Flexible devices, considered to be the next wave of electronics industry, require flexible encapsulation for protection while conforming to flexibility-needs in end applications. The characteristics of flexible electronics is not only reduced production cost but also thinner, lightweight and non-breakable which creates a new form application for the electronic devices. One such application is use to electronic devices in daily environment to monitor the vitals of one’s body. These devices are often expose to dust, sweat and moisture also they are generally subjected to flexing and folding motion which accrues stresses in those devices. These stresses and the harsh environment are often mitigated by using potting compounds, encapsulants to improve their survivability of the devices. In our study, we have chosen five different formulation of encapsulant subjected it various cure profiles to determine the adhesive strength of the various encapsulant. The benchmark peel strength was developed using a FE-model of the AU-biometric band and encapsulant peel strength at experimental conditions were compared to give us the best performing material. This paper includes the sample geometry which consists of five different encapsulants and two different substrate namely, polyimide and pet tested at four different cure schedule while the substrates were cleaned using two different cleaning methods. The encapulants are compared among each other to create a rank for possible future applications in FHE devices.

Quantifying the resistance to heat flow across well-bonded, planar interfaces is critical in modern electronics packaging architectures, particularly as device length scales are reduced and power demands continue to grow unabated. However, very few experimental techniques are capable of measuring the thermal resistance across such interfaces due to limitations in the required measurement resolution provided by the characterization technique (i.e., Rth < 0.1 mm 2 ·K/W in steady-state configurations) and restrictions on the thermal penetration depth that can be achieved as a result of the heating event that is typically imposed on a sample’s surface (for optical pump-probe thermoreflectance techniques). A recent numerical fitting routine for Frequency-domain Thermoreflectance (FDTR) developed by the authors 1 offers a potential avenue to rectify these issues if the transducer’s geometry can be confined. This work utilizes numerical simulations to evaluate the sensitivity of FDTR to a range of thermal boundary resistance (TBR) values as a function of the thermal resistance of adjacent material layers. Experimental measurements are performed across a handful of different material systems to validate our computational results and to demonstrate the the extent to which confined transducer geometries can improve our sensitivyt to the TBR across so-called “buried” interfaces when characterized with FDTR.

The present study is focused on the experimental characterization of two-phase heat transfer performance and pressure drops within an ultra-compact heat exchanger (UCHE) suitable for electronics cooling applications. In this specific work, the UCHE prototype is anticipated to be a critical component for realizing a new passive two-phase cooling technology for high-power server racks, as it is more compact and lighter weight than conventional heat exchangers. This technology makes use of a novel combination of thermosyphon loops, at the server-level and rack-level, to passively cool an entire rack. In the proposed two-phase cooling technology, a smaller form factor UCHE is used to transfer heat from the server-level thermosyphon cooling loop to the rack-level thermosyphon cooling loop, while a larger form factor UCHE is used to reject the total heat from the server rack into the facility-level cooling loop. The UCHE is composed of a double-side-copper finned plate enclosed in a stainless steel enclosure. The geometry of the fins and channels on both sides are optimized to enhance the heat transfer performance and flow stability, while minimizing the pressure drops. These features make the UCHE the ideal component for thermosyphon cooling systems, where low pressure drops are required to achieve high passive flow circulation rates and thus achieve high critical heat flux values. The UCHE’s thermal-hydraulic performance is first evaluated in a pump-driven system at the Laboratory of Heat and Mass Transfer (LTCM-EPFL), where experiments include many configurations and operating conditions. Then, the UCHE is installed and tested as the condenser of a thermosyphon loop that rejects heat to a pumped refrigerant system at Nokia Bell Labs, in which both sides operate with refrigerants in phase change (condensation-to-boiling). Experimental results demonstrate high thermal performance with a maximum heat dissipation density of 5455 (kW/m 3 /K), which is significantly larger than conventional air-cooled heat exchangers and liquid-cooled small pressing depth brazed plate heat exchangers. Finally, a thermal performance analysis is presented that provides guidelines in terms of heat density dissipations at the server- and rack-level when using passive two-phase cooling.

