High Temperature Thermal Cycling Research Articles

Ultraviolet pulsed laser irradiation of multi-walled carbon nanotubes in nitrogen atmosphere

Laser irradiation of randomly oriented multi-walled carbon nanotube (MWCNT) networks has been carried out using a pulsed Nd:YAG UV laser in nitrogen gas environment. The evolution of the MWCNT morphology and structure as a function of laser fluence and number of accumulated laser pulses has been studied using electron microscopies and Raman spectroscopy. The observed changes are discussed and correlated with thermal simulations. The obtained results indicate that laser irradiation induces very fast, high temperature thermal cycles in MWCNTs which produce the formation of different nanocarbon forms, such as nanodiamonds. Premelting processes have been observed in localized sites by irradiation at low number of laser pulses and low fluence values. The accumulation of laser pulses and the increase in the fluence cause the full melting and amorphization of MWCNTs. The observed structural changes differ from that of conventional high temperature annealing treatments of MWCNTs.

High-Performance Light-Emitting Diodes Encapsulated with Silica-Filled Epoxy Materials

Packaging materials have a great impact on the performance and reliability of light-emitting diodes (LEDs). In this study, we have prepared high performance LED devices through encapsulating LEDs by epoxy materials incorporated with filler powders. A set of evaluation methods have also been established to characterize the reliability of LED devices. No delamination or internal cracking between packaging materials and lead frames has been found for the encapsulated high performance LED devices after the package saturation with moisture and subsequent exposure to high-temperature solder reflow and thermal cycling. Four kinds of inorganic silica fillers, namely, quartz, fused silica, cristobalite, and spherical silica, and one kind of organic filler, that is, spherical silicone powder, were incorporated into the epoxy packaging materials to compare their effects on performance of LED devices. The properties of epoxy packaging materials and LED devices were studied by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), dynamic mechanical analysis (DMA), thermomechanical analyzer (TMA), ultravioletvisible spectrophotometer (UV-vis), scanning acoustic microscopy (SAM), and scanning electron microscopy (SEM). Except the spherical silicone powder filled epoxy materials, all the other filled systems showed lower equilibrium water sorption content and smaller water diffusion coefficient in both water sorption and moisture sorption tests. The coefficient of thermal expansion (CTE) values were also decreased with the addition of fillers, and the systems filled with quartz, fused, and filled with spherical silica gave the best performance, which exhibited the reduced CTE values both below and above Tg. The results of TGA essentially showed no difference between filled and unfilled systems. The glass transition temperature changed little for all the filled systems, except the one incorporated with spherical silicone. The modulus at room temperature increased with the addition of fillers. The light transmittance of filled epoxy materials varied with fillers after UV and thermal aging.

High Temperature Interconnect and Die Attach Technology: Au–Sn SLID Bonding

Au-Sn solid-liquid interdiffusion (SLID) bonding is a novel and promising interconnect and die attach technology for high temperature (HT) applications. In combination with silicon carbide (SiC), Au-Sn SLID has the potential to be a key technology for the next generation of HT electronic devices. However, limited knowledge about Au-Sn SLID bonding for HT applications is a major restriction to fully realizing the HT potential of SiC devices. Two different processing techniques-electroplating of Au/Sn layers and sandwiching of eutectic Au-Sn preform between electroplated Au layers-have been studied in a simplified metallization system. The latter process was further investigated in two different Cu/Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /Cu/Ni-P/Au-Sn/Ni/Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Si/SiC systems (different Au-layer thickness). Die shear tests and cross-sections have been performed on as-bonded, thermally cycled, and thermally aged samples to characterize the bonding properties associated with the different processing techniques, metallization schemes, and environmental stress tests. A uniform Au-rich bond interface was produced (the ζ phase with a melting point of 522°C). The importance of excess Au on both substrate and chip side in the final bond is demonstrated. It is shown that Au-Sn SLID can absorb thermo-mechanical stresses induced by large coefficient of thermal expansion mismatches (up to 12 ppm/K) in a packaging system during HT thermal cycling. The bonding strength of Au-Sn SLID is shown to be superb, exceeding 78 MPa. However, after HT thermal ageing, the ζ phase was first converted into the more Au-rich β phase. This created physical contact between the Sn and Ni atoms, resulting in brittle Ni <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> Sn <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> phases, reducing the bond strength. Density functional theory calculations have been performed to demonstrate that the formation of Ni <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> Sn <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> in preference to the Au-rich Au-Sn phases is energetically favorable.

