A Molecular Dynamics Study of Thermal Conductivity in Nanocomposites via the Phonon Wave Packet Method

The phonon wave packet technique is used in conjunction with the molecular dynamics simulations to directly observe phonon scattering at material interfaces. The phonon transmission coefficient of nanocomposites is examined as a function of the defect size, thin film thickness, orientation of interface to the heat flow direction. To generalize the results based on phonons in a narrow frequency range and at normal incidence, the effective thermal conductivity of the same nanocomposite structure is calculated using non-equilibrium molecular dynamics simulations for model nanocomposites formed by two mass-mismatched Ar-like solids and heterogeneous Si-SiCO2 systems. The results are compared with the modified effective medium formulation for nanocomposites.

Cu-Direct Laser Drilling of Blind Via-Hole in Multi-Layer PWBs: Influence of Surface Treatment on Drilled Hole Quality

This report describes the quality assessment of Blind Via Holes (BVHs) of Printed Wiring Boards (PWBs) drilled by a CO2 laser using Cu-direct drilling. In the Cu-direct drilling method, the copper foil and the build-up layer are melted at the same time, and the surface is treated to increase the laser energy absorbed by the copper foil since an untreated copper surface reflects most of the 10.6-μm-wavelength CO2 laser beam. However, there are few reports dealing with Cu-direct laser drilling of PWBs. In addition, when copper and resin with different processing thresholds are drilled at the same time, occurrences of a defect called overhang have been observed. So, in this report, first we propose a new method using thermography to measure the absorptance of a PWB surface for a CO2 laser. Moreover, we investigate how surface treatment of the outer copper foil influences the quality of a laser-drilled hole. Then, we observe the circumference of a point irradiated with the CO2 laser and explain how melting processes are different from surface treatment. Finally, based on the research we establish a method in order to cut down the overhang length as a parameter of drilled-hole quality. We also show that a high absorptance improves BVH quality.

Damage Accumulation and Life-Prediction Models for SnAgCu Leadfree Electronics Under Shock-Impact

Relative damage-index based on the leadfree interconnect transient strain history from digital image correlation, explicit finite-elements, cohesive-zone elements, and component’s survivability envelope has been developed for life-prediction of two-leadfree electronic alloy systems. Life prediction of pristine and thermally-aged assemblies, have been investigated. Solder alloy system studied include Sn1Ag0.5Cu, and 96.5Sn3.5Ag. Transient strains during the shock-impact have been measured using digital image correlation in conjunction with high-speed cameras operating at 50,000 fps. Both the board strains and the package strains have been measured in a variety of drop orientations including JEDEC horizontal drop orientation, vertical drop orientation and intermediate drop orientations. In addition the effect of sequential stresses of thermal aging and shock-impact on the failure mechanisms has also been studied. The thermal aging condition used for the study includes 125°C for 100 hrs. The presented methodology addresses the need for life prediction of new lead-free alloy-systems under shock and vibration, which is largely beyond the state of art. Three failure modes have been predicted including interfacial failure at the copper-solder interface, solder-PCB interface, and the solder joint failure. Explicit non-linear finite element models with cohesive-zone elements have been developed and correlated with experimental results. Velocity data from digital image correlation has been used to drive the attachment degrees of freedom of the submodel and extract transient interconnect strain histories. Explicit finite-element sub-modeling has been correlated with the full-field strain in various locations, orientations, on both the package and the board-side. The survivability of the leadfree interconnections under sequential loading (thermal aging and shock-impact) from simulation has been compared with pristine circuit assemblies subjected to shock-impact. Sequential loading changes the failure modes and decreases the drop reliability as compared to the room temperature experimental results. Damage index based survivability envelope is intended for component integration to ensure reliability in harsh environments.

