In-Situ Monitoring via Synchrotron Radiation Laminography of Thermal Fatigue Cracks at Die-Attached Joints Under Cyclic Energization Loading

Recently, due to the increasing heat density of printed circuit boards (PCBs), thermal fatigue damage in the joints has exerted a more significant influence on the reliability of electronic components. Accordingly, the development of a new nondestructive inspection technology is strongly desired by related industries. The authors have applied a synchrotron radiation X-ray micro-tomography system to the nondestructive observation of micro-cracks. However, the reconstruction of CT images is difficult for planar objects such as PCB substrates, due to insufficient X-ray transmission in the direction parallel to the substrates. In order to solve this problem, a synchrotron radiation laminography system was developed to relax size restrictions on the observation samples, and was applied to the three-dimensional nondestructive evaluation of several kinds of solder joints, which were loaded under accelerated thermal cyclic conditions via thermal shock tests. Moreover, the thermal fatigue crack propagation process that occurs under actual PCB energization loading conditions will differ from that under the usual acceleration test conditions. In this work, the possibility of in-situ monitoring of the thermal fatigue crack propagation process using the laminography system was investigated at die-attached joints subjected to cyclic energization loading, which is close to the actual usage conditions of PCBs. The optical system developed for use in the laminography system was constructed to provide a rotation stage with a tilt from the horizontally incident X-ray beam, and to obtain X-ray projection images via a beam monitor. In this manner, the X-ray beam is sufficiently transmitted through the planar specimen in all projections. The observed specimens included several die-attached joints, in which 3 mm square ceramic dies had been mounted on a 40 mm square FR-4 substrate using Sn-3.0wt%Ag-0.5wt%Cu solder. Consequently, the laminography system was successfully applied to the in-situ monitoring of thermal fatigue cracks that appeared in the solder layer under cyclic energization. This was possible because the laminography images obtained in the energization state have a quality that is equivalent to those obtained in a non-energized state, provided that the temperature distribution of the specimen is stable. In addition, the fatigue crack propagation process can be quantitatively evaluated by measuring the crack surface area and calculating the average crack propagation rate. However, in some cases, the appearance of thermal fatigue cracks was not observed in a solder layer that had been loaded by the accelerated thermal cycle test. This result strongly suggests that delamination occurred at the interface, which indicates that the corresponding fracture mode was significantly influenced by the type of thermal loading.

Impact of Tile Design on Thermal Performance of Open and Enclosed Aisles

In raised floor data centers, tiles with high open area ratio or complex understructure are used to fulfill the demand of today’s high-density computing. Using more open tiles reduces pressure drop across the raised floor with the potential advantages of increased airflow and lower noise. However, it introduces the disadvantage of increased non-uniformity of airflow distribution. In addition, there are various tile designs available on the market with different opening shapes or understructures. Furthermore, a physical separation of cold and hot aisles (containment) has been introduced to minimize the mixing of cold and hot air. In this study, three types of floor tiles with different open area, opening geometry, and understructure are considered. Experimentally validated detail models of tiles were implemented in CFD simulations to address the impact of tile design on the cooling of IT equipment in both open and enclosed aisle configurations. Also, impacts of under-cabinet leakage on the IT equipment inlet temperature in the provisioned and under-provisioned scenarios are studied. Finally, a predictive equation for the critical under-provisioning point that can lead to a no-flow condition in IT equipment with weaker airflow systems is presented.

Thermo-Optic Tuning Efficiency of Micro Ring Resonators on Low Thermal Resistance Silicon Photonics Substrates