An Experimental Study of Microstructure-Property Relationships in Thermal Barrier Coatings

The thermal-mechanical properties of thermal barrier coatings are highly influenced by the defects present in coating microstructure. The aim of this study was to meet the future needs of the gas turbine industry by further development of zirconia coatings through the assessment of microstructure-property relationships. A design of experiments was conducted for this purpose with current, spray distance, and powder feed rate as the varied parameters. Microstructure was assessed with SEM and image analysis was used to characterize porosity content. Evaluations were carried out using laser flash technique to measure thermal properties. A bi-layer beam curvature technique in conjunction with controlled thermal cycling was used to assess the mechanical properties, in particular their nonlinear elastic response. Coating lifetime was evaluated by thermo-cyclic fatigue testing. Relationships between microstructure and coating properties are discussed. Dense vertically cracked microstructure and highly porous microstructure with large globular pores were also fabricated. Correlations between parameters obtained from nonlinear measurements and lifetime based on a priori established microstructural analysis were attempted in an effort to develop and identify a simplified strategy to assess coating durability following sustained long-term exposure to high temperature thermal cycling.

Shear Toughness Evaluation of Solder Joints for the Reliability Tests

The effects of isothermal aging and the thermal cycling loading on the shear toughness of different solder materials and ball sizes have been explored. The difference between shear toughness values of traditional Sn/37Pb eutectic solder ball joints and the lead free Sn/3.0Ag/0.5Cu solders are chosen for discussion. The experiment measurements under the ball shear test (BST) have been compared and studied for both solder joints. The fracture behaviors of the solder joints under the high temperature aging and thermal cycling testing are examined by scanning electron microscope (SEM). The variation of shear toughness of different ball joints reveals that the high temperature aging and thermal cyclic loading reduce the shear toughness significantly. The measured shear toughness values indicate that the Sn/3.0Ag/0.5Cu solder joints have better ductility for the joints undergoing the high temperature aging and the thermal cycle loadings. Based on the measured results, the better reliability for the Sn/3.0Ag/0.5Cu ball joints is expected, due to the aging and cycling load testing.

Damage of lead-free solder joints in flip chip assemblies subjected to high-temperature thermal cycling

The damage mechanism of lead-free ball grid array (BGA) solder joints is studied whilst the magnitude of the damage was quantitatively evaluated to determine the impact of the thickness of inter-metallic compound (IMC) layer on the reliability of the joints. Five virtual test vehicle assemblies of flip chip (FC) which have 4–12μm thickness of IMC layer were subjected to the same accelerated high-temperature cycles utilising, in parts, the IEC standard 60749-25. The visco-plastic responses of the assemblies’ solder bump joints to the induced thermal load were simulated using Anand’s plasticity model. The results demonstrate that very thin and thick thickness of IMC layer impact the reliability of the FC solder joints. It was found that strain range at steady state joint damage is approximately 0.07–0.12. This finding indicates that strain controlled studies on bulk solder which are not up to these limits are inadequate to characterise damage behaviour and model damage evolution in the FC BGA solder bump joints. Further results show that the magnitude of the average accumulated visco-plastic energy density per cycle (wacc) depends on the volume percentage of solder and IMC in the joints. This dependency explains why the existing life prediction models based on wacc are not satisfactory in estimating the damage in solder joints. The authors propose the development of new prediction models which will be specific on the volume percentage of solder and IMC in the joints.

Influence of the intermixed interfacial layers on the thermal cycling behaviour of atmospheric plasma sprayed lanthanum zirconate based coatings

In application as a thermal barrier coating (TBC), yttria stabilised zirconia (YSZ) approaches some limits of performance. To further enhance the efficiency of gas turbines, higher temperature capability and a longer lifetime of the coating are needed for the next generation of TBCs. Pyrochlore oxides of general composition, A2B2O7, where A is a 3+ cation (La to Lu) and B is a 4+ cation (Zr, Hf, Ti, etc.) have high melting point, fair coefficient of thermal expansion, and low thermal conductivity which make them suitable for applications as high temperature thermal barrier coatings. Among those oxide materials lanthanum zirconate (LZ/La2Zr2O7) offers very attractive properties. This work describes the fabrication, microstructure and high temperature (1280°C) thermal cycling behaviour of lanthanum zirconate coatings with five different coating architectures, deposited using atmospheric plasma spray process. The coating architecture which had five layers with two intermixed interlayers had much longer life time than other considered architectures. The coatings were characterised using X-ray diffraction, energy dispersive spectrometry, optical and scanning electron microscopy, before and after thermal cycling tests, to study the coating failure mechanisms.