Experimental Study of the High Cycle Fatigue of Thin Film Metal on Polyethylene Terephthalate for Flexible Electronics Applications

Flexible electronics represent an emerging area in the electronics packaging and systems integration industry with the potential for new product development and commercialization in the near future. Manufacturing electronics on flexible substrates will produce low cost devices that are rugged, light, and flexible. However, electronic systems are vulnerable to failures caused by mechanical and thermal stresses. For electronic systems on flexible substrates repeated stresses below the ultimate tensile strength or even below the yield strength will cause failures in the thin films. It is known that mechanical properties of thin films are different from those of bulk materials; so, it is difficult to extrapolate bulk material properties on thin film materials. The objective of this work is to study the behavior of thin-film metal coated flexible substrates under high cyclic bending fatigue loading. Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are widely used substrates in the fabrication of microelectronic devices. Factors affecting the fatigue life of thin-film coated flexible substrates were studied, including thin film thickness, temperature, and humidity. A series of experiments for sputter-deposited copper on PET substrates were performed. Electrical resistance and crack growth rate were monitored during the experiments at specified time intervals. High magnification images were obtained to observe the crack initiation and propagation in the metal film. Statistical analysis based on design of experiments concepts was performed to identify the main factors and factor’s interaction that affect the life of a thin-film coated substrate. The results of the experiments showed that the crack starts in the middle of the sample and slowly grows toward the edges. Electrical resistance increases slightly during the test until the crack length covers about 90% of the total width of the sample where a dramatic increase in the resistance takes place.

The IP Landscape for MEMS Packaging

Early MEMS devices employed packages developed for conventional semiconductor microelectronics. Today, MEMS packages reflect the unique environment, mechanical, chemical and thermal requirements of MEMS devices themselves. A casual search of on-line databases reveals nearly 40,000 patents worldwide containing the words “MEMS” and “package.” While not all relevant, the number of IP documents easily overwhelms researchers, investors and IP practitioners. The authors systematically analyze the relevant IP and organize it by generic technology categories. A unique mapping methodology provides greater understanding of the landscape of IP in the MEMS packaging arena across a wide range of considerations including geography, IP development and ownership trends, infrastructure implications and application concepts. The authors also present a rudimentary valuation of IP within the MEMS packaging field based on citation analysis. Finally, the authors demonstrate a method to develop a strategic framework based on the IP landscape useful for investment, market development and strategic alliance planning.

Anomaly-Detection and Prognostication of Electronics Subjected to Shock and Vibration

Failures in electronics subjected to shock and vibration are typically diagnosed using the built-in self test (BIST) or using continuity monitoring of daisy-chained packages. The BIST which is extensively used for diagnostics or identification of failure, is focused on reactive failure detection and provides limited insight into reliability and residual life. In this paper, a new technique has been developed for health monitoring and failure mode classification based on measured damage precursors. A feature extraction technique in the joint-time frequency domain has been developed along with pattern classifiers for fault diagnosis of electronics at product-level. The Karhunen Loe´ve transform (KLT) has been used for feature reduction and de-correlation of the feature vectors for fault mode classification in electronic assemblies. Euclidean, and Mahalanobis, and Bayesian distance classifiers based on joint-time frequency analysis, have been used for classification of the resulting feature space. Previously, the authors have developed damage pre-cursors based on time and spectral techniques for health monitoring of electronics without reliance on continuity data from daisy-chained packages. Statistical Pattern Recognition techniques based on wavelet packet energy decomposition [Lall 2006a] have been studied by authors for quantification of shock damage in electronic assemblies, and auto-regressive moving average, and time-frequency techniques have been investigated for system identification, condition monitoring, and fault detection and diagnosis in electronic systems [Lall 2008]. However, identification of specific failure modes was not possible. In this paper, various fault modes such as solder inter-connect failure, inter-connect missing, chip delamination chip cracking etc in various packaging architectures have been classified using clustering of feature vectors based on the KLT approach [Goumas 2002]. The KLT de-correlates the feature space and identifies dominant directions to describe the space, eliminating directions that encode little useful information about the features [Qian 1996, Schalkoff 1972, Theodoridis 1998, Tou 1974]. The clustered damage pre-cursors have been correlated with underlying damage. Several chip-scale packages have been studied, with leadfree second-level interconnects including SAC105, SAC305 alloys. Transient strain has been measured during the drop-event using digital image correlation and high-speed cameras operating at 100,000 fps. Continuity has been monitored simultaneously for failure identification. Fault-mode classification has been done using KLT and joint-time-frequency analysis of the experimental data. In addition, explicit finite element models have been developed and various kinds of failure modes have been simulated such as solder ball cracking, trace fracture, package falloff and solder ball failure. Models using cohesive elements present at the solder joint-copper pad interface at both the PCB and package side have also been created to study the traction-separation behavior of solder. Fault modes predicted by simulation based pre-cursors have been correlated with those from experimental data.

