Creep response of joints bonded with single-layered pressure sensitive adhesives (PSAs) was investigated in this study. PSAs are becoming more and more popular in the electronic industry as bonding media because of their ease of design, fast accurate bonding, environmentally-friendly bonding and ease of reworking. Such adhesive bonds are expected to experience complex, sustained loading conditions in service; e.g. loading due to large mass components, shock, temperature, or alignment mismatch of substrates. Stress-strain behavior of PSA bonding assembly has been extensively studied through experiments and simulations, including the effects of loading conditions (loading rate and temperature), PSA configurations (thickness of adhesive and single/double-layered PSAs), and bonding substrate surface properties (substrate material and surface roughness). However, the literature regarding the creep response of PSA-bonded assemblies is lacking and there is no literature on modeling methodologies for the creep response of such bonding systems. Similar to the stress-strain behavior of PSA-bonded assemblies, the creep response includes transitions between multiple hardening and softening phases. Experimental results indicate that the secondary creep rate can change by up to two orders of magnitude after each transition, which is too significant to ignore when estimating the creep deformation of joints bonded with this material system. The number of transitions is related to the configuration of the PSA system, i.e. the single-layered PSA has one transition while double-layers PSAs may have multiple transitions due to the additional interface(s) introduced by the carrier layer. This unique secondary creep behavior comes from the competition between hydrostatic stress relaxation and strain hardening, caused by cavitation and fibrillation processes, respectively. The total stress applied on the joint is equal to the summation of deviatoric stress and hydrostatic stress. An advanced model based on the stress-strain ‘block’ model [5–7] is developed for evaluating the creep response. This model has the capability to control the initiation and growth of cavities in the bulk of the PSA and at the interface between PSA and substrate. This model is able to capture the nonlinear visco-plastic behavior of the PSA fibrils and estimate the effects of flexible carrier layer on the transitions in creep curves. The model prediction shows reasonable agreement with experimental results in terms of the characteristic features in creep strain histories.

In order to study the optimal N:Al flux ratio during the deposition of AlN, the effects of N:Al flux ratio on the crystal quality (crystallinity and surface roughness) of homoepitaxial AlN are investigated. The growth temperature ranges from 1600 K to 2000 K with an increment of 200 K. When the N:Al flux ratios are changed from 0.8 to 2.8, the good crystallinity is obtained at 1600 K with the N:Al flux ratio of 2.4, while it is obtained at 1800 K with the N:Al flux ratio of 2.4 and with the N:Al flux ratio of 2.0 at 2000 K. The crystallinity at 1800 K with N:Al flux ratio of 2.4 stands out among these three. At 1800 K with varied N:Al flux ratios, the minimum surface roughness is also obtained at the N:Al flux ratio of 2.4. Further more, the distribution of deposited Al atoms at 1800 K is explored, the result shows that the uniform distribution of Al atoms appears at N:Al flux ratio of 2.4.

Nowadays microelectronic packaging has become a billion dollar business. Due to the increased material and production costs per package, manufacturing yield loss in this state-of-art business is expected to be at a bare minimum which is tough to persevere in a high volume manufacturing environment. Additionally, high performance and varied power computing needs in the electronic business demands microprocessors with different form factors and complex package designs. One of the most common joint which is extensively used in such a complicated package is the polymer to metal bonding. In the latest technology products involving high package warpage, interfacial bonding has to be strong enough to withstand the dynamic warpage and high mechanical stresses associated with it and hence the reliability of polymer to metal adhesion is critical. In this paper, fundamental mechanisms related to adhesion phenomena of polymer-metal interface are proposed. Adhesive failure modes related to polymer-metal bonding and key variables influencing the bonding of silicone based polymer material to nickel electroplated on copper in an integrated circuit heat spreader assembly are experimentally studied. Factors modulating polymer to metal bonding including interfacial chemistry, surface contamination and material roughness are evaluated.

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.

Enhanced boiling is one of the popular cooling schemes in thermal management due to its superior heat transfer characteristics. This study demonstrates the ability of copper inverse opal (CIO) porous structures to enhance pool boiling performance using a thin CIO film with a thickness of ∼ 10 μm and pore diameter of 5 μm. The microfabricated CIO film increases microscale surface roughness that in turn leads to more active nucleation sites thus improved boiling performance parameters such as heat transfer coefficient and critical heat flux compared to those of smooth Si surfaces. The experimental results for CIO film show a maximum critical heat flux of 225 W/cm 2 (at 16.2°C superheat) or about 3 times higher than that of smooth Si surface (80 W/cm 2 at 21.6°C superheat). Optical images showing bubble formation on the microporous copper surface are captured to provide detailed information of bubble departure diameter and frequency.

