EFFECT OF ISOTHERMAL AGING ON SAC305 HARMONIC VIBRATION DURABILITY

The effect of isothermal aging on the harmonic vibration durability of Sn3.0Ag0.5Cu solder interconnects is examined. Printed wiring assemblies with daisy-chained leadless chip resistors (LCRs) are aged at 125°C for 0, 100, and 500 hours. These assemblies are instrumented with accelerometers and strain gages to maintain the same harmonic vibration profile in-test, and to characterize PWB behavior. The tested assemblies are excited at their first natural frequencies until LCRs show a resistance increase of 20%. Dynamic finite element models are employed to generate strain transfer functions, which relate board strain levels observed in-test to respective solder strain levels. The transfer functions are based on locally averaged values of strains in critical regions of the solder and in appropriate regions of the PWB. The vibration test data and the solder strains from FEA are used to estimate lower-bound material fatigue curves for SAC305 solder materials, as a function of isothermal pre-aging

Thermomechanical fatigue failure of interfaces in lead-free solders

The European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS) banned lead from electronic systems from July 1, 2006 onwards, which has led to much interest in leadfree solders in the past years. Among several lead-free solder alternatives, SnAgCu is a widely accepted replacement due to its better creep-fatigue resistance and microstructural stability. SnAgCu has been extensively studied in the past decade, however, there are still issues to be resolved concerning solder reliability, the underlying mechanisms of thermo-mechanical fatigue failure, fatigue life predictions and the overall effect of decreasing component size, driven by the ongoing miniaturization trend. This thesis aims to scientifically contribute to this subject by a coupled experimental-numerical approach. In solder joint reliability, the bump/pad interface has a crucial role, the quality of which is determined by the metallization and interfacial defects. Solder balls, solder paste and cast eutectic SnAgCu are reflowed on Cu, Ni/Au and Cu/Ni(V)/Au metallization layers and the substrate influence on the bulk and interfacial metallurgy is examined. The damage propagation at SnAgCu soldered joints on Cu and Ni/Au substrates are investigated and microstructure related damage localization is identified as the dominant failure mechanism. Therefore, continuum damage approaches are believed to be inadequate for solder joint reliability predictions. Nano-indentation and tensile testing is used for the mechanical characterization of SnAgCu. An assessment on indentation parameters for solders is conducted and the influence of the Ag content on material properties of SnAgCu is presented. One of the main causes of ball grid array (BGA) failure is thermo-mechanical fatigue crack propagation in the solder, which is almost always observed at the bump/pad junction. Motivated by this fact, a combined experimental-numerical study on the cyclic mechanical response of SnAgCu/Ni-Au interface is conducted. In this study, damage evolution at the bond/pad interface is characterized by dedicated fatigue tests. Local deformations leading to crack propagation are simulated by separation of interfaces through a cohesive zone approach. Solder joints are tested under cyclic shear and cyclic tension for different specimen sizes and strain amplitudes. Two different damagemechanisms are observed: local deformations in the bulk and at the bonding interface. The interfacial failure mode is typically favored at a high initial stress, and a small solder volume. Crack propagation is simulated by an irreversible linear traction-separation cohesive zone law accompanied by a non-linear interfacial damage parameter. Later, tensile and shear experiments are used to characterize the cohesive zone parameters for the normal and the tangential opening, respectively. Interfacial fatigue damage in BGA solders is caused by the difference in coefficient of thermal expansion (CTE) of the materials in the package. Apart from this thermal incompatibility in the package, Sn based solders are themselves prone to thermal fatigue damage due to the intrinsic thermal anisotropy of the ß-Sn phase. Thermal fatigue causes local deformations especially at the grain boundaries. Hence, the thermal fatigue response of bulk SnAgCu is investigated as well. Bulk SnAgCu specimens are thermally cycled between -40 and 125¿C and mechanically tested afterwards in order to quantify the thermal fatigue damage. A size dependent cyclic softening behavior is observed. Test specimens are individually modeled including the microstructure and local crystallographic orientations, on the basis of orientation imaging scans (OIM). Both thermal cycling and tensile testing are imposed as boundary conditions. Reproducing the experimental results in the simulations, parameters of a cohesive zone based intergranular fatigue damagemodel are identified. Finally, the intergranular damage law characterized in this study is combined with the bump/pad interfacial damage law, and a 2Dmicrostructure-incorporated fatigue life prediction tool is established. Using this tool, it is shown that the failure mode of a soldered joint depends extensively on its geometry. The model presented above is extended to 3D for a more complete description of the problem. To provide the microstructural input, a database containing OIM scans of several SnAgCu solder balls is constructed. A missing constituent in the model so far, interfacial defects, i.e. voids, are examined statistically using newly manufactured BGA packages, revealing information on their size, position and frequency. Combining all the data collected, i.e. material properties, microstructure, defects, local damage laws, a 3D slice model from a BGA package is constructed. The slice model contains a single solder ball connecting the board and the chip. A series of case studies is created using experimental input such as different microstructures and initial defects allowing a statistical analysis. Fatigue life of these models are predicted and the results are validated by failure distribution analyses of BGA packages provided by the industry. Here the critical solder ball assumption is made: if a solder ball fails, the electrical circuit of the BGA package is open, thus the package fails. Setting a critical damage value for the interfaces accumulating fatigue damage, a good agreement with the experiments and simulations is obtained. It is seen that microstructural modeling allows to predict and understand the scatter in the solder ball fatigue life observed in reality. Finally, the effect of solder ball size and geometry on interconnect reliability is dis cussed on the basis of numerical analyses. For this purpose, a geometry factor and a microstructure factor is defined, and their influence on damage evolution is discusse

