Thermo-mechanical Stress Research Articles

Assessment of the Risk of Crack Formation at a Hybrid Bonding Interface Using Numerical Analysis

Hybrid bonding technology has recently emerged as a promising solution for advanced semiconductor packaging technologies. However, several reliability issues still pose challenges for commercialization. In this study, we investigated the possibility of crack formation caused by chemical mechanical polishing (CMP) defects and the misalignment of the hybrid bonding structure. Crack formation and thermomechanical stress were analyzed for two common hybrid bonding structures with misalignment using a numerical simulation. The effects of annealing temperature and dishing value on changes in the non-bonding area and peeling stress were systematically analyzed. The calculated peeling stresses were compared to the bonding strength of each bonding interface to find vulnerable regions prone to cracking. The non-bonding area in the bonding structure increased with a decreasing annealing temperature and an increasing dishing value. To achieve a sufficient bonding area of more than 90%, the annealing temperature should be greater than 200 °C. During the heating period of the annealing process, the SiCN-to-SiCN bonding interface was the most vulnerable cracking site with the highest peeling stress. An annealing temperature of 350 °C carries a significant risk of cracking. On the other hand, an annealing temperature lower than 250 °C will minimize the chance of cracking. The SiCN-to-SiO2 bonding interface, which has the lowest adhesion energy and a large coefficient of thermal expansion (CTE) mismatch, was expected to be another possible cracking site. During cooling, the SiCN-to-Cu bonding interface was the most vulnerable site with the highest stress. However, the simulated peeling stresses were lower than the adhesion strength of the bonded interface, indicating that the chance of cracking during the cooling process was very low. This study provides insights into minimizing the non-bonding area and preventing crack formation, thereby enhancing the reliability of hybrid bonding structures.

Flexibilizing Mechanisms of Spinel-, Hercynite-, Galaxite-, and Chromite-Containing Basic Refractories for Cement Rotary Kilns

Background: Refractory linings of cement rotary kilns are subjected to severe thermomechanical stresses of cranking, ovality, tyre hooping/migration, and uneven thermal distribution. An insistent demand is to discover the significance of spinel, hercynite, galaxite, and chromite in magnesia refractories, as well as their flexibilizing mechanisms. Objectives: The objectives are to compare the fracture behavior of magnesia–spinel, – hercynite, -galaxite, and -chromite refractories and to unveil the flexibilizing mechanisms of different spinels. Methods: The wedge-splitting test is carried out to produce various fracture parameters. Their flexibilizing mechanisms are unveiled by performing microstructural observations and analyses. Results: Various fracture parameters are obtained, including specific fracture energy, brittleness number, characteristic crack length, and thermal-shock resistance parameter. Generally, magnesia–hercynite bricks and magnesia–galaxite bricks have demonstrated the obvious advantages of fracture resistance, which are more flexible than magnesia–spinel bricks and magnesia–chromite bricks. Conclusion: The flexibility of magnesia–spinel bricks is attributed to microcracks generated from the thermal mismatch between spinel grains and surrounding periclase, which is recognized as the thermal-expansion mismatch mechanism. In magnesia–hercynite and magnesia-galaxite refractories, the active-ion-diffusion mechanism is predominant beyond similar microcracks, to drive the flexibility by the continuous diffusion of Fe2+, Mn2+, and Mg2+ during high-temperature processes. In magnesia–chromite bricks, the pore rims contribute to the flexibility as a silicate-migration mechanism, after silicate envelopes first arise around chromite grains and then vanish into the surrounding magnesia by the suction of capillary force during the burning process.

Compliant Interconnects Based on Single Micrometer-sized Metal-Coated Polymer Spheres.