In this study heat transfer due to vaporization is investigated for low concentration binary mixtures of 2-propanol/water on nanostructured surfaces. The surfaces are comprised of zinc oxide (ZnO) nanocrystals grown by hydrothermal synthesis on a smooth copper substrate having an average roughness of 0.06 μm. Three nanostructured surfaces used in this study differ only in the duration of the hydrothermal synthesis consisting of 4, 10, and 24 hours of surface growth. Surface geometries were observed to be a function of hydrothermal synthesis time with an increase in area coverage, length, and diameter of nanocrystals with increase synthesis time. ZnO nanocrystals exhibit mean diameter of 500–700 nm, mean length of 1.7–3.3 μm and porosities of 0.04–0.58. Individual droplets between 2.5–3.9 mm in diameter consisting of a binary mixture of 2-propanol/water with concentration of either 0.01 M or 0.03 M were deposited at a minimum distance above the surface that would be sufficient for droplets to detach on their own due to gravity onto a nanostructured surface at temperatures between 110–140 °C. High speed video was used to record the deposition and vaporization process and through image analysis it was possible to measure heat transfer coefficients based on the wetted area, as well as other parameters. Through the video analysis it was observed that droplets which are approximately spherical, impact the surface and spread into a thin film with mean film thickness between 65–400 μm which then evaporated by film evaporation without nucleate boiling. Wettability of each of the surfaces was characterized through contact angle measurements from photographs of the droplet profile when the droplet profile was discernible. When profiles were not discernible due to hydrophilicity of some surfaces, contact angles were calculated by utilizing droplet volume and spread area. Contact angle measurements were performed on the surfaces before and after each experiment in order to document changes in wettability as a result of experimentation. Results from this experiment are compared to water droplet vaporization results from a previous experiment in order to determine whether 2-propanol enhances the heat transfer, and found that the heat transfer coefficient was increased by up to 128% in some cases. Heat transfer enhancement was found to be a function of droplet diameter as well as mixture concentration with 3.9 mm 0.01 M 2-propanol/water droplets showing larger enhancement. Potential uses of heat transfer in this application are also discussed.

The thermal contact resistance (TCR) is the crucial issue in the field of heat removal from systems like electronic equipment, satellite thermal control systems, and so on. To cope with the problem, a lot of studies have been done mainly for flat rough surfaces. However, as pointed out so far, there are still wide discrepancies among measured and predicted TCRs, even for similar materials. To investigate the key factors for the abovementioned discrepancies, a fundamental analysis was conducted in our previous study [1] using a simple contact surface model, which was composed of the unit cell model proposed by Tachibana [2] and Sanokawa [3]. Furthermore, by introducing a 2-D microscopic surface model, which consists of random numbers and Abbott’s bearing area curve, the effects of surface waviness and roughness on the temperature fields near the contact interface have been investigated microscopically [4]. In this study, based on a 1-D wavy surface model, a fundamental study has been conducted to predict TCR and the thermal contact conductance (TCC), which is a reciprocal of TCR, between wavy surfaces with the thermal interface material (TIM) under a relatively low mean nominal contact pressure of 0.1–1.0 MPa. From comparison between the calculated and measured results, it has been shown that, in spite of a simple 1-D analysis, the present model predicts the temperature drop at the contact interface, which is obtained as the product of TCR and the heat rate flowing through TIM, within some 10 to 60% error for a TIM with the thermal conductivity of 2.3 W/(m·K) and the initial thickness of 0.5, 1 and 2 mm.

Presented are the results of a 3-D numerical analysis of a composite heat spreader for immersion cooling of a 20 × 20 mm microprocessor. The spreader is comprised of two 0.5 mm thick Copper (Cu) laments separated by a layer of highly ordered pyrolytic graphite (HOPG), 0.25–1.0 mm thick. The exposed surface of the top Cu lament has an average roughness, Ra = 1.79 μm and is cooled by saturation nucleate pool boiling of PF-5060 dielectric liquid. Investigate is the impact of δ HOPG on the total power removed, the maximum temperature of the underlying chip, T max , and mitigating the chip hot spots. Increasing δ HOPG increases the total power removed, but also increases T max . The spreader with a 1.0 mm-thick δ HOPG is capable of removing 318 W, without exceeding 90% of the critical heat flux (CHF), at T max = 120°C. This power removal is significantly higher than that with an all Cu spreader of the same thickness of 90 W, but at much lower T max of 67°C. Composite spreaders with δ HOPG = 0.25, 0.5, and 0.75 mm are capable of removing up to 160 W at T max = 85°C, 228 W at 100°C, and 292W at 115°C, respectively. The HOPG suppresses the transmission of hot spots to the spreader surface and increasing δ HOPG does not mitigate the hot spots.