In this study, we proposed a flexible method to realize a cylindrical tuber encapsulant layer. Firstly, patterned LED substrate with silicone-wetting and silicone-repellency surfaces was prepared by depositing low surface energy modified nanosilica particles. Secondly, coating phosphor gel onto the LED chip. Thirdly, coating the encapsulant (silicone OE6550A/B) onto the phosphor gel layer. The silicone stops spreading at the border of silicone-wetting and silicone-repellency surfaces and its final contact angle can be adjusted to any value between the contact angle of silicone-wetting and silicone-repellency surface, so it is very easy to realize dome-shape silicone layer with contact angle of ∼90°. For COB-LEDs with LED array in series, the final silicone layer geometry is similar to the cylindrical tuber. The results show that compared to the conventional flat encapsulant layer, the proposed encapsulant layer can improve the light efficiency by > 60% for pure blue light and 13.6% for white light at correlated color temperature (CCT) of ∼5500K.

As electronic devices continue to shrink in size and increase in functionality, effective thermal management has become a critical bottleneck that hinders continued advancement. Two-phase cooling technologies are of growing interest for electronics cooling due to their high heat removal capacity and small thermal resistance (< 0.3 K-cm 2 /W) [1]. One typical example of a two-phase cooling method is droplet evaporation, which can provide a high heat transfer coefficient with low superheat. While droplet evaporation has been studied extensively and used in many practical cooling applications (e.g., spray cooling), the relevant work has been confined to spherical droplets with axisymmetric geometries. A rationally designed evaporation platform that yields asymmetric meniscus droplets can potentially achieve larger meniscus curvatures, which give rise to higher vapor concentration gradients along the contact line region and therefore yield higher evaporation rates. In this study, we develop a numerical model to investigate the evaporation behavior of asymmetrical microdroplets suspended on a porous micropillar structure. The equilibrium profiles and mass transport characteristics of droplets with circular, triangular, and square contact shapes are explored using the Volume of Fluid (VOF) method. The evaporative mass transport at the liquid-vapor interface is modeled using a simplified Schrage model [2]. The results show highly non-uniform mass transport characteristics for asymmetrical microdroplets, where a higher local evaporation rate is observed near the locations where the meniscus has high curvature. This phenomenon is attributed to a higher local vapor concentration gradient that drives faster vapor diffusion at more curved regions, similar to a lightning rod exhibiting a strong electric field along a highly curved surface. By using contact line confinement to artificially tune the droplet into a more curved geometry, we find the total evaporation rate from a triangular-based droplet is enhanced by 13% compared to a spherical droplet with the same perimeter and liquid-vapor interfacial area. Such a finding can guide the design and optimization of geometric features to improve evaporation in high performance electronics cooling systems.

Electronics in automotive underhood and downhole drilling applications may be subjected to sustained operation at high temperature in addition to high strain-rate loads. SAC solders used for second level interconnects have been shown to experience degradation in high strain-rate mechanical properties under sustained exposure to high temperatures. Industry search for solutions for resisting the high-temperature degradation of SAC solders has focused on the addition of dopants to the alloy. In this study, a doped SAC solder called SAC-Q solder have been studied. The high strain rate mechanical properties of SAC-Q solder have been studied under elevated temperatures up to 200°C. Samples with thermal aging at 50°C for up to 6-months have been used for measurements in uniaxial tensile tests. Measurements for SAC-Q have been compared to SAC105 and SAC305 for identical test conditions and sample geometry. Data from the SAC-Q measurements has been fit to the Anand Viscoplasticity model. In order to assess the predictive power of the model, the computed Anand parameters have been used to simulate the uniaxial tensile test and the model predictions compared with experimental data. Model predictions show good correlation with experimental measurements. The presented approach extends the Anand Model to include thermal aging effects.

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.