The rapid evolution of multifunctional electronics necessitates interconnection technologies appropriate for large dies with high-density and/or ultrafine pitch input/output pins. Existing technologies face numerous challenges, including demands for bonding equipment that can deliver extremely high force as well as thermo-mechanical stresses induced in the assembled packages due to mismatched thermal expansion of materials involved. This study proposes an approach to compliant interconnects comprising single micrometer-sized metal-coated polymer spheres, being joined to mating electrodes by sintering of Ag nano ink at low temperature (140 °C) and low pressure (∼15 mN/particle). Such an interconnection technology is expected to enhance the thermo-mechanical robustness of the assembled packages as well as be capable of high-density, ultrafine pitch interconnects. Our approach demonstrates control over conductive particles during assembly, achieving a 98% success rate in individual interconnects with a single captured particle. The use of sintered Ag not only secures free-standing particles on electrical pads (with an adhesion force above 2 μN) but also results in a 15% reduction in interconnect resistance, with measured resistance as low as 0.5 Ω, compared to interconnects without Ag ink. This method presents an alternative to metallurgical joints, particularly suited for high-density, ultrafine pitch applications, offering low bonding pressure and temperature, along with improved interconnect compliance to enhance the thermo-mechanical robustness of the packages.

Post‐Mortem Analysis of Building‐Integrated Flexible Thin Film Modules

ABSTRACTFlexible, lightweight thin film (TF) photovoltaic (PV) modules offer a unique opportunity for integration into non‐planar surfaces unable to support heavy weights. While such applications increase the potential for PV in urban areas, the reliability implications are yet to be investigated. Here, prototypes of corrugated rooftiles with integrated Cu (In,Ga)Se2 (CIGS) modules were investigated after 3 years of outdoor operation. Their performance before and after the outdoor exposure was compared and defects were localized. An unpackaging method was developed, allowing access to the solar cells for more detailed characterization of present defects without causing additional damage or changes to existing defects. To our knowledge, this was the first time such an unpackaging method was successfully applied to flexible TF PV modules. The relative efficiency loss ranged from 17% to 43%, mostly due to short‐circuit current (ISC) loss and series resistance (RS) increase. The predominant cause of the RS increase was the delamination at the interconnects, ascribed to thermomechanical stresses caused by outdoor temperature fluctuations. The ISC loss was mainly caused by localized delamination of CIGS from the molybdenum (Mo) back‐contact. The occurrence of such delaminated areas pointed to presence of high local stresses during outdoor operation, possibly due to thermal fluctuations, applied deformation and/or mechanical impact. Two other types of delamination defects were found with no observable impact on performance. These results show the necessity for further optimization in the material choice and processing of TF flexible modules, to avoid mechanical stress related failures upon integration into curved surfaces.

Bluestein–Gulyaev(B-G) waves in a piezo-thermoelastic half-space in the context of modified multi-phase-lag theory

In the years 1968–69, Bluestein and Gulyaev discovered a new kind of surface wave in a piezoelectric material known as B-G wave after their name. Although it was discovered in few decades ago, but till now, no work has been done related to B-G wave propagation based on modified multi-phase-lag theory. In the present work, the authors have investigated the propagation behavior of Bluestein-Gulyaev-type waves in a piezo-thermoelastic half-space in the context of modified multi-phase-lag theory. The normal mode technique is employed to obtain the principal variables, for example, displacements, temperature, electric potential, electric displacements, and thermo-mechanical stresses. The boundary conditions are adopted in such a way that the upper surface of the piezo-thermoelastic half-space is stress free and the half-space is implicated by a perfectly conducting electrode that is grounded (i.e., electrically short conditions at the free surface). Based on these boundary conditions, secular equations are derived for the proposed model. Some special cases have also been discussed, and it is found that the secular equation is in well-agreement with the pre-established thermoelasticity theory. Moreover, some analyses are made to highlight the important peculiarities of the problem. The mathematical framework of the present study is momentous for both theoretical and practical expositions for temperature sensors and piezoelectric surface acoustic wave (SAW) devices. Also present study helps the researchers who are engaged on this domain.