Multiscale Modeling of Hot Spots in GaN High Electron Mobility Transistors

A coupled Lattice Boltzmann (LB)-Finite Difference (FD) method is used to solve for the heat transport in a 6 finger GaN high electron mobility transistor. The LB method is used to capture relevant phonon physics near a microscopic heat generation region by solving the Boltzmann Transport Equation, while an FD model is used to capture the thermal transport at the macroscopic level. The coupling region between the LB and FD domains, which enables multiscale modeling, is discussed. The results of the multiscale models were compared to results generated from other numerical methods. An increasing departure from diffusion theory is observed with increasing dissipated power under the gray phonon model. This difference is attributed to a combination of boundary scattering effects as well as phonon confinement within the small dimensions of the hot spot.

Performance Study of a GaAs Based Laser Diode Chip in a Condensing Environment

This paper presents a performance study done on semiconductor laser diodes in a moisture condensing environment. Devices with laser diodes are used in a wide variety of electronic applications and in various climatic conditions. The motivation behind this study is a common environmental exposure, where a device using a laser diode is brought into a relatively humid environment (a building) from a cold, outside environment. Under such conditions, condensation occurs on various components of the device, including the diode, and could affect the laser output power. Reliability of the device is a critical concern since the laser diode and the lens are susceptible to failure due to such repetitive condensation conditions. The test vehicle chosen for this study was a 980nm laser diode. These are used in products for a broad range of markets, including data communications, aerospace, material processing, scientific and defense industries [1–3]. These products may be used in environmental conditions that could result in condensation within the product. A hermetic package could address this concern, but it is an expensive option. Nonhermetic packaging for the laser component could help lower the cost of these devices; however reliability is a potential concern. Prior research on laser diodes consists of various reliability measurements on 980nm lasers using stress tests (e.g. accelerated aging tests; thermal cycling tests) [3–6]. Reliability analysis of laser diodes specifically addressing condensation measurements has not been previously reported. A Military Standard Specification [MIL-STD-883E Method 1004.7] titled, ‘Moisture resistance test’ was used to conduct this reliability study [10]. An experimental setup was designed and fabricated. A photonic package with a 980nm laser diode was subjected to repetitive condensing cycles and laser output power was recorded as a function of time, temperature and humidity. The variation in laser output power due to condensation was observed and quantified. The focus of this paper is on performance degradation of the laser diode. The possible mechanisms for this degradation are currently being investigated.

Localized High-Resolution Stress Measurements on MEMS Structures

Three different methods of stress measurement with strong spatial resolution are presented. They base on stress relief techniques caused by focused ion beam milling, on altered electron backscattering by deformed lattices and on Stokes line shift measurements by Raman spectroscopy. The capability of these methods is demonstrated by their application to typical MEMS structures. A comparison between the methods is performed in order to outline potentials and limitations.

Evaluation of the Bonding Strength at the Three-Dimensional Vertex in Silicon-Resin Joints

Portable electric devices such as mobile phone and portable music player become compact and also their performance improves. High density packaging technology such as CSP (Chip Size Package) and Stacked-CSP is needed to realize advanced functions. CSP is a bonded structure composed of materials with different properties. A mismatch of material properties may cause stress singularity at the edge of interface, which lead to the failure of bonding part in structures. Singular stress field in residual thermal stresses occurs in a cooling process after bonding the joints at a high temperature. In the present paper, the strength of interface in CSP consisted of silicon and resin is investigated. Boundary element method and an eigen value analysis based on finite element method are used for evaluating the intensity of singularity of residual thermal stresses at a vertex in a three-dimensional joint. Three-dimensional boundary element program based on the fundamental solution for two-phase isotropic body is used for calculating the stress distribution in the three-dimensional joint. Angular function in the singular stress field at the vertex in the three-dimensional joint is calculated using eigen vector determined from eigen analysis. The strength of bonding at the interface in several silicon-resin specimens with different thickness of resin is investigated analytically and experimentally. Stress singular analysis applying an external force for the joints is firstly carried out. After that, singular stress field for the residual thermal stresses varying material property of resin with temperature is calculated. Combining singular stress fields for the external force and the residual thermal stress yields a final stress distribution for evaluating the strength of interface. A relationship between the external force for delamination in joints and the thickness of resin is derived. Finally, a critical intensity of singularity for delamination between silicon and resin is determined.