Silicon photonics has emerged as a scalable technology platform for future optotelectronic communication systems. However, the current use of SiO2-based silicon-on-insulator (SOI) substrates presents a thermal challenge to integrated active photonic components such as lasers and semiconductor optical amplifiers due to the poor thermal properties of the buried SiO2 optical cladding layer beneath these devices. To improve the thermal performance of these devices, it has been suggested that SiO2 be replaced with aluminum nitride (AlN); a dielectric with suitable optical properties to function as an effective optical cladding that, in its crystalline state, demonstrates a high thermal conductivity (∼100× larger than SiO2 in current SOI substrates). On the other hand, the tuning efficiencies of thermally-controlled optical resonators and phase adjusters, crucial components for widely tunable lasers and modulators, are directly proportional to the thermal resistance of these devices. Therefore, the low thermal conductivity buried SiO2 layer in the SOI substrate is beneficial. Moreover, to further improve the thermal performance of these devices air trenches have been used to further thermally isolate these devices, resulting in up to ∼10× increase in tuning efficiency. Here, we model the impact of changing the buried insulator on a SOI substrate from SiO2 to high quality AlN on the thermal performance of a MRR. We map out the thermal performance of the MRR over a wide range of under-etch levels using a thermo-electrical model that incorporates a pseudo-etching approach. The pseudo-etching model is based on the diffusion equation and distinguishes the regions where substrate material is removed during device fabrication. The simulations reveal the extent to which air trenches defined by a simple etch pattern around the MRR device can increase the thermal resistance of the device. We find a critical under-etch below which no benefit is found in terms of the MRR tuning efficiency. Above this critical under-etch, the tuning efficiency increases exponentially. For the SiO2-based MRR, the thermal resistance increases by ∼7.7× between the un-etched state up to the most extreme etch state. In the unetched state, the thermal resistance of the AlN-based MRR is only ∼4% of the SiO2-based MRR. At the extreme level of under-etch, the thermal resistance of the AlN-based MRR is still only ∼60% of the un-etched SiO2-based MRR. Our results suggest the need for a more complex MRR thermal isolation strategy to significantly improve tuning efficiencies if an AlN-based SOI substrate is used.

Estimation of Effective Thermal and Mechanical Properties of Particulate Thermal Interface Materials (TIMs) by a Random Network Model

Particulate thermal interface materials (TIMs) are commonly used to transport heat from chip to heat sink. While high thermal conductance is achieved by large volume loadings of highly conducting particles in a compliant matrix, small volume loadings of stiff particles will ensure reduced thermal stresses in the brittle silicon device. Developing numerical models to estimate effective thermal and mechanical properties of TIM systems would help optimize TIM performance with respect to these conflicting requirements. Classical models, often based on single particle solutions or regular arrangement of particles, are insufficient as real-life TIM systems contain a distriubtion of particles at high volume fractions, where classical models are invalid. In our earlier work, a computationally efficient random network model was developed to estimate the effective thermal conductivity of TIM systems [1,2]. This model is extended in this paper to estimate the effective elastic modulus of TIMs. Realistic microstructures are simulated and analyzed using the proposed method. Factors affecting the modulus (volume fraction and particle size distribution) are also studied.

True Concurrent Modulation of a Multi-Channel Ring Modulator Transmitter Driven by a Comb Laser

This paper presents experimental data of concurrent modulation of a multi-channel transmitter that uses carrier-injection ring modulators at 10Gb/s/channel that is optically driven by a quantum-dot comb laser with 50GHz channel spacing.

Development of Packaging and Modular Control of Thermoelectric Clothes Dryer, With Performance Evaluation

This paper provides an overview of a thermoelectric heat pump clothes dryer which was developed with the aim of reducing the significant primary energy consumption attributed to residential electric clothes drying in the United States (623 TBtu/yr). The use of thermoelectric modules in place of the conventional electric resistance heater resulted in a 40% reduction in the energy consumption of the system, compared to the minimum energy efficiency standard. This was achieved for the first time for a standard test load of 8.45 lb, using a clothes dryer prototype with a thermoelectric heat pump module as the sole heating mechanism. The current experimental prototype was developed after extensive modeling, system design and control optimization, and experimental system-level evaluation of control parameters. The demonstration of improved energy consumption has laid the foundation for future development of this technology.