RF-MEMS devices area complex systems governed by the interaction of a variety of forces, including electrostatics, solid deformation, fluid damping and contact. The performance and reliability of these devices is strongly dependent on device geometry and composition, and also on material microstructure and related properties. In this paper, we consider multiscale simulation of RF MEMS switched. At the device level, we introduce a comprehensive integrated numerical framework to simulate the major governing physics and their interactions. At the micron scale, we develop a mesoscale contact model to describe the history-dependent force-displacement relationships in terms of the surface roughness, the long-range attractive interaction between the two surfaces, and the repulsive interaction between contacting asperities (including elastic and plastic deformation). The inputs to this model are obtained from atomic level simulations and nanoscale surface topography characterization. The mesoscale contact model is integrated in the device-level simulation to predict the pull-in and pull-out behavior of these switches. The uncertainties associated with the simulation are quantified and propagated using a non-intrusive collocation method based on generalized Polynomial Chaos (gPC) expansions. With such a framework, we are able to predict the PDFs of pull-in and pull-out voltage, identify the critical factors that have the most influence on the quantities of interest, and therefore guide resource allocation and risk-informed decision-making.

This work is a comparative experimental study of two ways of heat transfer intensification inside the cold plate of an Indirect Liquid Cooled IT Server. The first way is classical, as surface extension or heat transfer coefficient enhancement by adding fins, turbulators, or surface roughness modification. The second is more recent, it is about using nanofluids. For comparison purpose, we have defined three criteria: thermal performance, hydraulic performance, and efficiency. According to those parameters, classical methods have shown better performances comparatively to nanofluids.

In the present paper, a new method is presented for evaluating heat-transfer materials. When electronic devices are mounted on a metallic heat sink, a heat-transfer material is required in order to handle the thermal flow from the devices while maintaining electrical insulation. However, the thermal characteristics of heat-transfer materials are strongly affected by their contact with the heat sink. The concept proposed in the present paper is to measure the thermal characteristics of a heat-transfer material placed on an aluminum plate that is machine milled to form precise grooves, which represent the surface roughness of the heat sink. When pressure is applied to the heat-transfer material, the material is pressed into these grooves, reducing the gaps between the two materials and consequently increasing the thermal conductance. By varying the shape of the grooves and the amount of pressure applied, the contact conditions can be precisely controlled. Thus, complex thermal parameters can be measured with good repeatability. In order to verify the effectiveness of the proposed method, an evaluation system was constructed, in which the temperature was controlled using flowing water. Tests were then carried out on two types of heat-transfer materials, with hardnesses of C10 and C75, as determined using an Asker C type hardness meter. Four aluminum plates with different groove shapes were used. For each combination of heat-transfer material and aluminum plate, an increase in the thermal conductance was clearly observed as the pressure increased, until saturation finally occurred at a particular pressure. This result was expected because, as the pressure increases, the gaps between the heat-transfer material and the aluminum plate are expected to decrease and finally disappear. A trade-off relationship between the thermal conductance of the heat-transfer material and its softness, the latter of which affects the pressure at which the gaps become filled, was also found to exist. The results obtained in the present study confirm the reliability and effectiveness of the proposed method. Based on an analysis of the coupled electrical and thermal phenomena in contact, we also propose the electrical method including the contact condition factor.

During service, micro-cracks form inside solder joints, making a microelectronic package prone to failure particularly during a drop. Hence, the understanding of the fracture behavior of solder joints under drop conditions, synonymously at high strain rates and in mixed mode, is critically important. This study reports: (i) the effects of processing conditions (reflow parameters and aging) on the microstructure and fracture behavior of Sn-3.8%Ag-0.7%Cu (SAC387) solder joints attached to Cu substrates, and (ii) the effects of the loading conditions (strain rate and loading angle) on the fracture toughness of these joints, especially at high strain rates. A methodology for calculating critical energy release rate, G C , was employed to quantify the fracture toughness of the joints. Two parameters, (i) effective thickness of the interfacial intermetallic compounds (IMC) layer, which is proportional to the product of the thickness and the roughness of the IMC layer, and (ii) yield strength of the solder, which depends on the solder microstructure and the loading rate, were identified as the dominant quantities affecting the fracture behavior of the solder joints. The fracture toughness of the solder joint decreased with an increase in the effective thickness of the IMC layer and the yield strength of the solder. A 2-dimensional fracture mechanism map with the effective thickness of the IMC layer and the yield strength of the solder as two axes and the fracture toughness as well as the fraction of different fracture paths as contour-lines was prepared. Trends in the fracture toughness of the solder joints and their correlation with the fracture modes are explained using the fracture mechanism map.

