2019 IEEE 69th Electronic Components and Technology Conference (ECTC), 2019
This paper describes the kinetic and microstructural mechanism of electromigration (EM) failure f... more This paper describes the kinetic and microstructural mechanism of electromigration (EM) failure found in low-profile solder joints where EM and intermetallic phase formation compete for the same volume of Sn. The low-profile solder joint used in our study was made of 20-25um thick solder situated in between a Cu pillar and a Ni coated Cu lead frame (LF). The samples were EM tested in a temperature range of 140-170oC with the current densities varying between 35-45 KA/cm2 in an oil bath to induce failure without Joule Heat induced artifacts. Our studies on EM failure kinetics and microstructural mechanism have produced two key findings. The first finding suggests that the EM diffusivity (Z*D) of diffusing species (Sn, Ni, Cu) in the solder matrix can be uniquely ranked from microstructural analysis, and it is estimated to be (Z*D) Cu> (Z*D) Sn>(Z*D) Ni. This difference in EM diffusivity causes Cu-Sn and Ni-Sn intermetallic compounds (IMC) to develop in distinctively different manners under EM, leading to different EM failure mechanisms. The second finding is that EM in low-profile solder joints consists of multiple failure stages: a) with EM-related voiding in Sn dominating at lower temperatures; while b) thermally-induced IMC growth and invasion competes with EM-induced Sn voiding at high temperatures leading to the complete failure of each joint.
2020 International Wafer Level Packaging Conference (IWLPC), 2020
Electromigration (EM) induced failure is inevitable in wafer-level chip scale package microelectr... more Electromigration (EM) induced failure is inevitable in wafer-level chip scale package microelectronic packages (WCSP), especially with the implementation of lead-free solders. Many factors contribute to EM failure such as joule heating and current crowding. EM can induce void formation, which can eventually lead to open-circuit failure. Due to its nature, EM is a critical failure concern to the microelectronic industry and can be influenced by current conditions. This study examines the failure mechanisms in solder joints implemented in WCSP packages in Direct Current (DC) and DC-pulse current conditions with varying Duty Factors (DF). DF represents the on-off time for DC to flow through the device under test (DUT). Further, a transient simulative study using finite element method (FEM) explores the failure mechanism and investigates the stress development with DC and DF conditions. Findings suggested that a lower duty factor yielded longer time to failure (TTF). Meanwhile, higher pulsed DC DF yielded a lower TTF than DC. This study aims to explain the failure mechanism with each DF. This study aims to explain this phenomenon and suggests the need for further exploration.
ABSTRACT This paper reports the findings supporting that solder joint grain structure within an a... more ABSTRACT This paper reports the findings supporting that solder joint grain structure within an assembly is not fixed but varies with the joint location and cooling rate. Also reported is the fact that such change in grain structure can make the joint fatigue resistance to be increased. These conclusions are made by comparing the grain structure and fatigue kinetics of the joints in BGA assembly with variation in cooling rate after thermal aging. Our study finds that the grain structure changes with cooling rate because of the kinetic interplay between plastic deformation by thermal strain and its relaxation process other than dislocation glide. The variation in the spontaneous plastic deformation and its storage in the solder promote recrystallization in one extreme, while the process of dynamic relaxation releases the stored energy and make the joint maintain its original grain structure in another extreme. Mechanical fatigue testing of these samples reveals that failure prone joint, corner joint, becomes immune to fatigue failure when its grain structure is changed to polygranular structure. The degree of reliability gain is not possible to quantify at the present moment because the failed joint is shifted to non-recrystallized joint and the failure kinetics is also affected by the amount of residual stress which is also affected by the cooling rate. However, there are many of indications that the joint with polygranular structure is much less susceptible to fatigue failure. Some of highlighting evidences are presented in this paper.
The geometry of interconnections used in current electronic systems is described, and issues rela... more The geometry of interconnections used in current electronic systems is described, and issues related to the reflow process that melts and re-solidifies the solder to make the interconnection are introduced.