Corroded Behavior of 310S Stainless Steel in Molten Al-Si Eutectic Alloy

Corroded behavior of 310S stainless steel by molten Al-Si eutectic alloy in high temperature thermal cycles of 495～620°C was investigated. SEM/EDX, XRD and metallographic technique were used for observing and analyzing the morphology, composition and internal microstructure of the corrosion specimens. Results showed: on the surface of 310S stainless steel, after corroded through 300 melting-solidification thermal cycles, a corrosion layer of about 3μm thick, composed mainly of Fe2Al5 phase and FeAl3 phase was formed. Besides, upon observation of internal microstructure of 310S stainless steel after corrosion, plenty intercrystalline precipitation was present, which resulted in relatively severe intercrystalline corrosion. This was considered the principal factor that compromised mechanical property of the specimen of 310S stainless steel.

Au-Sn SLID bonding for high temperature applications

Au-Sn solid-liquid-interdiffusion (SLID) bonding is a novel and promising interconnect technology for high temperature (HT) applications. In combination with Silicon Carbide (SiC) devices, Au-Sn SLID has the potential of being a key technology for the next generation of innovative, cost effective and environmentally friendly drilling and well intervention systems for the oil industry. However, limited knowledge about Au-Sn SLID bonding for combined HT and high power applications is a major restriction to fully realize the high temperature potential of SiC devices. This paper presents a comprehensive study of fluxless Au-Sn SLID bonding. Two different processing techniques – electroplating of Au / Sn layers and sandwiching of eutectic Au-Sn preform between electroplated Au layers – have been studied in a simplified metallization system. The latter process was further investigated in two different Cu / Si3N4 / Cu / NiP / Au-Sn / Ni / Ni2Si / SiC systems (different Au-layer thickness). Die shear tests and cross-sections have been performed on “as bonded”, thermally cycled and thermally aged samples to characterize the bonding properties associated with the different processing techniques, metallization schemes and environmental stress tests. A uniform Au-rich bond interface is produced (the ζ phase with a melting point of 522 °C). The importance of excess Au on both substrate and chip side in the final bond is demonstrated. It is shown that Au-Sn SLID can absorb thermo-mechanical stresses induced by large CTE mismatches (up to 12 ppm/K) in a packaging system during HT thermal cycling. The bonding strength of Au-Sn SLID is shown to be superb, exceeding 78 MPa. Importantly, Au-Sn SLID is shown to be an excellent interconnect technology for HT packaging.

Study of microstructural and mechanical properties of weld heat affected zones of 2024-T3 aluminium using Gleeble simulation

The effect of the microstructural properties on the mechanical properties of welding thermal cycles and post-weld heat treatment of the heat affected zone (HAZ) in 2024-T3 aluminium alloy has been investigated. Gleeble HAZ simulation, differential scanning calorimetry, TEM and tensile test have been utilised to investigate the regions representative of HAZ microstructures. The decay of strength in the weld HAZ is primarily due to the precipitation and coarsening of stable S phases. The welded HAZ in the region at peak temperature of 414°C has the lowest strength after natural aged temper. Post-weld T81 artificial aging (PWAA-T81) heat treatment at 190°C for 12 h has no effect on improving the HAZ strength; the HAZ strength of 2024-T3 alloy obtained by PWAA-T81 treatment is less than that obtained by natural aging, and its lowest strength is shifted to the region of the peak temperature, which is 452°C. Scanning electron microscopy observation reveals that the fracture mode changes from transgranular to intergranular failure when the 2024 specimen is exposed to a thermal cycle up to a peak temperature of 550°C. This is caused by the liquation of grain boundary segregates or formation of a eutectic structure while the specimen is subjected to high temperature thermal cycles during welding, which results in a decrease in the strength and ductility of the grain boundary. It is also shown that the decrease in ductility in this high temperature HAZ cannot be improved using the PWAA-T81 heat treatment.

State of the art of high temperature power electronics

High temperature power electronics has become possible with the recent availability of silicon carbide devices. This material, as other wide-bandgap semiconductors, can operate at temperatures above 500 °C, whereas silicon is limited to 150-200 °C. Applications such as transportation or a deep oil and gas wells drilling can benefit. A few converters operating above 200 °C have been demonstrated, but work is still ongoing to design and build a power system able to operate in harsh environment (high temperature and deep thermal cycling).