Metal lines used in integrated circuits (ICs) become narrow for raising the device performance. Due to scaling down of the ICs, current density and Joule heating are increased, which induces electromigration (EM) damage. EM is transportation phenomena of metallic atoms caused by electron wind under high current density. EM leads to hillock and void formation in the metal line, thus EM should be considered to evaluate the performances of the device safe. It is known that a value of threshold current density which is critical current density of the EM damage exists in via-connected and passivated lines. In this study, the effect of line geometry on the threshold current density is discussed in the case of taper-shaped line. The evaluation method of threshold current density is conducted based on numerical simulation technique with building-up processes of atomic density distribution in the metal line by using a governing parameter of EM damage. As the simulation results, threshold current density increased in the cases of shorter line length, lower temperature, and wider width in cathode side. Furthermore, a new parameter was proposed for simplified evaluation of the threshold current density in taper-shaped lines. The evaluation method is able to apply various line shapes and conditions and it is expected to use for confirmation of the reliability of the lines in circuit design processes.

Two-phase cooling has become an increasingly attractive option for thermal management of high-heat flux electronics. Cooling channels embedded directly on the back of the heat source (chip) facilitate two-phase boiling/evaporation effectiveness, eliminating many thermal resistances generated by more traditional, remote chip-cooling approaches. Accordingly, manifold-microchannel flow paths in embedded cooling systems can allow very high heat fluxes with low junction temperatures. But, the effect of the feeding manifold design, channel geometry, and the associated shear, stagnation zones, and centripetal accelerations with varying heat flux and mass flux are not well understood. This study builds upon our previous work and elucidates effects of channel geometry, mass flux, and outlet quality on the boiling/evaporation flow regimes in a manifolded microgap channel.

In this study, we proposed a bell shape phosphor layer geometry and the corresponding dual-step phosphor coating method for enhancing the angular color uniformity (ACU) of phosphor-converted white light-emitting diodes (pcLEDs). Numerical simulation based on Volume of Fluid (VOF) model was applied to predict phosphor geometries. Based on the simulated results, experiments were conducted to realize the phosphor geometries. The simulated results show that the VOF model can predict the phosphor geometries with an acceptable geometric deviation within 5%. The experimental results show that compared with the spherical cap phosphor layer geometry, the bell shape geometry can achieve better ACU performance, an optimal bell phosphor layer geometry with equal coating volume above and around the LED chip was achieved, for the corrected color temperature (CCT) of 4000 K, the angular CCT deviation of the optimal geometry is 62 K, while it is 382 K for the spherical cap geometry.

Flexible electronics provide new design options not afforded by rigid electronics in a variety of applications including wearable electronics, robotics and automotive systems. However, the processes for the manufacturing of complex electronic assemblies using fine-pitch components are not as well developed as those for rigid electronics. The lack of structural rigidity of flexible printed circuit cards requires attention to assembly configuration for double-sided flexible assemblies. In addition, mechanisms are needed to compensate for the deformation and warpage of the flexible substrate and components during assembly. In this paper, the stresses in solder joints of double-sided flexible assemblies have been measured during thermal excursions using x-ray micro-computed tomography in conjunction with digital volume correlation. The method allows for non-invasive measurement and does not require cross-sectioning of the part for the purpose of deformation and strain measurement. In addition, the measurements are not limited to the joints in the line of sight. The three-dimensional measurements of deformation and strain have been visualized on the geometry of the solder joints in the package. Digital volume correlation (DVC) method has been used to find the displacements and strains in interconnects of operational electronics. The x-ray microscopic computed tomography (μCT) system has been used to generate the 16 bit digital volume data. The x-ray detector has ability to image the x-ray attenuation of x-rays through the object. Reliability testing of SAC 305 solder interconnects has been performed on double-sided flexible circuit board using x-ray μCT by heating the package to 100°C. The flexible circuit board used in this experiment is of BGA 256-144 combination, two packages, A-PBGA256-1.0mm-17mm and A-CABGA144-1.0mm-13mm. A 3D printed fixture has also been used to support the flexible board and keep it flat while in the CT scan machine. The reference and deformed scans are then re-constructed 3D using Volume Graphics, and Digital Volume Correlation performed using MATLAB modules. Reliability of double-sided flexible printed circuit boards will be discussed and any crack, defects, or deformation in the solder interconnectivity which might occur while heating the package on flexible board is presented. The solder joint strains during thermal excursions are also compared between the flexible and rigid printed circuit assemblies.