Journal of Materials Science: Materials in Electronics, Jan 6, 2021
This paper reports experimental observations showing that a current flow produces an effect of st... more This paper reports experimental observations showing that a current flow produces an effect of strengthening a solder joint against a shear load. This conclusion is found from a single-joint shear test conducted on Sn-1Ag-0.5Cu wt% (SAC 105) joints with in situ current stressing varied from 700 to 1400 A/cm 2 at room temperature. To isolate the current effect from the Joule heat effect, the same type of tests were conducted at room temperature, 40, 50, and 80°C, without current applied to the joint, thus mimicking the condition of steady-state temperature resulting from Joule heating. Comparative testing was also conducted after aging the samples at 150°C for 200 h. These tests produced indications suggesting that the current flow causes the maximum shear load to increase, while the rise in temperature by the Joule heat effect results in the opposite effect. Experimentally, as much as a 9.5% increase in the maximum shear load was observed from the isothermally aged sample tested under a current density of 700 A/cm 2 , while an * 25% reduction was estimated to result from a temperature increase by Joule heating. The potential mechanism for these observations is discussed.
ZnO/graphene oxide core-shell nanoparticles (ZGNPs) were prepared by via in-situ chemical synthes... more ZnO/graphene oxide core-shell nanoparticles (ZGNPs) were prepared by via in-situ chemical synthesis and electrolysis. This report addresses the disagreement in the photocatalytic and photoluminescence behavior in this system. XRD, Raman, HRTEM, PL and UV-Vis were able to confirm ZnO nanoparticles and core-shell structures formation at various temperatures. A little shift was observed in Raman for ZGNPs after conjugating. PL showed a few peaks that were not related to the graphene, which depends on the synthesis route and defects. After conjugating between ZnO and Graphene, two mechanisms are suggested. The first is band structure modification in the boundary of core and shell. The second one is diffusion of functional group in GO as a dopant on ZnO surface.
A novel multifunctional core-shell, yolk-shell SiO 2 @TiO 2 : Eu 3+ /Sm 3+ and hollow TiO 2 : Eu ... more A novel multifunctional core-shell, yolk-shell SiO 2 @TiO 2 : Eu 3+ /Sm 3+ and hollow TiO 2 : Eu 3+ /Sm 3+ structures were successfully synthesized. The relationship between morphology/ions doping and multifunctional properties including photoluminescence and photocatalysis has been discussed in detail for the first time by systematic characterization techniques, including SEM, TEM, XRD, PL, UV-vis, FT-IR, BET and XPS. Upon ultraviolet (UV) excitation, the products show the characteristic red and orange-red emission lines of Eu 3+ and Sm 3+ , respectively. In addition, Judd-Ofelt intensity parameters (Ω 2 , Ω 4) were used to investigate the symmetry and coordination state of Eu 3+ ions in the products with different morphologies. The relative luminescence intensities were in the order: yolk-shell > core-shell > hollow structure. This phenomenon can be ascribed to the unique yolk-shell configuration, which possesses appropriate interior cavity used for multiple reflections and scattering, allowing more efficient utilization of the light. Moreover, compared with yolk-shell and hollow structure, the core-shell spheres Page 1 of 44 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering exhibit excellent photoactivity for the degradation of MO, because the existence of Ti-O-Si bonds increases the surface acidity of the sample, thereby activating the oxidation reaction. In addition, the electron activation in the TiO 2 matrix can be facilitated through intermediate oxygen atoms. And products doped with different RE 3+ ions exhibited distinguishable photocatalytic performance. The surfactant-free method provides a promising route towards the development of multifunctional materials for many applications in photocatalysis and photoluminescence.
Understanding multifaceted microstructural evolution mechanisms is a key enabling foundation that... more Understanding multifaceted microstructural evolution mechanisms is a key enabling foundation that will enable computational modeling and prediction of electronic system lifetimes before anything is built.
Expanding use of electronic systems in close proximity to biological systems, marine environments... more Expanding use of electronic systems in close proximity to biological systems, marine environments, and polluted air introduce elements that attack solder joints. Mechanisms that cause damage and their effects on the lifetime of electronic system are discussed.