On the reliability evaluation, the phenomenon of surface roughness on the Al surface of DBA (Direct Brazed Aluminum) on power device is confirmed. This surface roughness is generated by cyclic thermal stress under thermal cycle test. In this study, the surface roughness characteristic after thermal cycle test is executed to DBA substrate and the generating mechanism was investigated. Furthermore, the effect on the fatigue life of solder joint on DBA substrate is also investigated.

Hydrophobic surfaces with microscale roughness can be rendered ultrahydrophobic by the addition of sub-micron scale roughness. A simple yet highly effective concept of fabricating hierarchical structured surfaces using a single-step deep reactive ion etch process is proposed. Using this method the complexities generally associated with fabrication of two-tier roughness structures are eliminated. Experiments are conducted on two double-roughness surfaces with different surface roughness, achieved by varying the size of the microscale roughness features. The surfaces are characterized in terms of static contact angle and roll-off angle and compared with surfaces consisting of only single-tier microscale roughness. The robustness of the new hierarchical roughness surfaces is verified through droplet impingement tests. The hierarchical surfaces are more resistant to wetting than the single roughness surfaces and show higher coefficients of restitution for droplets bouncing off the surface. The droplet dynamics upon impingement are explored.

Recently, ceramics has been using widely as living body material. It is called “bioceramics”. Both strength and elasticity of a living body material need almost three times larger than that of elaborate bone. However, it is difficult to satisfy the condition. So bioceramics is inferior to an elaborate bone. The purpose of this research is to obtain a basic data for improving the mechanical characteristics of bioceramics by the Acoustic Emission (AE) method. It was discussed how average roughness depends on mechanical characteristic and AE parameter of bioceramics. The following results were obtained. The first, the bioceramics showed a positive correlation between sound velocity and Kurtosis, that is one of the important parameter for the surface roughness. The second, it was clarified that same relation was obtained between Kurtosis and such mechanical parameters as stress, Young’s modulus and hardness. The third, these results were in accordance with the theory of fracture mechanics for fine ceramics reported independently by Rice and Aoki.

With recent worldwide-use and the rapid spread of electric and electronic equipment, printed wiring boards (PWBs) are being miniaturized and made multifunctional. Miniaturizing the equipment and making it high performance are accompanied by an increase in the number of small diameter through holes for circuit connections available to the PWBs. Such tendency needs the large number of smaller diameter drilling with shorter pitches. Moreover, the improvement of drilled-hole wall quality for PWBs might be demanded by such trends. Here, PWBs are often made of fiber-reinforced plastics (FRP) for the insulating layers and copper foil for the circuit layers. A woven glass fiber cloth is generally used for PWBs. Therefore, delamination might occur during drilling and affect the surface roughness of the drilled-hole wall. Such wrong surface roughness of drilled-hole wall should be avoided because it might reduce the reliability of insulating between next holes by ion migration of copper plating progressing along the delamination. Revealing of its mechanism is expected to be more important with smaller diameter with smaller electric devices. However, it might be difficult because the micr-drilling of FRP shows is a complex phenomenon because it consists of different materials. Therefore, this paper investigates the proper method to improve drilled-hole wall quality by evaluating the drill temperature rising mechanism and the surface generation mechanism of micro-drilled-hole walls of PWBs made of GFRP. The following results were reached: (1) A drill temperature rising mechanism was conducted. (2) Drill temperature tends to increase with the frictional torque between the hole wall and the margin part of drill. (3) Frictional torque increases with the spring-back of the drilled-hole wall during drilling. (4) The surface roughness of the drilled-hole wall of the GFRP plate is affected by the breakdown of the glass fiber and is mainly caused by the cutting edge of the outer corner except the margin part of the drill. (5) A drilling method with higher fiber bonding strength, such as drilling with compressing PWBs in the thickness direction, effectively improves the drilled-hole wall quality.

