Electromigration Analysis of Power Modules by Electrical-Thermal-Mechanical Coupled Model

Power semiconductors and modules are basic components of electrical infrastructure and are currently widely used in applications such as power conversion devices, industrial equipment, railways, and automobiles. Power modules are being developed with the aim of downsizing and increasing power output. With the larger current densities and higher operating temperatures associated with downsizing and increasing power output, degradation of power modules can occur as a result of electromigration. Electromigration is a phenomenon where atoms move due to the momentum transfer between conducting electrons and metal atoms. In addition, atoms are also moved by mechanical stress gradients and temperature gradients, so it is necessary to take into consideration the combined effects of electrical, thermal, and mechanical stress. In this report, we describe an electrical-thermal-mechanical coupled analysis of electromigration in a bonding wire of a power module. First, the analysis is validated under the condition that the displacement of the wire surface is fixed. The distributions of vacancy concentrations and hydrostatic stress are almost equal to those in previous studies. Next, we present the influences of current density, temperature, and the displacement constraint on electromigration in a wire with a simplified shape. The analysis results confirm that the plasticity and creep should be taken into consideration in a bonding wire. This also confirm that vacancy concentration increase more rapidly by changing the displacement of the wire surface from the fixed condition to the free condition. Finally, we present analysis results for a bonding wire with the actual shape found in power modules. In this wire, a local concentration peak appear in the electrode terminal. The analysis results reveal that electromigration may affect not only void formation but also other failure phenomena in the bonding wire of power modules.

Validation of an Atomistic Field Theory for Contact Electrification Using a MEMS Load Cell

This work depicts an experimental method for the validation of an Atomistic Field Theory (AFT) model for contact electrification of dielectrics. The AFT model is used to simulate the effects of Triboelectric Nanogenerators (TENGs) for energy harvesting. Recently, the AFT model has shown that contact electrification can be described by the induced surface dipoles when two dissimilar materials are brought into close contact assuming that the crystal lattices are free of defects, no residual strain in the materials is present and that the experiment is performed in vacuum. These simulations have been used to predict the induced contact potential between MgO and BaTiO3. To validate the AFT model, a set of quasi-static experiments will be conducted to test two different operating modes of TENGs, which can be mirrored in the simulations. The first experiment is a micro-scale pull-in/pull-off test in which a pad of single crystal Si (SCSi) will be brought into and out of contact with a dielectric substrate (thermally grown SiO2). The second experiment will mimic the TENG during sliding operation. A SCSi microcantilever will be brought into contact with the dielectric surface and displaced in sliding mode. These experiments will be conducted using a custom, reusable MEMS load cell and an electrometer to monitor the interaction forces and induced charge on the surfaces. To obtain the required displacement resolution of the load cell, a high-speed Michelson interferometer will be used. This allows for higher load cell stiffness to accommodate for surface adhesion effects. The load cell will be calibrated using the well-known technique of hanging masses from the load cell. The relative distance between the interacting surfaces in both experiments will be controlled by a piezo stage with 1 nm resolution. Results from these experiments are to be compared to the AFT model results.

Flexible Valveless Pump for Bio Applications

In electromechanical actuators Lorentz force law is used to convert electrical energy into rotational or linear mechanical energy. In these conventional electromechanical actuators, rigid wires conducts the electrical current and as such the types of motion generated by these actuators are limited. Recent advances in liquid metal alloys permit designing electrical wires that are stretchable. These flexible wires have been used to fabricate various flexible connections, sensors and antennas. However, there have been very little efforts to use these stretchable liquid metal wires as actuators. Building upon our previous work in this area, we have made a flexible pump which can be used in bio applications. In this design we placed a flexible polymeric substrate filled by liquid metal Galinstan between two permanent magnets. Since the pump should convey the biological cells suspended along the fluid flow, utilizing check valves may increase the risk of clog in the inlet or outlet. Therefore, our design is based on the nozzle/diffuser concept. This new pump can be considered as a peristaltic and valve-less mechanical pumps which utilizes the Lorentz force law as the actuating mechanism.

Study of Angle-of-Attack (AoA) for Airfoil in Deterministic Lateral Displacement (DLD)

Deterministic lateral displacement (DLD) is a method of inertial size-based particle separation with potential applications in high throughput sample processing, such as the fractionation of blood or the purification of target species like viral particles or circulating tumor cells. Recently, it has been shown that symmetric airfoils with neutral angle-of-attack (AoA) can be used for high-throughput design of DLD device, due to their mitigation of vortex effects and preservation of flow symmetry under high Reynolds number (Re) conditions. While high-Re operation with symmetric airfoils has been established, the effect of AoA for airfoil on the DLD performance has not been characterized. In this study, we present a high-Re investigation with symmetric airfoil-shaped pillars having positive and negative 15 degree AoA. Both positive and negative AoA configurations yield significant flow anisotropy at higher flow rates. The stronger shift of the critical diameter (Dc) was observed with negative AoA, but not in positive AoA device. The most likely contributor may be the growing anisotropy that develops in the AoA device at higher flow rates. This study shows that high-Re DLD design with airfoil shaped pillars requires significant consideration for pillar orientation to control flow symmetry.