ABSTRACT A newly devised misfit management method was used to deftly balance the lattice strain i... more ABSTRACT A newly devised misfit management method was used to deftly balance the lattice strain in an InAs quantum dot (QD) based In0.15Ga0.85As p-i-n device. The photovoltaic behavior under AM0 conditions exhibited higher Voc and lower Jsc in the QD-based device as compared to a control p-n device without quantum dots. To our knowledge, both of these observations are new and seemingly conflict with some of the current understanding of quantum-dot-based PV device physics. The PV behavior under infrared illumination confirms that the quantum-confined carriers of the InAs islands in the intrinsic region contribute to the photocurrent.
On-off operation of electronic systems leads to thermal cycling, which introduces thermal and str... more On-off operation of electronic systems leads to thermal cycling, which introduces thermal and strain energy into solder joints, resulting in microstructural evolution mechanisms of recovery, recrystallization and damage nucleation. The thermal strain history is very sensitive to the package design. The effects of microalloying on microstructure evolution bring both a challenge and an opportunity to improve the reliability of solder joints.
The thermodynamic driving forces that govern non-equilibrium solidification and subsequent micros... more The thermodynamic driving forces that govern non-equilibrium solidification and subsequent microstructural evolution of solder joint microstructures are discussed in detail, including formation of intermetallic phases within, and at the interface that creates the bond between the solder, package, and board.
ABSTRACT The lattice misfit at the island-cap interface in two In0.15Ga0.85As p-i-n devices, with... more ABSTRACT The lattice misfit at the island-cap interface in two In0.15Ga0.85As p-i-n devices, with 5 layers of InAs quantum dots (QDs), was modified by depositing 2.1 and 3.2 ML of InAs while maintaining near identical capping layers. The device with 35 ± 3 nm island size distribution exhibited photoluminescence activity in the near infra-red range from 975 to 1150 nm while the device with 42 ± 12 nm size islands recorded lower PL intensity over a narrower range of 1000-1100 nm suggesting (a) increased island-cap interface misfit, (b) truncation of the islands, and (c) generation of structural defects.
In addition to thermal strains, other mechanical strains arising from handling (such as bending, ... more In addition to thermal strains, other mechanical strains arising from handling (such as bending, drop, or shock), and transport (vibration) add energy to joints that affects the rate and nature of microstructural evolution and damage generation.
2019 IEEE 69th Electronic Components and Technology Conference (ECTC), 2019
This paper describes the kinetic and microstructural mechanism of electromigration (EM) failure f... more This paper describes the kinetic and microstructural mechanism of electromigration (EM) failure found in low-profile solder joints where EM and intermetallic phase formation compete for the same volume of Sn. The low-profile solder joint used in our study was made of 20-25um thick solder situated in between a Cu pillar and a Ni coated Cu lead frame (LF). The samples were EM tested in a temperature range of 140-170oC with the current densities varying between 35-45 KA/cm2 in an oil bath to induce failure without Joule Heat induced artifacts. Our studies on EM failure kinetics and microstructural mechanism have produced two key findings. The first finding suggests that the EM diffusivity (Z*D) of diffusing species (Sn, Ni, Cu) in the solder matrix can be uniquely ranked from microstructural analysis, and it is estimated to be (Z*D) Cu> (Z*D) Sn>(Z*D) Ni. This difference in EM diffusivity causes Cu-Sn and Ni-Sn intermetallic compounds (IMC) to develop in distinctively different manners under EM, leading to different EM failure mechanisms. The second finding is that EM in low-profile solder joints consists of multiple failure stages: a) with EM-related voiding in Sn dominating at lower temperatures; while b) thermally-induced IMC growth and invasion competes with EM-induced Sn voiding at high temperatures leading to the complete failure of each joint.
2020 International Wafer Level Packaging Conference (IWLPC), 2020
Electromigration (EM) induced failure is inevitable in wafer-level chip scale package microelectr... more Electromigration (EM) induced failure is inevitable in wafer-level chip scale package microelectronic packages (WCSP), especially with the implementation of lead-free solders. Many factors contribute to EM failure such as joule heating and current crowding. EM can induce void formation, which can eventually lead to open-circuit failure. Due to its nature, EM is a critical failure concern to the microelectronic industry and can be influenced by current conditions. This study examines the failure mechanisms in solder joints implemented in WCSP packages in Direct Current (DC) and DC-pulse current conditions with varying Duty Factors (DF). DF represents the on-off time for DC to flow through the device under test (DUT). Further, a transient simulative study using finite element method (FEM) explores the failure mechanism and investigates the stress development with DC and DF conditions. Findings suggested that a lower duty factor yielded longer time to failure (TTF). Meanwhile, higher pulsed DC DF yielded a lower TTF than DC. This study aims to explain the failure mechanism with each DF. This study aims to explain this phenomenon and suggests the need for further exploration.