In this paper, the effect of pretest isothermal aging and in-test aging on the fatigue behavior of Sn3.0Ag0.5Cu alloy are examined using the microstructurally adaptive creep model (MACM) and the maximum entropy fracture model (MEFM). Compared to traditional constitutive models, the MACM considers the effect of thermal history. Two microstructural parameters, the average Ag 3 Sn particle size and the average primary-Sn cell size, are identified as critical to capturing the aging behavior of SnAgCu alloys and are incorporated into a modified Dorn creep form. The MEFM is next utilized to characterize the damage accumulation rate in the Sn3.0Ag0.5Cu solder alloy. The MEFM uses a single damage accumulation parameter, which connects the accumulated damage to the accumulated in-elastic dissipation. This parameter is independent of sample geometry, test temperature and strain rate. The concepts of static aging and dynamic aging are utilized to describe pretest aging and in-test aging. In 25° C tests, with longer static aging, a faster fatigue damage accumulation results. Through the relationship between the damage accumulation rate and the average primary-Sn cell size, the influence of microstructural evolution introduced by static aging on fatigue behavior is confirmed. In 100° C tests, the dynamic aging causes further rapid damage accumulation relative to the 25° C tests.

Topology optimization of an air-cooled heat sink considering heat conduction plus side-surface convection is presented. The optimization formulation is explained along with multiple design examples. A post-processing procedure is described to synthesize water-tight solid model computer-aided design (CAD) geometry from 3-D point-cloud data extracted from the optimization result. Using this process, a heat sink is optimized for confined jet impingement air cooling. A prototype structure is fabricated out of AlSi12 using additive layer manufacturing (ALM). The heat transfer and fluid flow performance of the optimized heat sink is experimentally evaluated, and the results are compared with benchmark plate and pin-fin heat sink geometries that are conventionally machined out of aluminum and copper. In two separate test cases, the experimental results indicate that the optimized ALM heat sink design has a higher coefficient of performance relative to the benchmark heat sink designs.

This study considers the optimization of a complex micro-scale cooling geometry that represents a unit-cell of a full heat sink microstructure. The configuration consists of a channel with a rectangular cross section and a hydraulic diameter of 100 μm, where the fluid flows between two cooling fins connected by rectangular crossbars (50 × 50 μm). A previous investigation showed that adding these crossbars at certain locations in the flow can increase the heat transfer in the microchannel, and in the present work we perform an optimization to determine the optimal location and number of crossbars. The optimization problem is defined using 12 discrete design parameters, which represent 12 crossbars at different locations in the channel that can either be turned off and become part of the fluid domain, or turned on and become part of the solid domain. The optimization was done using conjugate heat transfer computational fluid dynamics (CFD) simulations using Fluent 15.0. All possible 4096 configurations were simulated for one set of boundary conditions. The domain was discretized using about 1 million nodes combined for the fluid and solid domains and the computational time was around 1 CPU hour per case. The results show that further improvements in heat transfer are feasible at an optimized pressure drop. The results cover a range of pressure drops from 25 kPa to almost 90 kPa and the heat transfer coefficient varies from 60 to 120 kW/m2K. The configurations on the Pareto front show the trend that crossbars closer to the maximal fluid-solid interface result in a more optimal performance than crossbars positioned farther away. In addition to performing simulations for all possible configurations, the potential of using a genetic algorithm to identify the configurations that define the Pareto front was explored, demonstrating that a 80% reduction in computational time can be achieved. The results of this study demonstrate the significant increase in performance that can be obtained through the use of computational tools and optimization algorithms for the design of single phase cooling devices.

