Thermally-induced Stresses Research Articles

Optimizing the performance of a Coriolis-based angular rate sensor applying an analytical approach

This paper focuses on the optimization of a novel angular rate sensor element based on the Coriolis force working principle. The device is resonantly excited and consists of two mechanically coupled oscillators representing the drive and the sense unit. In order to minimize energy losses during operation, the device employs a single point suspension to the substrate. This is especially advantageous when choosing an antiphase torsional motion for the sense mode. Furthermore, thermally-induced stress resulting in undesired drift effects of the device is minimized. The excitation frequency of the electrostatic drive unit was chosen to be in the range from 10 to 15 kHz, according to automotive requirements. The optimization process started with a complete parameterization of the sensor geometry, providing the basis for an analytical model. This was set up via the so-called deformation algorithm, applying the Ritz method. Next, the eigenfrequencies and mode shapes of the sensor were calculated analytically and compared with FEM results. The inclusion of the Coriolis force induced response of the sense unit yielded signal values from the differential capacitive pickup. An advanced hill climbing algorithm was used, varying two geometrical parameters simultaneously in such a way that the difference in drive and sense frequencies was kept at a constant value of 200 Hz. Based on this procedure an optimized design was found with an increase in signal level of about 450% as compared to earlier versions (e.g. from 3 to 17 aF/°/s). In a last step, fabrication related perforation holes which are typical for surface micromachined devices were included in the model. For this configuration, a frequency matching step was performed by FEM calculations. Resulting stiffness values were fed into the analytical model yielding a final output signal of the sensor of 16 aF/°/s.

Dynamic analysis of a thermal–induced stress in an elastic circular plate

Stress is a phenomenon that could cause a lot of destruction to engineering structures if there are no adequate in-built absorbers in such structures. Buildings, bridges and such other structures must therefore be protected from excessive stress in other to maintain their shapes and forms and hence to guarantee the life span of these structures because of the negative effects this phenomenon may have on them if left unchecked. In this work we study the magnitude of a thermal-induced stress in a circular elastic plate of radius b with an indented circular hole of radius a at the center. The magnitudes of the normal and tangential components for various span ratio a b are computed. The results of this analysis show a definitive relationship between the stress profile and variability in span ratio. Journal of the Nigerian Association of Mathematical Physics Vol. 8 2004: pp. 57-60

Evolution of Contact Resistance during the Bonding Process of NCA Flip-Chip Interconnections

Non-conductive adhesive (NCA) flip-chip interconnects are emerging as an attractive alternative to lead or lead-free solder interconnects due to their environmental friendliness, lower processing temperatures, and extendability to fine-pitch applications. The electrical connectivity of an NCA interconnect relies solely on the pure mechanical contact between the integrated circuit (IC) bump and the substrate pad; the electrical conductivity of the contact depends on the mechanical contact pressure, which in turns depends to a large extent on the cure shrinkage characteristics of the NCA. Therefore, it is necessary to monitor the evolution of the electrical conductivity which could reflect the impact of cure- and thermal-induced stresses during the curing and cooling process, respectively. In this article, in situ measurement of the development of contact resistance during the bonding process of test chips was developed by using a mechanical tester combined with a four-wire resistance measurement system. A drop of resistance induced by the cure stress during the bonding process is clearly observed. With decreasing bonding temperature, the drop of contact resistance induced by cure shrinkage becomes larger, while the cooling-induced drop of resistance becomes smaller. The evolution of contact resistance agrees well with experimental observations of cure stress build-up. It is found that vitrification transformation during the curing of the adhesive could lead to a large cure stress and result in the reduction of the contact resistance. Furthermore, no obvious changes were observed when the applied load was removed at the end of bonding.