ABSTRACT This paper reports the findings supporting that solder joint grain structure within an a... more ABSTRACT This paper reports the findings supporting that solder joint grain structure within an assembly is not fixed but varies with the joint location and cooling rate. Also reported is the fact that such change in grain structure can make the joint fatigue resistance to be increased. These conclusions are made by comparing the grain structure and fatigue kinetics of the joints in BGA assembly with variation in cooling rate after thermal aging. Our study finds that the grain structure changes with cooling rate because of the kinetic interplay between plastic deformation by thermal strain and its relaxation process other than dislocation glide. The variation in the spontaneous plastic deformation and its storage in the solder promote recrystallization in one extreme, while the process of dynamic relaxation releases the stored energy and make the joint maintain its original grain structure in another extreme. Mechanical fatigue testing of these samples reveals that failure prone joint, corner joint, becomes immune to fatigue failure when its grain structure is changed to polygranular structure. The degree of reliability gain is not possible to quantify at the present moment because the failed joint is shifted to non-recrystallized joint and the failure kinetics is also affected by the amount of residual stress which is also affected by the cooling rate. However, there are many of indications that the joint with polygranular structure is much less susceptible to fatigue failure. Some of highlighting evidences are presented in this paper.
The geometry of interconnections used in current electronic systems is described, and issues rela... more The geometry of interconnections used in current electronic systems is described, and issues related to the reflow process that melts and re-solidifies the solder to make the interconnection are introduced.
Journal of Materials Science: Materials in Electronics, Jan 6, 2021
This paper reports experimental observations showing that a current flow produces an effect of st... more This paper reports experimental observations showing that a current flow produces an effect of strengthening a solder joint against a shear load. This conclusion is found from a single-joint shear test conducted on Sn-1Ag-0.5Cu wt% (SAC 105) joints with in situ current stressing varied from 700 to 1400 A/cm 2 at room temperature. To isolate the current effect from the Joule heat effect, the same type of tests were conducted at room temperature, 40, 50, and 80°C, without current applied to the joint, thus mimicking the condition of steady-state temperature resulting from Joule heating. Comparative testing was also conducted after aging the samples at 150°C for 200 h. These tests produced indications suggesting that the current flow causes the maximum shear load to increase, while the rise in temperature by the Joule heat effect results in the opposite effect. Experimentally, as much as a 9.5% increase in the maximum shear load was observed from the isothermally aged sample tested under a current density of 700 A/cm 2 , while an * 25% reduction was estimated to result from a temperature increase by Joule heating. The potential mechanism for these observations is discussed.
ZnO/graphene oxide core-shell nanoparticles (ZGNPs) were prepared by via in-situ chemical synthes... more ZnO/graphene oxide core-shell nanoparticles (ZGNPs) were prepared by via in-situ chemical synthesis and electrolysis. This report addresses the disagreement in the photocatalytic and photoluminescence behavior in this system. XRD, Raman, HRTEM, PL and UV-Vis were able to confirm ZnO nanoparticles and core-shell structures formation at various temperatures. A little shift was observed in Raman for ZGNPs after conjugating. PL showed a few peaks that were not related to the graphene, which depends on the synthesis route and defects. After conjugating between ZnO and Graphene, two mechanisms are suggested. The first is band structure modification in the boundary of core and shell. The second one is diffusion of functional group in GO as a dopant on ZnO surface.
