Reliability evaluation of stacked die BGA assemblies under mechanical bending loads

This thesis presents a reliability evaluation of stacked die ball grid array (BGA) assemblies under mechanical bending loads. The test specimens used in this investigation were four die stacked BGAs assembled on printed circuit boards (PCBs) with eutectic tin-lead solder and gold over nickel finishes, both as-reflowed and after aging. The failure envelopes of both types of specimen were quantified in terms of PCB flexural strain and strain rate. The experimental data from cyclic bending tests at three strain amplitudes with a constant strain rate have been used to determine the effect of strain amplitudes on cycles to failure. The experimental data from cyclic bending tests were combined with the data from impact tests to determine the effect of strain rate to cycles to failure. The failure sites associated with each test condition were identified, and failure site transition phenomena are reported and discussed

MODELING RATE DEPENDENT DURABILITY OF LOW-Ag SAC INTERCONNECTS FOR AREA ARRAY PACKAGES UNDER TORSION LOADS

The thesis discusses modeling rate-dependent durability of solder interconnects under mechanical torsion loading for surface mount area array components. The study discusses an approach to incorporate strain-rate dependency in durability estimation for solder interconnects. The components under study are two configurations of BGAs (ball grid array) assembled with select lead-free solders. A torsion test setup is used to apply displacement controlled loads on the test board. Accelerated test load profile is experimentally determined. Torsion test is carried out for all the components under investigation to failure. Strain-rate dependent (Johnson-Cook model) and strain-rate independent, elastic-plastic properties are used to model the solders in finite element simulation. Damage model from literature is used to estimate the durability for SAC305 solder to validate the approach. Test data is used to extract damage model constants for SAC105 solder and extract mechanical fatigue durability curve

Optimization for finite element modeling of electronic components under dynamic loaDing

Usage of electronic components in the U.S. ARMY applications is becoming more challenging due to their usage in harsh environments. Experimental verification of these components is expensive and it can yield information about specific locations only. This research outlines the finite element modeling methodology for these electronic components that are subjected to high acceleration loads that occur over extremely short time such as impact, gun firing and blast events. Due to their miniature size these finite element models are computationally expensive. An optimization engine was presented to have an efficient analysis procedure that provides a combination of accuracy, computational speed and modeling simplicity. This research also involves experimental testing of the electronic components mounted on the circuit boards. Testing was conducted at different strain levels in order to study the behavior of boards. Finite element models were developed for these tests and compared with experimental results

Development of convective reflow-projection moire warpage measurement system and prediction of solder bump reliability on board assemblies affected by warpage

Out-of-plane displacement (warpage) is one of the major thermomechanical reliability concerns for board-level electronic packaging. Printed wiring board (PWB) and component warpage results from CTE mismatch among the materials that make up the PWB assembly (PWBA). Warpage occurring during surface-mount assembly reflow processes and normal operations may cause serious reliability problems. In this research, a convective reflow and projection moire warpage measurement system was developed. The system is the first real-time, non-contact, and full-field measurement system capable of measuring PWB/PWBA/chip package warpage with the projection moire technique during different thermal reflow processes. In order to accurately simulate the reflow process and to achieve the ideal heating rate, a convective heating system was designed and integrated with the projection moire system. An advanced feedback controller was implemented to obtain the optimum heating responses. The developed system has the advantages of simulating different types of reflow processes, and reducing the temperature gradients through the PWBA thickness to ensure that the projection moire system can provide more accurate measurements. Automatic package detection and segmentation algorithms were developed for the projection moire system. The algorithms are used for automatic segmentation of the PWB and assembled packages so that the warpage of the PWB and chip packages can be determined individually. The effect of initial PWB warpage on the fatigue reliability of solder bumps on board assemblies was investigated using finite element modeling (FEM) and the projection moire system. The 3-D models of PWBAs with varying board warpage were used to estimate the solder bump fatigue life for different chip packages mounted on PWBs. The simulation results were validated and correlated with the experimental results obtained using the projection moire system and accelerated thermal cycling tests. Design of experiments and an advanced prediction model were generated to predict solder bump fatigue life based on the initial PWB warpage, package dimensions and locations, and solder bump materials. This study led to a better understanding of the correlation between PWB warpage and solder bump thermomechanical reliability on board assemblies.Ph.D.Committee Chair: Dr. Ume, I. Charles; Committee Member: Dr. Book, Wayne; Committee Member: Dr. Kim, Yeong; Committee Member: Dr. Pan, Jiahui; Committee Member: Dr. Sitaraman, Suresh; Committee Member: Dr. Wu, C. F. Jef

