In-plane Coefficient Of Thermal Expansion Research Articles

Fabrication and Characterization of Chromium-Coated Graphite Fibers Reinforced Copper Matrix Composites

Milled form of mesophase pitch-based graphite fibers were coated with a chromium layer using chemical vapor deposition technique, and Cr-coated graphite fiber/Cu composite with 50 vol.% fibers was fabricated by hot-pressing. The composite was characterized with scanning/transmission electron microscopies and by measuring thermal properties, including thermal conductivity and coefficient of thermal expansion (CTE). The results showed that the milled fibers were preferentially oriented in a plane perpendicular to the pressing direction, leading to anisotropic thermal properties of the composite. The Cr coating reacted with graphite fiber and formed to a continuous and uniform Cr3C2layer. This carbide layer established a good metallurgical interfacial bonding in the composite, which can improve the thermal properties effectively. The in-plane thermal conductivity and CTE of the composite reached to 392 W/mK and 6.5 ppm/K respectively, making the composite suitable for being electronic packaging materials.

High performance polymer/AlN composites for electronic substrate application

An ideal high performance poly(etheretherketone) (PEEK)/AlN composite that has unique combination of anisotropic linear coefficient of thermal expansion (CTE) and dielectric properties was developed for the use in electronic packaging substrate as an alternative of conventionally used epoxy/E-glass substrate. The out-of-plane and in-plane CTEs of the composites were very close to that of copper and silicon chip, respectively. The dielectric constants of the composites are almost independent with increasing frequency. The dissipation factor decreased approximately 50% compared to the pure PEEK. These results reveal that the composite may be the futuristic candidate for high performance electronic packaging substrate.

Preparation and properties of polyimide nanocomposites with negative thermal expansion nanoparticle filler

Nanocomposites with tunable coefficient of thermal expansion (CTE) were prepared by incorporating cubic zirconium tungstate (ZrW2O8) nanoparticles at various volume percentages in a polyimide (PI). Rod-shaped nanoparticles of cubic ZrW2O8, which has isotropic negative thermal expansion, were synthesized using a hydrothermal method. The interfacial interaction between the PI and ZrW2O8 was enhanced by covalently bonding different organic moieties, including a short aliphatic silane and PI oligomer, to the surface of ZrW2O8. Structure–property relationships for the PI–ZrW2O8 nanocomposites were investigated for thermal degradation, glass transition, tensile and thermal expansion properties. Addition of ZrW2O8 nanoparticles did not alter the thermal degradation and glass transition temperature of the base PI. The addition of ZrW2O8 nanoparticles increased the Young's modulus of the polymer, indicating stiffening of the polyimide matrix. The increase was higher for nanocomposites with engineered interfaces due to the efficient load transfer achieved through the presence of linker groups. The addition of ZrW2O8 reduced the in-plane CTE of the base PI at all loadings studied. The CTE of the base PI was reduced by around 22% with the addition of ZrW2O8 at 15 volume% loading.

Thermal expansion behavior of through-silicon-via structures in three-dimensional microelectronic packaging

Thermo-mechanical reliability is an important issue for the development and deployment of the through-silicon-via (TSV) technology in three-dimensional (3D) microelectronic packaging. The mismatch in coefficient of thermal expansion (CTE) between the array of copper (Cu) lines and the surrounding silicon (Si), upon temperature variation, affects the overall thermal expansion behavior of the whole TSV structure itself and generates an internal stress state. In this work we use the finite element method to numerically study the effective in-plane CTE of the Si/Cu composite structure. A 3D unit-cell approach is undertaken, which takes into account uniformly distributed TSVs in the Si chip. Results of the temperature-dependent effective CTE can be used as model input for simulating larger-scale 3D packages where the Si/Cu TSV structure is treated as a homogeneous material. We also examine the evolution of stress and deformation fields, and identify potential reliability concerns associated with the thermal loading.