Sequential thermomechanical stress and cracking analysis of photovoltaic modules with full and half-cut cells

The transition from conventional full-cell patterns to half-cell modules in the photovoltaic (PV) industry promises enhanced stability and efficiency. This study investigates the thermomechanical behaviour and stress distribution within half-cell and full-cell PV modules during the manufacturing and operational phases. Using finite element modeling, the research simulates sequentially the manufacturing and operating processes such as laser cutting, soldering, lamination, mechanical loading and thermal cycling. The Extended Finite Element Method (XFEM) predicts crack initiation and propagation in the crack-sensitive regions in PV modules during their entire life. Key findings highlight stress concentrations during manufacturing, influenced by parameters like laser power, scribing length and Silicon wafer size. Mechanical loading (ML) and thermal cycling (TC) induce additional stresses, impacting module reliability. The results are useful for the optimization of material properties and the development of advanced design strategies contributing to the durability and efficiency of PV systems.

Power Cycling Performance of 3.3 kV SiC-MOSFETs and the Impact of the Thermo-Mechanical Stress on Humidity Induced Degradation

Recently, silicon carbide (SiC) power modules of the 3.3 kV voltage class became available and are a promising candidate to replace silicon power modules in traction applications. However, the more than three times higher Young’s Modulus compared to silicon leads to a reduced lifetime under thermo-mechanical stress. This could pose a significant obstacle in their implementation, since traction applications are particularly demanding in their mission profiles with respect to load cycling, but also to environmental conditions. Thus, the thermo-mechanical stress is not just limiting the lifetime itself, but might also promote the humidity induced degradation due to delamination or micro-cracks. In this work, multiple power cycling tests at different temperature swings on 3.3 kV SiC MOSFET chips in a power module were performed, to assess their ruggedness under thermo-mechanical stress. Before or afterwards, these modules were tested under standard HV-H3TRB conditions to verify the interaction between thermo-mechanical and humidity stress on the robustness of the modules.

Influence of Material Properties on Ruggedness Evaluation of Package Architectures for SiC Power Devices

Button shear tests at different temperatures between different mold compounds and Cu-leadframes have been performed to evaluate the adhesion of mold compounds to the inner surfaces of SiC-power devices. The results at different temperatures show the behavior of different material and layer stack combinations under storage at room temperatures as well as under operating conditions with elevated temperatures, where adhesion and thus the hermeticity of the device against moisture is significantly lowered. It has been shown that different thermomechanical properties of the mold compounds as well as the usage of different adhesion promoter materials have a significant effect on the adhesion properties of the mold compound to the leadframe surface of the SiC power devices, which have direct impact on the ruggedness and lifetime stability. Furthermore, these results have been correlated to thermomechanical simulations of the package architecture of SiC power devices under stress. Different wire bond architectures have been evaluated in simulations with and without die top delamination taken into account, showing that EMC delamination may play a major role in the lifetime stability of SiC power devices under thermomechanical stress like simulated in TCT- , IOL- and PTC-testing.

Analysis of micro-defect-induced cracking in the IPyC layer of TRISO particle with the XFEM

PurposeWe use the extended finite element method (XFEM) to model the whole process of initiation and propagation of cracks in the inner dense pyrolytic carbon (IPyC) layer of tri-structural isotropic (TRISO) particle induced by the microdefect in an irradiation-induced thermomechanical coupling environment and study the effect of microdefect sizes on the propagation path.Design/methodology/approachThe irradiation-induced thermal–mechanical coupling analysis is first conducted for the representative volume element (RVE) of the TRISO particle by using the conventional finite element method (CFEM) so that the stress distribution is obtained. The stress results are then restored for the enriched elements, and the simulation of crack initiation and propagation is eventually carried out by using the XFEM.Findings1. As a crack initiates in the IPyC layer, it will terminate at the free edge of the RVE TRISO particle in the end. 2. The size of the microdefect has a significant impact on the propagation path.Originality/valueThe ceramic dispersion microencapsulated (CDM) fuel is a good accident-resistant fuel whose safe operation is crucial to the safety and reliability of the whole nuclear reactor. It is of great scientific significance and practical value to study the irradiation-induced thermomechanical coupling stress distribution and cracking behavior in the IPyC layer of TRISO particles for the CDM fuel. Crack initiation and propagation analysis is challengeable for this complex multi-layer structure. This can help understand the failure mechanism of TRISO particles and evaluate the operation safety of the reactor.