We demonstrate the Lid-Integral Silicon Coldplate topology as a way to bring liquid cooling closer to the heat source IC. It allows to eliminate one thermal interface material (TIM2), to establish and improve TIM1 during packaging, to use wafer-level processes, and to ease integration in 1st level packaging. We describe the integration, and analyze reliability aspects of this package using modeling and test vehicle builts. To compare the impact of geometry, materials and mechanical coupling on warpage, strains and stresses, we simulate finite element models of five different topologies on an organic LGA carrier. We measure the thermal performance in terms of thermal resistance from coldplate base to inlet liquid and obtain 15mm 2 K/W at 30 kPa pressure drop across the package. We build two different topologies using silicon coldplates and injection molded lids. Gasket-attached coldplates pass an 800 kPa pressure test, direct-attached coldplates fracture in the coldplate. The results advise to use a compliant layer between coldplate and the manifold lid and promise a uniformly thick TIM1 layer in the Si-Si matched topology. The work shows the feasibility of composite lids with integrated silicon coldplates in high heat flux applications.

Under the DARPA-sponsored ICECool Applications program, a microchannel cooling system using a 50-50 ethylene glycol-water mixture was optimized for cooling a high-power GaN-on-Diamond Monolithic Microwave Integrated Circuit (MMIC). Automated multi-objective optimization of the microchannel passages yielded an optimized design with a predicted thermal resistance of 22.4 K·cm 2 /kW at a pressure drop of only 121.4 kPa for an inlet temperature of 40°C. These values were corroborated by a coupled thermofluid analysis that included a detailed treatment of both the gate region and microchannel cooling geometry. Several versions of prototype coolers were fabricated, with one set consisting of pairs of coolers joined at their heated faces. These cooler pairs were used in heat exchange tests to characterize the average thermal resistance and the flow performance of the coolers. The performance testing results were consistent with the analytic predictions. Based on the analytical and experimental results, the system may be operated at inlet temperatures as high as 65°C without exceeding the transistor junction temperatures of 240 °C required for 10 6 hour mean-time-to -failure. The higher inlet temperature ameliorates system penalties associated with rejection of waste heat to ambient heat sinks.

Even as the use of flexible graphite heat spreaders becomes ubiquitous in mobile electronics, numerically quantifying the heat dissipation remains a challenge. The rapid pace of development of mobile devices has deterred the industry from establishing standards, and rules of thumb are few, as are closed-form solutions. Users have requested numerical methods and tools to simplify the selection of flexible graphite heat spreaders from among the standard thicknesses and grades, as well as to quantify the effect of changing heat transfer area and configuration. In the presence of adjacent layers — adhesives, dielectrics, or still air gaps — the thin nature of the materials and the high, orthogonal thermal conductivity ratios of the graphite combine to create a complex conjugate heat transfer problem. Although the thinnest of these sheets constitute but a tiny fraction of the thickness of a cell phone or tablet, their dominant role in the heat transfer requires that they not be neglected in the calculations. Some CFD software guidelines advise using multiple meshing layers to capture fully the heat transfer in these spreaders, while others (primarily FEA based) provide a plate element that negates the need for discretization. In the former, a fully meshed spreader confounds the goal of a quick calculation, but the flexibility of 3D solution also demands meticulous attention to the details, provides “an answer” that is easy to misinterpret, and in the hands of an unskilled user, invites error. The goal of this project is to establish the guidelines for computing heat spreading in graphite, including cell dimension ratio, mesh density, spreading radius, and transport capacity and to marry the orthogonal properties of the material with the row-column format of a spreadsheet or matrix software. It also reviews methods for addressing the non-orthotropic situations such as angled plates, and the curved surfaces seen in the case of graphite wraps and flexible hinges. There are cases in which a simple contact resistance value adequately represent a graphite thermal interface material, but others that require an accounting for the lateral conductivity that increases the efficacy of the TIM. Finally, the error of the calculation is assessed for a simple representative geometry.