State dependent pyroelectric and thermal expansion coefficients in a PZT wafer

A PZT wafer is subjected to a temperature increase from the reference temperature 20–111 °C under no electric field. During the temperature increase, the variations of remanent polarization in thickness direction and remanent in-plane strain are measured. From the measurements, the values of pyroelectric coefficient in thickness direction and in-plane thermal expansion coefficient are estimated. The temperature experiment is repeated starting at various different values of initial remanent polarization, and the dependency of pyroelectric and thermal expansion coefficients on initial remanent polarization is obtained. In the tested range of temperature, it is found that the pyroelectric coefficient can be given as a linear function of remanent polarization and that the in-plane thermal expansion coefficient is given as a linear function of remanent in-plane strain or as a quadratic function of remanent polarization.

Spontaneous molecular orientation of polyimides induced by thermal imidization (6). Mechanism of negative in-plane CTE generation in non-stretched polyimide films

The mechanism of negative coefficient of thermal expansion (CTE) generation for non-stretched polyimide (PI) films is proposed in this work. Negative CTE behavior was observed in some miscible binary blend films composed of a major fraction of a rod-like semi-crystalline PI derived from pyromellitic dianhydride (PMDA) with p-phenylenediamine (PDA) and flexible PIs based on 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA) whereas homo PMDA/PDA PI film shows a considerably low but a positive CTE value. The results suggest that the negative CTE generation is related to not only a considerably high extent of in-plane orientation of the PMDA/PDA chains but also to the crystallinity of the blends. The present work revealed that some other PIs, a poly(ester imide), and a polybenzoxazole system also display negative CTE and these systems also possess extremely high extents of in-plane chain orientation without exception. In addition to CTE, the morphologies were monitored as a function of imidization temperature for two PI systems, PMDA/2,2′-bis(trifluoromethyl)benzidine and PMDA/m-tolidine by wide-angle X-ray diffraction, FT-IR spectroscopy, birefringence, and film density measurements. The results suggested that the negative CTE phenomenon occurs when PI films possess very high extents of in-plane orientation and a less crystalline morphology simultaneously, thereby significant thermal expansion can be allowed to the thickness direction.

Investigation of thermal expansion of 3D-stitched C–SiC composites

Carbon fiber reinforced silicon carbide (C–SiC) composites are promising materials for a severe thermo-erosive environment. 3D-stitched C–SiC composites were fabricated using liquid silicon infiltration. The infiltration was carried out at 1450–1650 °C for 10–120 min in vacuum. Coefficient of thermal expansion (CTE) of the composites was determined in in-plane and through-thickness directions in the temperature range from room temperature to 1050 °C. The in-plane CTE varies in the range (0.5–2) × 10 −6/°C, while that in the through-thickness direction, it varies in the range (1.5–4) × 10 −6/°C. The effect of siliconization conditions is higher in the through-thickness direction than in the in-plane direction. The CTE values are lower than the values reported for chemical vapor impregnation based 3D C–SiC composites. An extensive microstructure study was also carried out to understand the thermal expansion behavior of the composites. It was found out that CTE behavior is closely related to the composition of the composite which in turn depends upon siliconization conditions. The best conditions were 1650 °C and 120 min.

Thermoelastic properties of sandwich materials with pin-reinforced foam cores

Pin-reinforced foam is a novel type of sandwich core materials formed by inserting pins (trusses) into a foam matrix to create a truss-like network reinforced foam core. Upon loading, the pins deform predominantly by local stretching whilst the deformation of foam is governed by local bending. This paper presents a theoretical study on the thermoelasticity of pin-reinforced foam sandwich cores. To calculate the effective thermoelastic properties of pin-reinforced foam cores, the energy-based homogenization approach is employed to develop a micromechanicsbased model, calibrated by the existing experimental data. It is found that the stiffness of the sandwich core is mainly governed by pin reinforcements: the foam matrix contributes little to sandwich stiffness. Compared with traditional foam cores without pin reinforcements, the changes in in-plane thermal expansion coefficients are not vigorous as a result of pin reinforcements, while the throughthickness thermal expansion coefficient changes significantly. It is also demonstrated that it is possible to design materials with zero or negative thermal expansion coefficients under such a context.