Very high temperature creep behavior of a single crystal Ni-based superalloy under complex thermal cycling conditions

Very high temperature thermal cycling has been performed on the single crystal superalloy MC2 to evaluate the effect of periodic overheating on creep behavior. The experiments consist of alternately performing 1 min dwell time at 1100°C and 1150°C for every 15 min during creep test at 1050°C/120 MPa. Both thermal cycling and prior γ′-rafting appear to be deleterious to the cyclic creep properties. The observed non-isothermal creep behavior is correlated with γ′-dissolution/coalescence processes, especially during overheatings where γ′ micro-rafts seem to play a significant role.

Ultrasonic Bonding of Electrodes of Rigid and Flexible Printed Circuit Boards with Non-Conductive Film (NCF)

This study investigated the feasibility of ultrasonic bonding of a rigid printed circuit board (RPCB) to a flexible printed circuit board (FPCB) with a non-conductive film (NCF) for improving the long-term reliability and lowering the manufacturing cost. Peel tests of the joints were carried out with increasing bonding time to optimize the bonding condition. High-temperature storage and thermal cycling tests were carried out to evaluate the reliability. The RPCB was successfully bonded to the FPCB with NCF using a transverse ultrasonic bonding. The optimum time taken for bonding was 3 sec. The joints with NCF showed a good reliability after the high-temperature storage test at 125°C for 1,000 h and the thermal cycling test for 1,000 cycles.

Microstructure Evolution and the Constitutive Relations of High-Temperature Solders

This study characterized the temperature-dependent constitutive parameters (yield strength, ultimate tensile strength, elastic modulus, strain hardening exponent) from the mechanical behavior of five high-temperature solders, 95Sn-5Sb, 95Pb-5In, 90Pb-10Sn, 92.5Pb-5Sn-2.5Ag, and 93Pb-3Sn-2In-2Ag, chosen such that T m > 518 K. To model appropriately their mechanical responses under high-temperature thermal cycling, where the temperatures exceed 473 K, the material’s parameters must be determined as a function of temperature. Uni-axial tensile tests were, therefore, carried out between 298 K and 473 K to determine the constitutive behavior of each solder. 95Sn-5Sb exhibited the highest strength over the temperature range tested except near 473 K. Pb-based alloys with a higher degree of solid solution (>5%) showed greater strengthening than those primarily strengthened by coarse precipitates. Additionally, microstructure changes in 90Pb-10Sn and 95Sn-5Sb were shown to be responsible for unexpected mechanical behavior at elevated temperatures.

High-temperature property of carbon/carbon composite joints bonded with ternary Ti–Si–C compound

Carbon/carbon composites were joined using Ti–Si–C compound as interlayer and high-temperature shear strength and thermal shock resistance of these joints were investigated in this paper. In order to improve thermal shock resistance of the joints, a SiC transition layer was introduced between carbon/carbon substrates and interlayer. Microstructures and phase composition of the as-received joints were characterized by electron backscattered diffraction and X-ray diffraction. Mechanical strength and thermal shock resistance of the joints were measured by high-temperature shear test and thermal cycle test, respectively. The shear test results show that, with test temperature increasing from 273 K to 1673 K, shear strength of joints gradually increases to a maximum value of 46 MPa at 1473 K, then drops suddenly at higher temperature. Joints with SiC transition layer exhibit better thermal shock resistance than the ones without SiC transition layer and strength retention can be kept up to 85%, even after 30 thermal cycles from 1673 K to room temperature.

Monodisperse droplet generation and rapid trapping for single molecule detection and reaction kinetics measurement

On-chip monodisperse droplet analysis systems are ideal for low concentration and single molecule applications because they partition reagents into identical picoliter or smaller reactor volumes that can be observed in real-time. We present a novel trapping method with droplet stopping times of approximately 38 ms that is applicable to most on-chip droplet generators. The technique greatly extends optical interrogation times without droplet motion or coalescence; and will allow on-chip single molecule detection of nanoparticle emitters with simple optics. The method maintains droplet monodispersity without chemistry-altering surfactants, and has been shown to preserve a stationary droplet stream through repetitive high-temperature thermal cycling with no additional energy input required to maintain droplet position.