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that the observed material behavior degradations of Sn-Ag-Cu (SAC) lead free solders during room temperature aging (25 C) and elevated temperature aging (50, 75, 100, 125, and 150 C) were unexpectedly large. The measured stress-strain data demonstrated large reductions in stiffness, yield stress, ultimate strength, and strain to failure (up to 50%) during the first 6 months after reflow solidification. In this study, we have used both accelerated life testing and finite element modeling to explore how prior isothermal aging affects the overall reliability of PBGA packages subjected to thermal cycling. In the experimental work, an extensive test matrix of thermal cycling reliability testing has been performed using a test vehicle incorporating several sizes (5, 10, 15, 19 mm) of BGA daisy chain components with 0.4 and 0.8 mm solder joint pitches (SAC305). PCB test boards with 3 different surface finishes (ImAg, ENIG and ENEPIG) were utilized. In this paper, we concentrate on the reporting the results for a PBGA component with 15 mm body size. Before thermal cycling began, the assembled test boards were divided up into test groups that were subjected to several sets of aging conditions (preconditioning) including 0, 6, and 12 months aging at T = 125 °C. After aging, the assemblies were subjected to thermal cycling (−40 to +125 °C) until failure occurred. The Weibull data failure plots have demonstrated that the thermal cycling reliabilities of pre-aged assemblies were significantly less than those of non-aged assemblies. A three-dimensional finite element model of the tested 15 mm PBGA packages was also developed. The cross-sectional details of the solder ball and the internal structure of the BGA were examined by scanning electron microscopy (SEM) to capture the real geometry of the package. Simulations of thermal cycling from −40 to 125 C were performed. To include the effects of aging in the calculations, we have used a revised set of Anand viscoplastic stress-strain relations for the SAC305 Pb-free solder material that includes material parameters that evolve with the thermal history of the solder material. The accumulated plastic work (energy density dissipation) was used is the failure variable; and the Darveaux approach to predict crack initiation and crack growth was applied with aging dependent parameters to estimate the fatigue lives of the studied packages. We have obtained good correlation between our new reliability modeling procedure that includes aging and the measured solder joint reliability data. As expected from our prior studies on degradation of SAC material properties with aging, the reliability reductions were more severe for higher aging temperature and longer aging times.

This study considers CFD simulations with conjugate heat transfer performed in the framework of designing a complex micro-scale cooling geometry. The numerical investigation of the three-dimensional, laminar flow (Reynolds number smaller than 480) and the solid conduction is done on a reduced model of the heat sink micro-structure to enable exploring a variety of configurations at a limited computational cost. The reduced model represents a unit-cell, and uses periodic and symmetry boundary conditions to mimic the conditions in the entire cooling manifold. A simulation of the entire heat sink micro-structure was performed to verify the unit-cell set-up, and the comparison demonstrated that the unit-cell simulations allow reducing the computational cost by two orders of magnitude while retaining accurate results. The baseline design for the unit-cell represents a configuration used in traditional electronic heat sinks, i.e. a simple channel geometry with a rectangular cross section, with a diameter of 50 μ m , where the fluid flows between two cooling fins. Subsequently three types of modified geometries with feature sizes of 50 μ m were considered: baffled geometries that guide the flow towards the hotspot region, geometries where the fins are connected by crossbars, and a woodpile structure without cooling fins. Three different mass-flow rates were tested. Based on the medium mass-flow rate considered, the woodpile geometry showed the highest heat transfer coefficient with an increase of 70% compared to the baseline geometry, but at the cost of increasing the pressure drop by more than 300%. The crossbar geometries were shown to be promising configurations, with increases in the heat transfer coefficient of more than 20% for a 70% increase in pressure drop. The potential for further optimization of the crossbar configurations by adding or removing individual crossbars will be investigated in a follow up study. The results presented demonstrate the increase in performance that can be obtained by investigating a variety of designs for single phase cooling devices using unit-cell conjugate heat transfer simulations.

Interconnects that can deform under monotonous and/or repeated loading are increasingly important to a new class of electronic devices used for wearable applications. Such interconnects integrate different material sets such as polymers and metallic conductors and are subjected to large strain levels. A typical method to overcome the material incompatibility involves the conductor in the form of a serpentine or an out-of-the plane buckled geometry. In this paper, we demonstrate a novel combination of interconnect materials that enables significant improvement in the interconnect stretchability using Indium over the state-of-the-art without affecting the system performance. This was achieved without the necessity of the serpentine interconnects geometry that significantly improves the routing density. The manufacturing method used for this approach is also described. Finally, we discuss the cost competitiveness of the materials and the manufacturing method to assess the commercial viability of this approach. (5nm)