Heat-cycle endurance and in-plane thermal expansion of Al2O3/Al substrates formed by aerosol deposition method

A new heat radiation substrate for high power modules has been proposed by utilizing Al2O3 thick films formed by the aerosol deposition method (ADM) on Al substrates. Dense Al2O3 thick films (AD-Al2O3) could be obtained by the ADM at room temperature. A heat-cycle endurance test between -80°C and 180°C revealed that the AD-Al2O3/Al substrates were superior to the conventional direct brazed aluminum (DBA) substrates using AlN ceramics bounded to Al substrates. No lamination was observed between the AD-Al2O3 films and Al substrates after heat-cycle test in spite of the difference of thermal expansion coefficient between Al2O3 and Al. The in-plane thermal expansion coefficient of AD-Al2O3 films was almost the same as that of Al substrates, indicating that plastic deformation was induced in the AD-Al2O3 films consisting of nano-sized grains to relax the stress caused by the difference of thermal expansion coefficient.

Anisotropy of thermal expansion in Mg- and Mg4Li-matrix composites reinforced by short alumina fibers

The thermal expansion of Mg- and Mg4Li-matrix composites reinforced with 10 vol.% short Saffil fibers (SF) has been investigated at temperatures of RT-360 °C relating to the fiber orientation (in-plane and cross-plane). The in-plane coefficients of thermal expansion (CTEs) of composites are primarily influenced by the matrix yielding which results in significantly lower in-plane thermal expansivity of Mg/SF composite than that of Mg4Li/SF. In-plane and cross-plane CTEs of the composite studied behave complementarily: the lower the in-plane CTE, the larger the cross-plane CTE and vice versa.

Design Optimization and Reliability of PWB Level Electronic Package

Abstract As the electronic packaging industry develops technologies for fabrication of smaller, faster, economical and reliable products, thermal management and design play an important role. Temperature fluctuations caused by either power consumption or environmental changes, along with the resulting thermal expansion mismatch between the various packages materials result in deformation stresses in packages/assemblies especially in solder interconnects. Increased power dissipation and density in modern electronics system requires efficient and intelligent design and thermal management strategies to ensure the reliability of electronic products. In the past reliability issues related to optimization of electronic packages were dealt with by coupling analysis tools with optimization solvers. In this paper, ANSYS APDL code is used with a built-in optimization tool for optimization of electronic packages, and for improving the solder joint life and arriving at optimal design. It has been shown that, design optimization would enormously decrease the lead time. The finite element tool ANSYS is used to estimate the cycles to fatigue failure of solder joint of the package coupled with optimization module present in the solver for providing the details on determining optimal design parameters that affect the product reliability. Four model characteristics: printed wiring board (PWB) core in-plane Young’s modulus, PWB core in-plane coefficient of thermal expansion, PWB core thickness, and the standoff solder joint height are chosen as the optimization inputs (design variables) that ensure higher reliability and improved performance of the assembled product. The objective junction of the paper is to minimize average plastic work to improve the fatigue life of solder joints of the package. Subapproximation, design of experiment and central composite design based response surface modeling methodologies are used to study the effects of each design variables on the fatigue life.

Thermal expansion behavior of carbon fiber reinforced chemical-vapor-infiltrated silicon carbide composites from room temperature to 1400 °C

Thermal expansion behavior of C/SiC composites with three different preform architectures from room temperature (RT) to 1400 °C was investigated. It was found that they related to changes of interfacial thermal stress and microstructure due to interaction of fibers and matrix. The longitudinal coefficients of thermal expansion (CTE) of 1D and 3D C/SiC composites and in-plane CTE of 2D C/SiC composites changed in such similar way that increasing linearly with increasing temperature, reaching maximum values at about 900 °C, and fluctuating from 900 to 1400 °C. However, they had different increasing rates below 900 °C. In addition, transverse CTEs of 1D and 3D C/SiC increased linearly and both changed in a similar way as bulk CVD SiC material at low temperature range. And then, they increased rapidly from 1200 to 1400 °C after a decrease at 900 °C.