Development of a Dual-Axis Convective Gyroscope With Low Thermal-Induced Stress Sensing Element

This paper describes the design, simulation, and fabrication of a dual-axis gyroscope, whose working principle is based on the thermal convective and thermoresistive effects in lightly doped p-type silicon. The sensor configuration consists of a piezoelectric pump and a microthermal sensing element that is packaged in an aluminum case with a diameter of 14 mm and a length of 25 mm. The novel structure of the sensing element reduces the thermal-induced stress up to 89% as compared with the previous design. The sensor has been fabricated by micro- electromechanical systems technology, and completely packaged and characterized. The measured sensitivities of the gyroscope for the X-axis and Y-axis were 0.082 and 0.078 mV/deg/s, respectively. The cross sensitivities between the two input axes were less than 0.26%, and the nonlinearity was smaller than 0.5% full scale in the range of plusmn200deg/s. The resolution was 0.2deg/s at a measurement frequency of 1 Hz. The noise equivalent rate was 0.18deg/s/radicHz, which is equivalent to an angle random walk of 10.8deg/radich in a 65-Hz bandwidth. The offset drift was 360deg/h in 12-h measurement.

자유냉각된 폴리카보네이트 평판에서의 열에 의한 복굴절

Simulations of the thermally-induced residual stresses and birefringence in freely quenched plates of polycarbonate were performed by using the linear viscoelastic and photoviscoelastic constitutive equations for the mechanical and optical properties, respectively, and the first order rate equation for volume relaxation. The predictions for the birefringence showed good agreement with experimental measurements. However, for initial temperature close to the glass transition temperature, some differences existed around the surface layer. Based on the simulation, the influences of various cooling conditions on the residual stress and birefringence in plates were investigated. The residual stress and birefringence increased with increasing initial temperature, decreasing coolant temperature and increasing heat transfer coefficient of coolants.

Shear strength of plate girder web panel at elevated temperature

This paper presents a theoretical model to predict the failure loads of plate girders with transverse stiffeners subjected to a specified constant temperature or isothermal condition. The work is based on classical plate girder theory at normal ambient conditions subjected to increasing load. The equations incorporating the temperature-dependent material properties and thermally-induced axial stress arising from surrounding structural restraint are derived. They can be used for the case when the plate girder is subjected to pure shear or to a combination of shear and axial stresses. To simplify the analysis, uniform temperature distribution is assumed within the full depth of web panel. Solution procedures are presented for the fire resistance analysis of web plate girder, either in terms of the shear strength corresponding to isothermal condition, or the limiting temperature for a prescribed set of constant shear force and axial thermal restraint. For both procedures, the predictions agree well with the finite element analyses.

Thermophysical properties of the Ni-based alloy Nimonic 80A up to 2400 K, II

Numerical simulation of fluid flow, heat transfer, solidification or thermal induced stresses have gained a tremendous significance in steel working industry branches. With the advent of adequate computing power, full three-dimensional calculation of the determining physical equations have become possible. A major drawback of these simulation techniques is the lack of accurate thermophysical properties. By the means of a fast pulse heating technique thermophysical data for the solid and liquid material required for the simulations can be measured. Important input parameters for the heat transfer equation are heat capacity, heat of fusion, density and thermal conductivity. Since direct measurements of thermal conductivity of alloys in the liquid state are almost impossible, its estimation from the electrical conductivity using the Wiedemann–Franz law is very useful.

Processing and properties of ferroelectric relaxor lead scandium tantalate Pb(Sc1/2Ta1/2)O3 thin films

Thin films of Pb(Sc1/2Ta1/2)O3 (PST) were fabricated using an optimized chemical solution deposition procedure involving the de-hydration of scandium acetate and the addition of up to 30% excess lead in solution. The choice of substrate was found to impact the thermal induced stress in the films as confirmed by wafer bending and in-plane grazing angle x-ray diffraction measurements. The presence of either a compressive or a tensile in-plane stress led to a reduction in the temperature of the dielectric maximum, whereas the value of the dielectric maximum remained relatively unchanged; its value reduced by an order of magnitude compared with ceramic samples. It is concluded that mechanical stress alone is not the sole factor in the reduction of PST thin film permittivity. Microstructural features resulting from processing induced defects or an incomplete transformation to the relaxor state may be responsible for this commonly observed phenomenon.