A novel multifunctional core-shell, yolk-shell SiO 2 @TiO 2 : Eu 3+ /Sm 3+ and hollow TiO 2 : Eu ... more A novel multifunctional core-shell, yolk-shell SiO 2 @TiO 2 : Eu 3+ /Sm 3+ and hollow TiO 2 : Eu 3+ /Sm 3+ structures were successfully synthesized. The relationship between morphology/ions doping and multifunctional properties including photoluminescence and photocatalysis has been discussed in detail for the first time by systematic characterization techniques, including SEM, TEM, XRD, PL, UV-vis, FT-IR, BET and XPS. Upon ultraviolet (UV) excitation, the products show the characteristic red and orange-red emission lines of Eu 3+ and Sm 3+ , respectively. In addition, Judd-Ofelt intensity parameters (Ω 2 , Ω 4) were used to investigate the symmetry and coordination state of Eu 3+ ions in the products with different morphologies. The relative luminescence intensities were in the order: yolk-shell > core-shell > hollow structure. This phenomenon can be ascribed to the unique yolk-shell configuration, which possesses appropriate interior cavity used for multiple reflections and scattering, allowing more efficient utilization of the light. Moreover, compared with yolk-shell and hollow structure, the core-shell spheres Page 1 of 44 ACS Paragon Plus Environment ACS Sustainable Chemistry & Engineering exhibit excellent photoactivity for the degradation of MO, because the existence of Ti-O-Si bonds increases the surface acidity of the sample, thereby activating the oxidation reaction. In addition, the electron activation in the TiO 2 matrix can be facilitated through intermediate oxygen atoms. And products doped with different RE 3+ ions exhibited distinguishable photocatalytic performance. The surfactant-free method provides a promising route towards the development of multifunctional materials for many applications in photocatalysis and photoluminescence.
Understanding multifaceted microstructural evolution mechanisms is a key enabling foundation that... more Understanding multifaceted microstructural evolution mechanisms is a key enabling foundation that will enable computational modeling and prediction of electronic system lifetimes before anything is built.
Expanding use of electronic systems in close proximity to biological systems, marine environments... more Expanding use of electronic systems in close proximity to biological systems, marine environments, and polluted air introduce elements that attack solder joints. Mechanisms that cause damage and their effects on the lifetime of electronic system are discussed.
ABSTRACT A newly devised misfit management method was used to deftly balance the lattice strain i... more ABSTRACT A newly devised misfit management method was used to deftly balance the lattice strain in an InAs quantum dot (QD) based In0.15Ga0.85As p-i-n device. The photovoltaic behavior under AM0 conditions exhibited higher Voc and lower Jsc in the QD-based device as compared to a control p-n device without quantum dots. To our knowledge, both of these observations are new and seemingly conflict with some of the current understanding of quantum-dot-based PV device physics. The PV behavior under infrared illumination confirms that the quantum-confined carriers of the InAs islands in the intrinsic region contribute to the photocurrent.
On-off operation of electronic systems leads to thermal cycling, which introduces thermal and str... more On-off operation of electronic systems leads to thermal cycling, which introduces thermal and strain energy into solder joints, resulting in microstructural evolution mechanisms of recovery, recrystallization and damage nucleation. The thermal strain history is very sensitive to the package design. The effects of microalloying on microstructure evolution bring both a challenge and an opportunity to improve the reliability of solder joints.
The thermodynamic driving forces that govern non-equilibrium solidification and subsequent micros... more The thermodynamic driving forces that govern non-equilibrium solidification and subsequent microstructural evolution of solder joint microstructures are discussed in detail, including formation of intermetallic phases within, and at the interface that creates the bond between the solder, package, and board.
ABSTRACT The lattice misfit at the island-cap interface in two In0.15Ga0.85As p-i-n devices, with... more ABSTRACT The lattice misfit at the island-cap interface in two In0.15Ga0.85As p-i-n devices, with 5 layers of InAs quantum dots (QDs), was modified by depositing 2.1 and 3.2 ML of InAs while maintaining near identical capping layers. The device with 35 ± 3 nm island size distribution exhibited photoluminescence activity in the near infra-red range from 975 to 1150 nm while the device with 42 ± 12 nm size islands recorded lower PL intensity over a narrower range of 1000-1100 nm suggesting (a) increased island-cap interface misfit, (b) truncation of the islands, and (c) generation of structural defects.
In addition to thermal strains, other mechanical strains arising from handling (such as bending, ... more In addition to thermal strains, other mechanical strains arising from handling (such as bending, drop, or shock), and transport (vibration) add energy to joints that affects the rate and nature of microstructural evolution and damage generation.
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Papers by Choong-Un Kim