Characterization of an Isolated Hybrid Cooled Server With Failure Scenarios Using Warm Water Cooling

Modern day data centers are operated at high power for increased power density, maintenance, and cooling which covers almost 2 percent (70 billion kilowatt-hours) of the total energy consumption in the US. IT components and cooling system occupy the major portion of this energy consumption. Although data centers are designed to perform efficiently, cooling the high-density components is still a challenge. So, alternative methods to improve the cooling efficiency has become the drive to reduce the cooling cost. As liquid cooling is more efficient for high specific heat capacity, density, and thermal conductivity, hybrid cooling can offer the advantage of liquid cooling of high heat generating components in the traditional air-cooled servers. In this experiment, a 1U server is equipped with cold plate to cool the CPUs while the rest of the components are cooled by fans. In this study, predictive fan and pump failure analysis are performed which also helps to explore the options for redundancy and to reduce the cooling cost by improving cooling efficiency. Redundancy requires the knowledge of planned and unplanned system failures. As the main heat generating components are cooled by liquid, warm water cooling can be employed to observe the effects of raised inlet conditions in a hybrid cooled server with failure scenarios. The ASHRAE guidance class W4 for liquid cooling is chosen for our experiment to operate in a range from 25°C – 45°C. The experiments are conducted separately for the pump and fan failure scenarios. Computational load of idle, 10%, 30%, 50%, 70% and 98% are applied while powering only one pump and the miniature dry cooler fans are controlled externally to maintain constant inlet temperature of the coolant. As the rest of components such as DIMMs & PCH are cooled by air, maximum utilization for memory is applied while reducing the number fans in each case for fan failure scenario. The components temperatures and power consumption are recorded in each case for performance analysis.

An Investigation of Moisture-Induced Interfacial Delamination in Plastic IC Package During Solder Reflow

In this study, the moisture induced delamination behavior of a plastic ball grid array package under the solder reflow process was investigated by the finite element analysis. The entire moisture history of the PBGA package was simulated for preconditioning at moisture sensitivity level 1 and the subsequent exposure to a soldering reflow. A fracture mechanics based analysis was used to investigate the combined effects of temperature, moisture and vapor pressure on the delamination behavior at the die/molding compound and die/die attach interfaces during solder reflow. For determining the total strain energy release rate and total stress intensity factor under a multiphysics environment like reflow, researchers commonly used the principle of superposition to combine the results from individual thermal stress, hygroscopic stress and vapor pressure induced stress analyses. In this study, a new method was proposed to obtain the total strain energy release rate and total stress intensity factor under the multi-physics environment in a single fracture analysis instead of three. Two different methods-virtual crack closure technique (VCCT) and crack tip opening displacement method (CTOD) were employed and compared in studying the variation of strain energy release rates during lead-free solder reflow. The relationship between the strain energy release rate and crack length was also obtained. The developments of the stress intensity factors due to individual effect of thermal mismatch, hygroscopic swelling and vapor pressure were calculated. The mode mixity was also determined under different temperatures and crack length.

The Effect of Surface Texture on Thermal Sensation and Comfort

The study investigated how the material roughness of a tablet computer surface can affect thermal sensation and comfort of users fingers and palms at different surface temperatures. Three levels of pattern spacing were tested, and it was shown that rough material surface provided higher thermal comfort comparing to a smooth surface. In addition, the surface temperature of the material also moderates participants′ physical sensation of the roughness of the materials. The results of the study have shown evidences of the potentials to use materials with spatial patterns to improve thermal comfort while dissipating heat from electronic devices.

The Dependence of Moisture Induced Die Stresses Upon Moisture Properties of Polymer Materials in Electronic Packages

Polymer materials have been widely used in electronic packaging with many advantages such as: lower cost, light weight and good performance. They however suffer a major drawback that results in a number of challenges for reliability engineers and researchers, in which polymer materials are quite sensitive to moisture absorption when exposed to humid environment, causing many failure modes in electronic packages such as: popcorn cracking, delamination or corrosion. It is well-known that finite element simulation is a powerful tool to evaluate the effects of moisture on electronic package reliability. In this study, three moisture properties (diffusivity, saturated concentration, and coefficient of moisture expansion) were experimentally characterized. The obtained results were then used to perform moisture diffusion simulations on various types of electronic package. Finally, a numerical study was conducted on the dependence of the moisture effects (weight gains, die stresses) upon each moisture property of polymeric components of three kinds of electronic packages (Quad Flat Package, Plastic Ball Grid Array, and Flip Chip on Laminate). The results of the study provided valuable insights into how moisture induced die stresses vary with each moisture property of polymeric components in the packages.