Thermal cycling characterization of an integrated low-inductance GaN eHEMT power module

To exploit the potential of wide-bandgap semiconductors in high-frequency applications, innovative packaging designs are developed to minimize the parasitic inductance of power modules. This study presents an integrated power module with a hybrid PCB/DBC structure, which uses top-side cooling prepackaged GaN enhancement-mode high-electron-mobility transistors. The module achieves a remarkably low parasitic inductance of 2.65 nH. However, there is relatively scarce research on the reliability of this heterostructure, particularly its sensitivity to thermomechanical stress due to the coefficients of thermal expansion mismatch among material interfaces. In this work, the thermal cycling characteristics of the integrated power module are comprehensively investigated. Electrical and thermal parameters were periodically and separately measured offline on a simplified package to monitor the health conditions and decouple possible synergy and competition effects among the failure modes from all packaging components. A thorough failure analysis was conducted using nondestructive visual inspections and scanning acoustic microscopy, complemented by destructive cross-sectional examination and scanning electron microscopy. The findings identified the delamination of the DBC upper copper layer, which exhibited a conchoidal fracture interface, as the primary factor that contributed to the failure of the power module with increased thermal resistance. Furthermore, the study dissected its initiation and propagation mechanisms. This investigation provides valuable insights for the development of more reliable low-inductance power module designs.

Simulation of Thermo-Mechanical Stresses After a Quench in the 15 T Test Facility Dipole Magnet

Simulation of Thermo-Mechanical Stresses After a Quench in the 15 T Test Facility Dipole Magnet

Wear mechanisms and material deposition of high-performance polymer composites for hydrogen compression

The transistion to renewable energy production is a main component in achieving the climate targets defined in the 2015 Paris Agreement. Power-to-gas concepts, where electrical energy is stored as hydrogen gas, require the design and operation of state-of-the-art reciprocating compressor technology to realize high pressure ratios and high volumetric flow rates for gas supply demands. The main challenges in designing machines suited for these operating conditions are high thermo-mechanical stresses acting on the piston sealing rings. High shear loads and minimal ring gap sizes demand material pairings with excellent wear resistance and sliding properties. This work investigates the life cycle limiting wear mechanisms present in the piston compressor and develops advanced concepts for piston rings in cylinder tribo-contacts. PI and PPS polymers reinforced with carbon fiber were modified with dry lubricating PTFE to fulfil the requirements of this highly stressed tribo-system. Tribo-tests were performed against steel countersurfaces with a linear oscillating tribometer in pin-on-plate configuration. The failure mechanisms were analysed utilizing scanning electron microscopy on cross-sections cut into the depth of the countersurface material with focused ion beam milling. 2D scanning transmission electron microscopy mapping supported by energy dispersive X-ray spectroscopy were performed to analyze the depth profile of the deposited tribo-layer elements on the wear track.

Microstructural Modifications and Failure Mechanisms of an Aluminum‐Based Abradable Coating System during Isothermal and Cyclic Aging

Abradable systems are used in the aeronautical industry to improve the efficiency of gas turbine. Those materials are exposed in service to temperature up to 450 °C. The increase in gas turbine efficiency requires to increase the operating temperature and therefore the service temperature of abradable coating. The present study focuses on the isothermal and cyclic thermal aging of the Al–Si abradable coating system in a laboratory air at high temperature up to 500 °C. The investigation encompasses the microstructural evolution, phase transformation, and the formation of cracks, along with their interrelated effects. During aging, silicon particles precipitate in the abradable top coat. In addition, coarsening of those particles is observed and the coarsening kinetics appears to be faster in cyclic thermal aging conditions compared to isothermal aging. During cyclic and isothermal aging, brittle aluminides develope at the abradable top–coat/bond–coat interface, due to the interdiffusion of Al and Ni species. During cyclic aging, thermal cycles create thermomechanical stress at the top‐coat/bont‐coat interface due to coefficient of thermal expansion mismatch between the Al‐Si deposit and intermetallic phases. The stress generated results in the formation of cracks and porosities at the top–coat/bond–coat interface resulting in a dramatic failure of the system.