Thermomechanical fatigue failure of interfaces in lead-free solders

The European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS) banned lead from electronic systems from July 1, 2006 onwards, which has led to much interest in leadfree solders in the past years. Among several lead-free solder alternatives, SnAgCu is a widely accepted replacement due to its better creep-fatigue resistance and microstructural stability. SnAgCu has been extensively studied in the past decade, however, there are still issues to be resolved concerning solder reliability, the underlying mechanisms of thermo-mechanical fatigue failure, fatigue life predictions and the overall effect of decreasing component size, driven by the ongoing miniaturization trend. This thesis aims to scientifically contribute to this subject by a coupled experimental-numerical approach. In solder joint reliability, the bump/pad interface has a crucial role, the quality of which is determined by the metallization and interfacial defects. Solder balls, solder paste and cast eutectic SnAgCu are reflowed on Cu, Ni/Au and Cu/Ni(V)/Au metallization layers and the substrate influence on the bulk and interfacial metallurgy is examined. The damage propagation at SnAgCu soldered joints on Cu and Ni/Au substrates are investigated and microstructure related damage localization is identified as the dominant failure mechanism. Therefore, continuum damage approaches are believed to be inadequate for solder joint reliability predictions. Nano-indentation and tensile testing is used for the mechanical characterization of SnAgCu. An assessment on indentation parameters for solders is conducted and the influence of the Ag content on material properties of SnAgCu is presented. One of the main causes of ball grid array (BGA) failure is thermo-mechanical fatigue crack propagation in the solder, which is almost always observed at the bump/pad junction. Motivated by this fact, a combined experimental-numerical study on the cyclic mechanical response of SnAgCu/Ni-Au interface is conducted. In this study, damage evolution at the bond/pad interface is characterized by dedicated fatigue tests. Local deformations leading to crack propagation are simulated by separation of interfaces through a cohesive zone approach. Solder joints are tested under cyclic shear and cyclic tension for different specimen sizes and strain amplitudes. Two different damagemechanisms are observed: local deformations in the bulk and at the bonding interface. The interfacial failure mode is typically favored at a high initial stress, and a small solder volume. Crack propagation is simulated by an irreversible linear traction-separation cohesive zone law accompanied by a non-linear interfacial damage parameter. Later, tensile and shear experiments are used to characterize the cohesive zone parameters for the normal and the tangential opening, respectively. Interfacial fatigue damage in BGA solders is caused by the difference in coefficient of thermal expansion (CTE) of the materials in the package. Apart from this thermal incompatibility in the package, Sn based solders are themselves prone to thermal fatigue damage due to the intrinsic thermal anisotropy of the ß-Sn phase. Thermal fatigue causes local deformations especially at the grain boundaries. Hence, the thermal fatigue response of bulk SnAgCu is investigated as well. Bulk SnAgCu specimens are thermally cycled between -40 and 125¿C and mechanically tested afterwards in order to quantify the thermal fatigue damage. A size dependent cyclic softening behavior is observed. Test specimens are individually modeled including the microstructure and local crystallographic orientations, on the basis of orientation imaging scans (OIM). Both thermal cycling and tensile testing are imposed as boundary conditions. Reproducing the experimental results in the simulations, parameters of a cohesive zone based intergranular fatigue damagemodel are identified. Finally, the intergranular damage law characterized in this study is combined with the bump/pad interfacial damage law, and a 2Dmicrostructure-incorporated fatigue life prediction tool is established. Using this tool, it is shown that the failure mode of a soldered joint depends extensively on its geometry. The model presented above is extended to 3D for a more complete description of the problem. To provide the microstructural input, a database containing OIM scans of several SnAgCu solder balls is constructed. A missing constituent in the model so far, interfacial defects, i.e. voids, are examined statistically using newly manufactured BGA packages, revealing information on their size, position and frequency. Combining all the data collected, i.e. material properties, microstructure, defects, local damage laws, a 3D slice model from a BGA package is constructed. The slice model contains a single solder ball connecting the board and the chip. A series of case studies is created using experimental input such as different microstructures and initial defects allowing a statistical analysis. Fatigue life of these models are predicted and the results are validated by failure distribution analyses of BGA packages provided by the industry. Here the critical solder ball assumption is made: if a solder ball fails, the electrical circuit of the BGA package is open, thus the package fails. Setting a critical damage value for the interfaces accumulating fatigue damage, a good agreement with the experiments and simulations is obtained. It is seen that microstructural modeling allows to predict and understand the scatter in the solder ball fatigue life observed in reality. Finally, the effect of solder ball size and geometry on interconnect reliability is dis cussed on the basis of numerical analyses. For this purpose, a geometry factor and a microstructure factor is defined, and their influence on damage evolution is discusse