Optimization of PZT Diaphragm Pump for the Convective Gyroscope

In this paper, we present the optimization of the PZT diaphragm pump for application in gas gyroscope. A circular flow inside the sealed case is simulated in detail by utilizing 3D compressible flow with the interaction of fluid-solid phase and the transient analysis is employed. The working principle and the effect of the jet-pump integrated inside the sensor are explained and validated by experiments using anemometry technique. The results verified that configuration of the pump is optimized and the peak velocity of the flow at the sensing element is 3.5m/sec after starting the pump 3.6ms. A novel structure of the sensing element of the gas gyroscope, consists of thermistor and heater, is also reported. The thermistor is heated by a separate heater, whose power is supplied independently form that of thermistor. This design allows low voltage on the thermistor, therefore the noise is reduced. Both heater and thermistor are optimized in order to reduce the thermal induced stress which occurred in the old thermistors at working temperatures. The thermal stress appeared in p-type silicon thermistors reduced the performance of sensor by 7.5%, which is calculated and experimentally confirmed.

Mullite-rich plasma electrolytic oxide coatings for thermal barrier applications

A study has been undertaken of the characteristics exhibited by mullite-rich plasma electrolytic oxide coatings grown on aluminium alloys by using silicate-rich electrolytes. It is found that they can be grown at a higher rate, and to a greater thickness, than alumina PEO coatings on aluminium. The thermal conductivity of these coatings has been measured using a steady-state method. It is shown to be of the order of 0.5 W m − 1 K − 1 , which may be compared with ∼ 1.5 W m − 1 K − 1 for pure alumina PEO coatings and ∼ 10–15 W m − 1 K − 1 for dense polycrystalline mullite. Coupled with excellent substrate adhesion and good mechanical properties, this relatively low conductivity makes these coatings attractive for thermal barrier applications. Furthermore, they are shown to exhibit a relatively low global stiffness (∼ 40 GPa), which will reduce the magnitude of thermally-induced stresses and improve the resistance to spallation during temperature changes.

Thermal characterization and modeling of power hybrid converters for distributed power systems

We have developed and experimentally validated a 3D model for thermal analysis and reliability-conscious design of hybrid power converters, using the commercial finite-element solver COMSOL™. The model was tuned using an accurate packaged MOSFET model and a specially-designed test board for multi-point temperature measurements. Measured and modeled temperatures showed good agreement for various dissipated power levels. A simplified version of the packaged MOSFET was then used in a bridge configuration to build a thermal model of the converter. After thermal studies, we performed preliminary finite-element analysis of the thermally-induced stress distributions.

Enhancement in Ultrathin Oxide Growth by Thermal-Induced Tensile Stress

Rapid-thermal oxide grown under the condition that a certain portion of the substrate wafer was covered by another wafer with special shape was studied. It is interesting to find that in the ultrathin oxide regime, the thickness of oxide with covered wafer is even larger than that without. Thermal-induced tensile stress is believed to be the origin of the above enhanced oxidation rate. A novel ultrathin oxide grown at a low temperature of 800/spl deg/C is demonstrated. The capacitance-voltage and current-voltage characteristics of MOS capacitors with oxides grown with and without cover wafers under the same oxide thickness were compared.