WITHDRAWN: Metrology of warpage in silicon wafers using X-ray diffraction mapping

X-ray Diffraction (XRD) mapping is a non-destructive metrology technique that enables the reconstruction of warpage induced on a Silicon wafer through thermo-mechanical stress. Here, we mapped the wafer’s warpage using a methodology based on a series of line scans in the x and y directions and at different 90-degree rotations of the same sample. These line scans collect rocking curves from the wafer’s surface, recording the diffraction angle (ω) deviated from the Bragg angle due to surface misorientation. The surface warpage reflects in XRD measurements by inducing a difference between the measured diffraction angle and the reference Bragg angle (ω − ω0) and rocking curve broadening (FWHM) By collecting and integrating the rocking curves (RCs) and FWHM broadening from the whole surface and multiple rotations of the wafer, we could generate 3D maps of the surface function f(x) and the angular misorientation (warpage). The warpage exhibits a convex shape, aligning with optical profilometry measurements reported in the literature. The lab-based XRDI has the potential to be developed to map the wafer’s warpage in a shorter time and in situ, as can be perfectly performed in Synchrotron radiation source.

PARTICLE SWARM OPTIMIZATION AND CFD ANALYSIS OF A HEAT SINK FOR FORCED CONVECTIVE COOLING OF A DC/AC INVERTER

Power inverters play a crucial role in converting DC voltage and current to AC voltage and current. Its efficient operation relies on effective thermal management of the semiconductor devices, such as the IRF 3205 MOSFET chip which facilitate the switching operations to prevent thermo-mechanical stress that could to failure. This research developed and optimize a heat sink for an IRF 3205 MOSFET chip in a 3.5 KVA power inverter, using the Particle Swarm Optimization technique and experimented with a 3.5 KVA 24 V power inverter operating a 1.5HP air-conditioning unit. The heat sink geometry has 26 fins, 4 mm fin thickness, 40 mm fin height, 3 mm fin spacing, 2 mm fin base thickness, 179 mm length, 80 mm width and a thermal resistance of 60.75 °C/W. The thermal performance was evaluated using ANSYS Computational Fluid Dynamics (CFD) and results showed maximum base temperature of 65.85°C and experimental temperature of 64°C after two hours of steady operation dissipating heat of 45W. The results from both numerical and experimental data demonstrate the effectiveness of the optimized heat sink in maintaining the chip's temperature below its maximum working threshold of 170°C.

Dynamic failure of magnesia-carbon refractories under uniaxial compressive load

The thermo-mechanical stress, which was a combination of thermal stress from rapid temperature changed and dynamic mechanical stress from scrap impact, accelerated the damage to MgO–C refractories during service. The influence of graphite content (8 wt% and 16 wt%), heat treatment temperature, and oxidation level (partially oxidized or not) on dynamic mechanical stress damage behavior was investigated using the Split-Hopkinson pressure bar test at various loading velocities. The results indicated the SHPB test could form dynamic stress, and the parameters obtained post-test could characterize the dynamic mechanical properties of MgO–C refractories. Furthermore, all of the specimens' dynamic compressive strength clearly demonstrated the strain rate hardening effect; this phenomenon was particularly evident in the low-graphite specimens. Additionally, the thermally treated specimens dissipated impact energy by destroying the whole volume, whereas the oxidized specimens suffered damage layer by layer, and the specimens’ dynamic mechanical characteristics were weakened by oxidization or heat treatment.

Ceramic Meta-Aerogel with Thermal Superinsulation up to 1700°C Constructed by Self-Crosslinked Nanofibrous Network via Reaction Electrospinning.