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

A STUDY OF THE THERMAL CYCLING PERFORMANCE OF SOLDER JOINTS IN AREA ARRAY PACKAGING

For both the electronics manufacturer and consumer, reliability is an essential characteristic defining the quality of the electronic component and system. Gradual degradation of the electronic components decreases efficiency of the system, and lack of reliability can lead to a significant loss. Efforts at achieving better quality and reliability of electronic components involve the inspection of solder joints in area array packaging. It is of note that solder interconnections are the vulnerable parts of circuit board assemblies (CBA), because they are mainly subjected to various assembly process during electronic manufacturing as well as environmental exposure failures during service. Therefore, the reliability of solder joints is a major concern during the entire life of an area array packaging in order to minimize the electronic failure rate that may lead to large losses. This thesis aims to provide a solution that helps to overcome some of the challenges that can occur during the reliability inspection of solder joints in area array packaging. Firstly, by successfully developing a non-destructive monitoring methodology to study the performance of solder joints under thermal cycling test. The quality of the solder joints in this research work from growth to failure was monitored by using a type of ultrasonic inspection called acoustic micro imaging (AMI). Results indicate that provided a suitable AMI parameters is applied, one can generate a 3D reconstruction of the solder joints images to allow and assess the solder joints’ behaviour in flip chip packages. AMI inspection of solder joints show good agreement with the results obtained that was used to examine how the reliability was affected by the geometry and position of the joints. An automatic segmentation technique was developed that allow to characterize and extract distinctive features of solder joints on different area array packages; such features include mean intensity, structural similarities model and histogram intensity of the region of interest of solder joints. The validation experimental results have been statistically implemented using novel geometrical and time domain features extraction methods like area, form factor and standard deviation. The result from these methods were used to extrapolate the solder joint’s fatigue life at normal operating conditions. Moreover, the analysis of variance (ANOVA) was employed to determine the percentage contribution of solder joints parameters on the acquired images. The results indicated that the thickness of the printed circuit board can affect solder joint reliability