Analysis of demolding in micro metal injection molding

In this paper micro metal injection molding (μMIM) is being studied to produce 316L stainless steel microstructures. Experimental work showed that demolding failure was a serious hindrance to the success of μMIM as a suitable technology to produce small microstructures. Thus, a theoretical analysis of microstructure demolding in μMIM is needed in order to understand the demolding behavior theoretically and how to avoid demolding failure in μMIM. In this paper, theoretical analysis of the demolding of 24×24 (totally 576) microstructures was conducted with the help of Abaqus finite element analysis software. The analysis was involved in two factors that possibly lead to demolding failure: one is the shear stress during ejection of the microstructure due to contact pressure between the microstructure and the microcavity, the other is thermally-induced stress due to cooling of the microstructure. The analysis shows that the following factors have significant influence on the demolding: aspect ratios of the microstructure, the coefficient of friction between the microstructure and the microcavity, demolding temperature and holding pressure. And the microstructure that is farther away from the centerline of the round part with microstructures undergoes higher stress and more easily subject to breakage. The moment when the microstructure is the most possibly subject to breakage during demolding is always the onset of ejection.

Reliability Prediction for 95.5Sn3.9Ag0.6Cu Solder Bump and Thermal Design for Lead Free System in Package with Polymer-Based Material

This paper demonstrates the thermal-induced mechanical problems resulted from various temperature profiles of reliability test for a system-in-package (SIP) assembly process. The package includes two flip chip mounted chips (underfilled), two memory CSPs, some passive SMDs and 4-layer BT substrate. The flip-chip specimen was taken and the Moiré Interferometry was used as methodology to verify the developed Finite Element Model and material property. It also shows that the developed finite element model is capable to simulate the JEDEC standard JESD22-A104 reliability thermal cycle test and then to predict solder fatigue life and to summarize design rules for thermal optimization of package based on the creep model and viscoplastic model of solder while the SIP package design is proceeded. Thermal design for SIP depends on the placement of FC chip (high power) and memory CSP components. Passive SMDs are also included to study the effect of thermal-induced stress. A series of comprehensive parametric studies were conducted in this paper.

A Dual Axis Accelerometer Utilizing Low Doped Silicon Thermistor

This paper presents the development of a dual axis convective microaccelerometer, whose working principle is based on the convective heat transfer and thermo-resistive effect of lightly-doped silicon. Different with developed convective accelerometer, the sensor utilizes a novel structure of the sensing element which can reduce 93% of thermal-induced stress. Moreover, the thermistors are made from low-doped p-type silicon, which has the TCR higher than that of metals and poly-silicon convective accelerometer. By using numerical method, the chip dimensions and the package size are optimized. The sensitivity of the sensor was simulated; other characteristics such as frequency response, shock resistance, noise problem are also deeply investigated. The sensor has been fabricated by MEMS process and characterized by experiments.

앵커로 보수한 콘크리트 계면 거동의 고찰

Recently, the damaged concrete structures are often strengthened or repaired using the polymer concrete or the polymer cement mortar. In the repaired concrete structures at early ages, internal stresses could be developed due to the differential drying shrinkage of the repair material. Due to the difference of the thermal coefficients of the repair material and existing concrete, additional stresses also could be developed as the structures are subjected to the ambient temperature changes. Theses environmentally-induced stresses can sometimes be large enough to cause damage to the structures, such as debonding of the interface between the two materials. In this study, a rational procedure was developed where anchors can be designed and installed to prevent damages in such structures by thermally-induced stresses. Finally, through the experimental study and numerical study, the effects of the repair method using anchors with debonding was investigated and discussed the results.

Axial tensile stress–strain characterization of ITER model coil typeNb3Sn strands in TARSIS