Thermal insulation under extreme conditions requires the materials to be capable of withstanding complex thermo-mechanical stress, significant gradient temperature transition, and high-frequency thermal shock. The excellent structural and functional properties of ceramic aerogels make them attractive for thermal insulation. However, in extremely high-temperature environments (above 1500°C), they typically exhibit limited insulation capacity and thermo-mechanical stability, which may lead to catastrophic accidents, and this problem is never effectively addressed. Here, a novel ceramic meta-aerogel constructed from a crosslinked nanofiber network using a reaction electrospinning strategy, which ensures excellent thermo-mechanical stability and superinsulation under extreme conditions, is designed. The ceramic meta-aerogel has an ultralow thermal conductivity of 0.027W m-1 k-1, and the cold surface temperature is only 303°C in a 1700°C high-temperature environment. After undergoing a significant gradient temperature transition from liquid nitrogen to 1700°C flame burning, the ceramic meta-aerogel can still withstand thousands of shears, flexures, compressions, and other complex forms of mechanical action without structural collapse. This work provides a new insight for developing ceramic aerogels that can be used for a long period in extremely high-temperature environments.

Modal Analysis of a Fire-Damaged Masonry Vault

This study analyzes the thermo-mechanical behavior of a brick vault and the effect of a fire on its dynamic characteristics. Based on the results of an experimental test of a real barrel vault with a net span of 161 cm and a net rise of 46.5 cm, an accurate numerical model to simulate the behavior of the brick-and-mortar structure under thermo-mechanical stresses has been implemented. The comparison of the evolution of the displacement in the keystone and the temperatures at various points of the vault allows us to affirm that the adopted micro-modeling approach presents a good accuracy and a feasible computational effort. Finally, this study shows, from a numerical point of view, how the variation in the structure’s eigenfrequencies can be predicted for extreme situations, such as fire damage. This aspect can be critical to develop effective intervention and prevention strategies, which can be useful for the preservation of our valuable cultural and historic resources.

Simulation of Mechanical Stresses in BaTiO3 Multilayer Ceramic Capacitors during Desoldering in the Rework of Electronic Assemblies Using a Framework of Computational Fluid Dynamics and Thermomechanical Models.

Multilayer ceramic capacitors (MLCCs) are critical components when thermal processes such as reflow desoldering are used during rework of electronic assemblies. The capacitor's ferroelectric BaTiO3 body is very brittle. Therefore, thermomechanical stresses can cause crack formation and create conductive paths that may short the capacitor. In order to assess the thermally induced mechanical stresses onto an MLCC during reflow desoldering, simulations were carried out, which make use of a framework of computational fluid dynamics and thermomechanical models within the ANSYS software package. In the first step, CFD simulations were conducted to calculate the transient temperature field in the surrounding of the MLCC component, which was then used as an input for FEM simulations to compute the arising mechanical stresses inside the MLCC. The results of the simulations show that the major contribution to mechanical stresses within the MLCC component comes from the mismatch in thermal expansion between the printed circuit board and the MLCC. The temperature gradients along the MLCC component are rather small and account only for moderate internal stresses within the brittle BaTiO3 body.

Low-temperature strain-free encapsulation for perovskite solar cells and modules passing multifaceted accelerated ageing tests

Perovskite solar cells promise to be part of the future portfolio of photovoltaic technologies, but their instability is slow down their commercialization. Major stability assessments have been recently achieved but reliable accelerated ageing tests on beyond small-area cells are still poor. Here, we report an industrial encapsulation process based on the lamination of highly viscoelastic semi-solid/highly viscous liquid adhesive atop the perovskite solar cells and modules. Our encapsulant reduces the thermomechanical stresses at the encapsulant/rear electrode interface. The addition of thermally conductive two-dimensional hexagonal boron nitride into the polymeric matrix improves the barrier and thermal management properties of the encapsulant. Without any edge sealant, encapsulated devices withstood multifaceted accelerated ageing tests, retaining >80% of their initial efficiency. Our encapsulation is applicable to the most established cell configurations (direct/inverted, mesoscopic/planar), even with temperature-sensitive materials, and extended to semi-transparent cells for building-integrated photovoltaics and Internet of Things systems.