Anisotropic intrinsic and extrinsic stresses in epitaxial wurtzitic GaN thin film on γ-LiAlO 2 (1 0 0)

A wurtzitic GaN thin film with a thickness of 130 nm is heteroepitaxially deposited on γ-LiAlO 2(1 0 0) substrate by ion-beam-assisted molecular beam epitaxy (IBA-MBE). Structural properties of the film are characterized by high-temperature X-ray diffraction in the temperature range of 25–600 °C. The experimental approach demonstrates a possibility to determine residual stresses in anisotropic epitaxial thin film at high temperature using diffraction. The mechanical and thermal behavior of the film is influenced by the anisotropic nature of GaN ( 1 1 ¯ 0 0 ) crystallographic plane oriented parallel to the substrate surface. The diffraction measurements show that the specific mismatch of the in-plane thermal expansion coefficients result in anisotropic compressive in-plane residual stresses in the film, with the stress in the a-direction significantly larger than in the c-direction over the whole temperature range. Moreover, since no signs of plastic deformation are detected, the temperature dependence of residual stresses allows to extrapolate intrinsic stresses formed in the film during the deposition.

Simultaneous determination of experimental elastic and thermal strains in thin films

A methodology is presented that allows the separation and quantification of temperature-dependent phenomena influencing the mechanical behaviour of thin films. The approach is based on high-temperature X-ray diffraction and supposes a structural characterization of the thin film and the substrate during temperature cycling. The diffraction data are used to determine simultaneously elastic strains and in-plane thermal expansion coefficients (TECs) in the thin film as well as in-plane TECs of the substrate. The dependency of the experimental elastic strains on the virtual thermal strains provides an opportunity to identify the elastic behaviour as well as to resolve temperature-dependent phenomena influencing the residual stress state in the film. The approach is demonstrated on three model structures: Al/Si(001), AlN/Al2O3(0001) and Mg-implanted GaN/Al2O3(0001).

Environmentally protected hot-stage atomic force microscope for studying thermo-mechanical deformation in microelectronic devices

A commercial atomic force microscope (AFM) was equipped with a hot stage for conducting thermal cycling experiments up to 398 K, as well as a vacuum and purge system to provide a protective environment during heating. Two different hot-stage configurations, one for studying features in the plane of a microelectronic device, and the other for studying features on its cross section, were developed. It is shown that the AFM retains its calibration with no significant introduction of errors at temperatures up to 398 K. Two applications of in situ hot-stage atomic force microscopy, related to microelectronic devices, have been demonstrated. First, the in-plane coefficient of thermal expansion of a low dielectric constant (low-k) thin film dielectric material used in back-end interconnect structures was measured. Second, the equipment was used to conduct in situ studies of deformation of Cu thin film interconnect lines at the back end of silicon chips, under thermo-mechanical loads simulating those imposed on chip-level interconnect structures by a microelectronic package. The design of a bimetallic thermo-mechanical loading stage, which was used for the latter experiments in conjunction with the hot stage, is also discussed.

Anisotropic thermal expansion behavior of thin films of polymethylsilsesquioxane, a spin-on-glass dielectric for high-performance integrated circuits.

Thin films of poly(methylsilsesquioxane) (PMSSQ) are candidates for use as interdielectric layers in advanced semiconductor devices with multilayer structures. We prepared thin films of PMSSQ with thicknesses in the range 25.0-1151.0 nm by spin-casting its soluble precursor onto Si and GaAs substrates with native oxide layers and then drying and curing the films under a nitrogen atmosphere at temperatures in the range 250-400 degrees C. The out-of-plane thermal expansion coefficient alpha(perpendicular) of each film was measured over the temperature range 25-200 degrees C using spectroscopic ellipsometry and synchrotron X-ray reflectivity, while the in-plane thermal expansion coefficient alpha(parallel) of each film was determined over the temperature range 25-400 degrees C by residual stress analysis. PMSSQ films cured at higher temperatures exhibited reduced thermal expansion, which is attributed to the denser molecular packing and higher degree of cross-linking that arises at higher temperatures. Surprisingly however, all the PMSSQ films were found to exhibit very strong anisotropic thermal expansion; alpha(perpendicular) and alpha(parallel) of the films were in the ranges 140-329 ppm/ degrees C and 12-29 ppm/ degrees C respectively, depending on the curing temperature. This is the first time that cured PMSSQ thin films have been shown to exhibit anisotropic thermal expansion behavior. This anisotropic thermal expansion of the PMSSQ thin films might be due to the anisotropy of cross-link density in the films, which arises because of a combination of factors: the preferential orientation of methyl groups toward the upper film surface and the preferential network formation in the film plane that occurs during curing of the confined film. In addition, the film electron densities were determined using synchrotron X-ray reflectivity measurements and the film biaxial moduli were obtained using residual stress analysis.

