. The interconnection between the solar PV cells is a very important part of the PV module assembly, and its failure can adversely affect the performance and reliability of the PV module. The interconnection failure has been mostly linked to the crack initiation and propagation in the solder joints used to connect the ribbon interconnection to the cell. This research focuses on the study of the thermal failure of PV module solder joint to determine the optimum ribbon interconnection designs that will give improved thermo-mechanical reliability. It develops a virtual reliability qualification process for the assessment of the life expectancy of PV module interconnections. The FEM simulations in ABAQUS 2019 software are implemented to investigate failure of the solder joints in different ribbon interconnection designs under anticipated life cycle loading conditions and high temperature lamination process. For the first time, the extended finite element method (XFEM) technique is used to determine the crack initiation temperature, crack location, direction and growth rate in solder joint of PV module interconnection under lamination process. Furthermore, the research used the Developed Morrow Energy Density lifetime model to determine the number of cycles to creep-fatigue failure, and then it defined a new generic exponent factor using the Coffin–Manson–Arrhenius model to estimate the lifetime for the designs under different thermal cycling conditions. The research also combines the numerical results of XFEM and creep-fatigue investigation to determine the failure lifetime of PV Module interconnection designs. The results show that the Multi-Busbar interconnection design improves solder joint creep-fatigue life (up to 15%) and consequently provides higher thermo-mechanical reliability for the solar PV modules compared to other studied designs (Conventional and the Light Capturing Ribbon interconnections). The results of this PV module interconnections study can be used for evaluating potential design changes and to facilitate design for reliability validation of different configurations for improving the long-term PV module system reliability.Faculty of Science and Engineering, University of Wolverhampton

Dynamic Mechanical and Failure Properties of Solder Joints

Ph.DDOCTOR OF PHILOSOPH