Reduced pressure chemical vapor deposition of Ge thick layers on Si(0 0 1), Si(0 1 1) and Si(1 1 1)

We have carried out an in-depth structural study of Ge thick layers grown using the so-called “LT/HT” approach on Si(0 0 1), Si(0 1 1) and Si(1 1 1) wafers. The low temperature (400 °C) adopted for the first Ge layer plastically relaxes the strain in the Ge film without 3D islanding. The high temperature (750 °C) used for the growth of the second, topmost, Ge layer lowers the dislocation density and reduces the overall deposition time. High temperature thermal cycling (in-between 750 and 890 °C) was called upon to further reduce the amount of defects in the layers. Ge growth rates on (0 0 1) were consistently higher than the ones on (1 1 1), which were themselves higher than on (0 1 1) (be it at 400 °C, 100 Torr or at 750 °C, 20 Torr). The surfaces of Ge (0 0 1) layers were rather flat, as attested by their small root mean square (rms) roughness and Z ranges (<1 and 10 nm, respectively). By contrast, Ge (0 1 1) (Ge (1 1 1)) thick layers were really rough, with rms roughness and Z ranges typically 30 (60) times higher than on (0 0 1). Almost all Ge layers were in a tensile-strain configuration, with macroscopic degrees of strain relaxation in-between 101% and 106% (slightly higher on (1 1 1) than on (0 0 1) and on (0 1 1), however). Misfit dislocations were found (by cross-sectional transmission electron microscopy) to be confined in the first few hundreds of nanometres of 2.5 μm thick Ge layers on Si(0 0 1). As far as 2.5 μm thick Ge layers on (0 1 1) and (1 1 1) are concerned, numerous {1 1 1} stacking faults were present at the Ge/Si interface and also threading all the way to the surface. The threading “defect” density, which is close to 10 7 cm −2 on (0 0 1), is around 8×10 8 cm −2 on (0 1 1) and roughly 2×10 9 cm −2 on (1 1 1). The higher defect density on (0 1 1) and (1 1 1) is likely due to 60° misfit dislocations dissociation (into 30° and 90° partial dislocations). For such surface orientations, the 90° partial dislocation is expected to lead with in its trail the 30° partial dislocation (hence the stacking faults multiplication).

Microstructural and mechanical consequences of thermal cycles on a high zirconia fuse-cast refractory

A qualitative and quantitative study of microstructural evolution and mechanical damage was carried out on a high zirconia fuse-cast refractory containing some intergranular glassy phase during high temperature thermal cycles. Microscopy, X-ray diffraction, resonance frequency and acoustic emission were investigated to relate the microstructural evolution to the damage as function of temperature. A detailed description of the mechanisms responsible for the evolution was achieved and three parameters were clearly identified as contributing to the overall behaviour. The glassy phase above T g accommodates strains and stresses of the structure and heals some cavities or cracks. The zirconia phase transformation induces both cracking and healing, or closure. The thermal expansion mismatch between zirconia and glass, responsible for stress generation during cooling at low temperature, induces cracking and crack closure during heating. Another surprising observation was the delayed cracking after few days at room temperature resulting in stress relaxation, even on very small specimens. The overall behaviour is in the end not reversible, and this was clearly shown by in situ observation of cracking.

Detection of failure sites by focused ion beam and nano-probing in the interconnect of three-dimensional stacked circuit structures

Focused ion beam (FIB) and nano-probing were applied for failure analysis of three-dimensional stacked circuits with copper through-silicon-vias between the stacked chips. The failure analysis was done after high temperature storage and thermal cycling tests. Passive voltage contrast in FIB allowed to pinpoint the open sites. FIB cross-sections showed the presence of opens at the bottom of the copper vias. The failure cause was suspected to be an interlayer particle, which was confirmed by optical profilometry. Nano-probing was used on another sample to pinpoint the failure location through the measurement of the local resistance within the daisy chains. The failure was traced out to be related with surface contamination.

Dissolution of oxygen precipitates in germanium-doped Czochralski silicon during rapid thermal annealing

The thermal stability of the oxygen precipitates in Czochralski silicon (Cz-Si) crystal with germanium doping has been investigated with rapid thermal annealing. It was found that the grown-in oxygen precipitates could be dissolved easily in germanium-doped Cz-Si (GCz-Si) than in conventional Cz-Si. After prolonged high-temperature thermal cycle, it was found that the germanium doping inclined to dramatically reduce the thermal stability of oxygen precipitates in Cz-Si crystal, either generated at low temperature (800 °C) or formed at high temperature (1000 °C). It is proposed that the germanium doping in Cz-Si could result in the oxygen precipitates with small size and plate shape, which reduce their thermal stability.