Influence of processing conditions on the thermal and mechanical properties of SU8 negative photoresist coatings

The thermal and mechanical properties of a new negative photoresist, SU8, were characterized. The influence of curing conditions, such as baking temperature, baking time and UV dosage, on the thermal and mechanical properties of the resultant coatings was studied in detail. It was found that the glass-transition temperature (Tg) of the coatings was coincident with the baking temperature over the temperature range of 25 °C–220 °C for coatings being baked for just 20 min. However, the Tg reached a limiting value (about 240 °C) once the cross-linking reaction was complete, and would not increase further with the baking temperature. The peak temperature of the dimension versus temperature plots, where heat shrinkage occurred, was about a factor of 1.16 times higher than the baking temperature for the temperature range studied. Both the Tg and the shrinkage temperature were affected by the baking time. The thermal expansion coefficients (TEC), including the volumetric TEC (αv), the in-plane TEC (α1) and the out-of-plane TEC (α2), were measured by a pressure–volume–temperature (PVT) apparatus and thermal–mechanical analyzer (TMA). Great residual stress could be generated during the process, and the change in residual stress with the environmental humidity was investigated using vibrational holographic interferometry.

Linear thermoelastic characterization of anisotropic poly(ethylene terephthalate) films

All nine independent elastic constants have been determined for a biaxially stretched poly(ethylene terephthalate) (PET) film using novel mechanical methods. The orthotropic directions and the in-plane Poisson's ratios were first characterized using vibrational holographic interferometry of tensioned membrane samples. The out-of-plane Poisson's ratio was obtained by measuring the change in tension with the change in pressure for constant strain conditions. Pressure–volume–temperature (PVT) equipment was used to measure the bulk compressibility as well as the volumetric thermal expansion coefficient (TEC). The in-plane Young's moduli were obtained by tensile tests, while the out-of-plane modulus was calculated from the compressibility and other elastic constants that describe the in-plane behavior. The in-plane TECs in the machine and transverse directions were determined using a thermal mechanical analyzer (TMA). The out-of-plane TEC was determined using these values and the volumetric TEC determined via PVT. The resulting compliance matrix satisfies all of the requirements of a positive-definite energy criterion. The procedure of characterization utilized in this article can be applied to any orthotropic film. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2937–2947, 2002

Synthesis and properties of polybenzoxazole–clay nanocomposites

A novel polybenzoxazole (PBO)/clay nanocomposite has been prepared from a PBO precursor, polyhydroxyamide (PHA) and an organoclay. The PBO precursor was made by the low temperature polycondensation reaction between isophthaloyl chloride (IC) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane with an inherent viscosity of 0.5 dl/g. The organoclay was formed by a cation exchange reaction between a Na +-montorillonite (Na +-Mont) clay and an ammonium salt of dodecylamine. The PHA/clay was subsequently thermal cured to PBO/clay. Both X-ray diffraction and transmission electron microscope analyzes showed that the organoclay was dispersed in the PBO matrix in a nanometer scale. The in-plane coefficient of thermal expansion (CTE) of PBO/clay film decreased with increasing amounts of organoclay. The CTE of PBO/clay film containing 7 wt% clay was decreased by 21% compared to the pure PBO film. Both of the glass transition temperature ( T g) and the thermal decomposition temperature of PBO/clay increased with increasing amounts of organoclay. The thermal decomposition temperature and the T g of PBO/clay containing 7 wt% clay increased to 12 and 16 °C, respectively.

A Technique for Deducing In-Plane Modulus and Coefficient of Thermal Expansion of a Supported Thin Film

Abstract A technique for determining the in-plane modulus and the coefficient of thermal expansion (CTE) of supported thin films has been developed. The modulus and CTE are calculated by solving two coupled equations that relate the curvature of film samples deposited on two different substrates to the thermal and mechanical properties of the constituents. In contrast with the conventional method used to calculate modulus and CTE, which involves differentiation of the thermal stress in the film, this new technique does not require the differentiation of the thermal stress, and can also provide the temperature-dependence of the in-plane CTE and elastic modulus of supported thin films. The data reduction scheme used for deducing CTE and elastic modulus is direct and reliable.