For a few years there has been an increasing effort to study the impact of (bending) strainon the transport properties of superconducting wires. As the stress distribution,originated by differences in the thermal expansion and electromagnetic load, isthe driving factor for the final strains, the axial and transverse stiffness of thestrand play a crucial role in the final performance. Since the strain state of theNb3Sn filaments in strands determines the transport properties, basic experimentalstress–strain data are required at the strand level for accurate modellingand analysis and eventually for optimizing cable and magnet design. Weperformed axial tensile stress–strain measurements on several types ofNb3Sn strands used for the manufacture of the International Experimental Thermonuclear Reactor(ITER) central solenoid and toroidal field model coils and a powder-in-tube processed wire.In total 48 wire samples were tested at boiling helium, boiling nitrogen and at roomtemperature. We present the computation of the stress–strain characteristic with astraightforward 1D model using an independent materials database, obtaining a goodagreement with the experimental results.The details from the take-off origin of the measured stress–strain curves are discussed andthe data are evaluated with respect to some commonly used functions for fittingstress–strain curves. The measurements are performed in the new setup TARSIS (testarrangement for strain influence on strands). A double extensometer connected to thesample enables us to determine the strain level whereas a load cell is used to monitor thestress level.For higher levels of applied stress (100 MPa), we found typically a higher strainfor bronze route wires compared to a powder-in-tube and internal tin typeof strand. Stress–strain results are essential to assess more accurately theimpact of thermal and electromagnetic induced stress on the strain state of theNb3Sn filaments for wires from various manufacturing processes.

Modeling and experimental verification of the 300 mm Ar anneal process

We demonstrate a new model for simulating thermal and gravitational induced stress appearing in state of the art 300 mm argon annealing processes. Purpose of this work is to predict the allowable ramp rates in a vertical batch furnace system. The gravitational induced bending stress is modeled by using methods from structural mechanics like common finite element modeling. Calculation of the important specular reflecting radiation component – transporting extra radiation energy to the wafer center between the spacing of two horizontally oriented wafers – was achieved by using finite volume methods known from fluid dynamics. A reduction of the total resolved shear stress could be determined from our model if specular reflection is taken into account and thus allowing higher ramp rates. We furthermore demonstrate new experimental data for the upper yield stress of 300 mm Si-ingots with different boron and nitrogen co-doping levels. Especially at high temperatures, the upper yield stress of high ohmic material drops down significantly thus making it necessary to use planar support geometries to diminish the gravitational induced bending stress.

Experiments and theory of thermally-induced stress relaxation in amorphous dielectric films for MEMS and IC applications

Plasma-enhanced chemical vapor deposited (PECVD) silane-based silicon oxide (SiO x ) films were chosen as an example to study the thermally-induced stress relaxation phenomena of amorphous dielectric films. Wafer curvature was measured optically both during and after various thermal conditions, including temperature cycling, constant peak temperature annealing, and varying peak temperature annealing experiments. From these measurements, we are able to obtain a complete stress evolutional history in the thin film, and further derive a series of mechanical/material properties such as viscoelasticity, density and viscosity, coefficient of thermal expansion (CTE), etc. We found that the stress in the amorphous silicon oxide films is sensitive to both viscosity and density changes. Stress relaxation was then theoretically investigated by a viscoelastic model, which was associated with a defects-based microstructural causal mechanism. Our theoretical model elucidates defects movement and consequent microstructure rearrangement as a source of damping, accompanied by viscous flow. This theory was applied to explain a series of experimental results, including stress hysteresis generation and reduction, stress relaxation, and CTE changes, etc.

Effect of residual stress on the energy release rate of wedge and DCB test specimens

Changes in the available interfacial strain energy release rate caused by residual stresses (e.g. thermal, hygroscopic and piezoelectric induced stresses) in a triple-layered specimen are analyzed using linear elastic fracture mechanics and simple beam theory. The strain energy release rate, caused by either tensile or compressive in-plane residual stresses in the adhesive, is derived. The contributions of the thermal residual and mechanical stresses to the global energy release rate are also analyzed using both a steady-state fracture analysis and a thermoelastic laminate analysis. Excellent agreement is obtained with a finite element analysis conducted for comparison and validation. The thermally induced energy release rate, G T , is found to be independent of crack length, but is a function of residual stress level and the geometric and material parameters of the specimen. Although the contribution due to residual stresses is expected to be small for typical double cantilever beam specimens, the residual-stress-induced energy release rate can be non-negligible for subcritical debonding in wedge test specimens exposed to aggressive environments.