Numerical simulations for reliability assessment of lead-free solder interconnections in BGA packages

This work presents the results of computer-aided numerical simulations for the reliability assessment of lead-free solder interconnections in BGA packages. The finite element and Monte Carlo methods were employed for the macroscale structural and the mesoscale microstructural simulations, respectively. The major reliability tests for electronic component boards, i.e. thermal cycling, power cycling and drop impact tests, were simulated via the finite element method. The results provide a feasible tool for a better understanding of the observed failure modes in the reliability tests. The lifetime predictions based on the simulation results are helpful for the lifetime estimations of the BGA packages. The temperature effects on the drop impact reliability of the BGA packages were successfully elucidated by the finite element numerical experiments. In addition, a new algorithm was developed in order to predict dynamic recrystallization in solder interconnections during thermal cycling. The approach was realized by combining the Potts model based Monte Carlo method and the finite element method. The correlation between real time and Monte Carlo simulation time was established with the help of the in situ test results. Recrystallization with the presence of intermetallic particles in the solder matrix was simulated by introducing the energy amplification factors in the particle-affected deformation regions. The present algorithm predicted both the incubation period of the recrystallization as well as the growth tendency of the recrystallized regions in a way consistent with the experimental findings

Investigation into Solder Joint Failure in Portable Electronics Subjected to Drop Impact

Ph.DDOCTOR OF PHILOSOPH

Board level drop testing of advanced IC packaging

Master'sMASTER OF ENGINEERIN

An Investigation of Reliability of High Density Electronic Package-to-Board Interconnections from the Perspective of Solder Joint Metallurgy

The integration and miniaturization trend of the electronic packaging leads to much finer pitch of the device and package lead terminations. Several reliability concerns and issues that were previously not encountered are now surfacing. The objective of this thesis work is to investigate the reliability of the package-to-board interconnection from the perspective of solder joint metallurgy. It was carried out with several advanced packages such as CSP, WLCSP and leadless ceramic packages on organic laminate PWBs using tin-silver-copper based interconnection materials. The assemblies were subjected to several loading conditions and levels such as thermal, mechanical, and environmental stresses. As expected, the board level reliability (BLR) of electronic assemblies strongly depended on microstructure and morphology of the solder joints. Dispersion strengthening effect of the intermetallic compounds (IMCs), coarsening of the IMC particles, strain rate hardening, solder fatigue, and recrystallization of Sn grains in the highly stressed areas were observed. These were found to directly impact Pb-free solder joint reliability. Appropriate thermal aging can improve joint reliability up to 50% due to coarsening of the IMC particles. In addition, other factors such as dissolution of metals, interfacial reactions, IMC spalling, and cross interaction of surface materials on the two sides of the joints were also observed and discussed. The effects can be expressed as a series of interactive relationships: materials (pad surface materials and solder alloy composition) and/or soldering process lead to microstructure change in bulk solder and/or at interface, which in turn leads to joint reliability variation

Dynamic Mechanical and Failure Properties of Solder Joints

Ph.DDOCTOR OF PHILOSOPH

Rapid Assessment of BGA Fatigue Life Under Vibration Loading

Ball Grid Array (BGA) packages are a relatively new package type and have rapidly become the package style of choice. Much high density, high I/O count semiconductor devices are now only offered in this package style. Designers are naturally concerned about the robustness of BGA packages in a vibration environment when their experience base is with products using more traditional compliant gull or J leaded surface mount packages. Because designers simply do not have the experience, tools are needed to assess the vibration fatigue life of BGA packages during early design stages and not have to wait for product qualification testing, or field returns, to determine if a problem exists. This dissertation emphasizes a rapid assessment methodology to determine fatigue life of BGA components. If time and money were not an issue, clearly one would use a general-purpose finite element program to determine the dynamic response of the printed wiring board in the vibration environment. Once the response of the board was determined, one would determine the location and value of the critical stress in the component of interest. Knowing the critical stress, one would estimate the fatigue life from a damage model. The time required building the FEA model, conducting the analysis, and post-process the results would take at least a few days to weeks. This is too time-consuming, except in the most critical applications. It is not a process that can be used in everyday design and what-if simulations. The rapid assessment approach proposed in this research focuses on a physics of failure type approach to damage analysis and involves global and local modeling to determine the critical stress in the component of interest. A fatigue damage model then estimates the life. Once implemented in software, i.e. the new version of CALCE_PWA, the entire fatigue life assessment is anticipated to be executed by an average engineer in real time and take only minutes to generate accurate results