Thermally conductive composites as compared to metals have reduced density, decreased corrosion, oxidation, and chemical resistance, as well as adjustable properties to fit a given application. However, there are several challenges that need to be addressed before they can be successfully utilized in heat sink design. The interface between the device and thermal product that is used to cool it is an important factor in the thermal network designs of microelectronics cooling. Depending on the thermal interface conditions and material properties, the contact pressure and thermal stress level can attain undesirable values. In this paper, we investigate the effect of thermal interface between the fin and base plate on thermalstructural behavior of heat sinks. A coupled-field (thermal-structural) analysis using finite element method is performed to predict temperature as well as stress fields in the region of interface. In addition temperature and heat transfer rate predictions is supported through analytical results. Effect of various interface properties (such as air gap with rough surface and gaps filled with interface material) on the resulting thermal-structural response of the pin fin is investigated with respect to four interface materials combinations.

Silicon is the most widely used semiconductor material for MEMS and electronic applications. The strength of silicon has a critical impact on the reliable and accurate function of these components. In packaging assembly, reliability tests as well as in operation life the silicon die can be exposed to high stresses. For a theoretical risk assessment of die cracking due to the mentioned loadings a failure criterion is needed, which is capable to account the processing history. Therefore, the experimental characterization via fracture tests, analysis of different Si surface structures by Laser-Scanning-Microscopy (LSM) and Transmission-Electron-Microscopy (TEM) and a numerical approach using Finite Element Simulation was combined to allow failure prediction. In the fracture tests the influences of the backside grinding and the stress relief processes were investigated. The test dies were fabricated using the Dicing-before-Grinding (DBG) process with different mesh sizes of the grinding wheel. Additionally dry polishing and plasma etching were applied as stress relief processes. Bending tests are commonly used for the strength characterization of brittle materials like ceramics and silicon. The ball-ring-test was conducted to exclude edge effects on the strength, which are still under investigation in an additional part of the study. It was found that the determined fracture stresses follow a three-parametric Weibull distribution. The surface topography and roughness, respectively, was measured via LSM. TEM was applied to determine the structure of the surface-near damage-layer. In addition to the experimental investigations numerical calculations by means of Finite Element Simulations were performed in order to calculate critical fracture stresses from testing including anisotropic and geometric nonlinear effects. Furthermore, a submodeling-technique was applied to understand the effects of experimentally observed surface states on the critical stress, i.e. roughness, grinding marks, flaws, and poly Si-layers. Detailed modeling included local modeling of surface roughness on the critical strength.

The influence of surface roughness on flow boiling heat transfer and pressure drop in microchannels is experimentally explored. The microchannel heat sink employed in the study consists of 10 parallel, 25.4 mm long channels with nominal dimensions of 500 μm × 500 μm. The channels were produced by saw-cutting. Two of the test piece surfaces were roughened to varying degrees with electrical discharge machining (EDM). The roughness average, R a , varied from 1.4 μm for the as-fabricated, saw-cut surface to 3.9 and 6.7 μm for the two roughened EDM surfaces. Deionized water was used as the working fluid. Experiments indicate that the surface roughness has little influence on boiling incipience and only a minor impact on saturated boiling heat transfer coefficients at lower heat fluxes. For wall heat fluxes above 1500 kW/m 2 , the two EDM surfaces (3.9 and 6.7 μm) have similar heat transfer coefficients that were 20 to 35% higher than those measured for the saw cut surface (1.4 μm). Analysis of the pressure drop measurements indicates that only the roughest surface (6.7 μm) has an adverse effect on the two-phase pressure drop.

Recently, electronics systems have become more complex resulting in a need for system design method that considers multiple physical phenomenons. However, it is difficult at the electronic system design level to reduce the design time and realize high precision. We have developed a collaborative highspeed thermal/circuit design method with a modularized model [1–2]. In this paper, we reconsider the optimization method of this design system. We obtain the local minimum solution for a very high-density device system. However, because the constraints associated with both thermal and electrical design become very strong, we applied an agent-oriented optimization method in defining the boundary conditions between device module and board module to ease the constraints on the thermal layout design. This method is characterized by making the design for device module design intelligent. In a case study, we achieved more than a 20% improvement in the design solution in the Pareto curve. We also reduced the design time by more than 10 times (one weight pattern: about 30sec, Pareto solution set: about 600sec). This enables the designer of electronics system to compare Pareto solution sets within a short time and to evaluate many different layout designs during the rough design phase of a system design.