Packaging Challenges of Thin High Bandwidth POP

Components for Smartphone has been the biggest driving force of IC industry for years, and one of the most important IC is application processor (AP). AP needs to work with low power double data rate (LPDDR), the mobile DRAM together for the primary processing of cellular phone and other smart functions. At the beginning, they were packaged separately and then mounted onto printed circuit board (PCB) very close to each other. Nowadays, AP for flagship Smartphone is packaged with a variety of PoP (package on package) structures to shorten the communication distance between AP and LPDDR as well as to save more rooms for battery. High bandwidth package on package (HBW-POP) is the most popular structure among them. As compared to other substrate based PoP, HBW-POP provides the most top side pin count while keeps larger ball pitch for system assembly house to mount LPDDR packaged by fine-pitch ball grid array (FBGA) on top of it. And compared to novel Fan-Out based PoP, HBW-POP has lower cost for AP packaging. In addition, maximum package height of HBW-POP has been shrinking. It is because when LPDDR is mounted onto HBW-POP, the combination is always the tallest chips on the PCB, which determines how slim specific Smartphone can be. HBW-POP consists of 3 parts to encapsulate AP die, and they are top 2-layer substrate, middle molding and bottom 3-layer substrate. Each part has its own coefficient of thermal expansion (CTE) and rigidity, and the warpage performance of HBW-POP is important to align the warpage behavior of LPDDR. The warpage of HBW-POP needs to align with FBGA properly during reflow for good joint, but when HBW-POP becomes thinner, the rigidity of its different parts is changed, which result in different warpage behavior during the reflow. In this paper, we will review the challenges of thin HBW-POP packaging, meanwhile we will explore possible solutions to address each challenge. The study includes the screening of different thickness combination of the 3 parts of HBW-POP, and the optimization of the rigidity and CTE of them. Design of Experiments (DOE) are conducted to find solutions which can meet warpage target, and finally, we present more different tests to prove the reliability of our results.

Scaling Behavior in Electrohydrodynamic Jetting of Polymeric Solutions

An electrohydrodynamic (EHD) jet forms when a leaky-dielectric liquid issuing out of a needle is accelerated and stretched by electrostatic forces. Stability and scaling behavior of the EHD jet of polymeric solutions depend on electrostatics, fluid mechanics and rheology of the liquid. While EHD jetting of Newtonian liquids have been described in the literature, the effect of non-Newtonian rheology on EHD jetting is still not well-understood. Therefore, we present a detailed experimental investigation of the stability and scaling behavior of EHD jets of polymeric solutions that exhibit non-Newtonian flow behavior. The stability of cone-jet was analyzed by varying flow rate, electric field and polymer concentration. Experiments were performed for polymeric solutions of polycaprolactone (PCL) dissolved in acetic acid. Our experiments show that non-Newtonian viscoelastic behavior can significantly alter the stability characteristics of the EHD jet. We have found that increase of elasticity of polymeric solutions results in enhanced jet stability. Finally, we present the dependence of experimentally measured diameter dj of the EHD jet on the flow rate Q. Experimentally measured diameter of the EHD jet scales as dj ∼ Q0.65 for both Newtonian and non-Newtonian viscoelastic liquids, which can be attributed to dominant inertia forces in our experiment.

Analysis of Combined Electroosmotic and Pressure Driven Flow of Multilayer Immiscible Fluids in a Narrow Capillary

This paper presents the analytical solution of a combined electroosmotic and pressure driven flow of multilayer immiscible fluids in a narrow capillary. The mathematical model is based in the Poisson-Boltzmann equation and the modified Navier-Stokes equations. In the steady-state analysis, we consider different conditions at the interfaces between the fluids as potential differences, surface charge densities and electro-viscous stresses balances, which are discussed in detail. Results show the transport phenomena coupled in the description of velocity distribution, by the analyzing of the dimensionless parameters obtained, such as: potential differences, surface charge densities, electrokinetic parameters, term involving the external pressure gradient, ratios of viscosity and of dielectric permittivity. Here, the presence of a net electric charges balance at the interfaces breaks the continuity of the electric potential distributions and viscous shear stresses, modifying the flow field; thus, the electrical conditions established at the interfaces play an important role on the flow behavior. The present work, in which the velocity field is described, aims to be an important contribution in the development of theoretical models that provide a better understanding about labs-on-a-chip design.

Retracted: “Insights Into the Mechanical Properties of Bi-Layer Germanene Coupled by Covalent Bonding” [ASME 2019 International Mechanical Engineering Congress and Exposition, Volume 10: Micro- and Nano-Systems Engineering and Packaging, Salt Lake City, Utah, USA, November 11–14, 2019, Conference Sponsors: ASME, ISBN: 978-0-7918-5947-6, Copyright © 2019 by ASME. Paper No. IMECE2019-12129, pp. V010T12A023; 8 pages; doi: 10.1115/IMECE2019-12129]

The above referenced paper has been removed from publication. (March 11, 2020) Copyright © 2020 by ASME

Measurement of the Residual Stress Distribution in the 3D-Stacked Electronic Modules by Embedded Strain Sensors

In this study, micro-scale strain sensors were embedded in a silicon chip to measure the variation of the local stress distribution around thin film interconnections used for 3D semiconductor modules. Piezoresistive effect of single crystal silicon was applied to the sensors. The stress sensitivity at room temperature was 1.3 MPa/Ω. Even though the stress sensitivity varied as a strong function of temperature, it was confirmed that this sensor can measure the stress distribution quantitatively in the temperature range from room temperature to 80°C at most. And the variation of the residual stress in electroplated copper thin films during heat treatment was investigated from the viewpoint of the change of stress amplitude during a thermal cycle. From the obtained results, it was confirmed that the recrystallization occurred when the films were annealed at only 200°C, and the change of the micro textue of the films caused the change of their residual stress. Therefore, it is very important to control the micro texture during electroplating for assuring long term reliability of interconnections.

Study of Low-Frequency Narrow Bandwidth Surface Acoustic Wave Sensor for Liquid Applications

Surface acoustic wave (SAW) devices have been applied as telecommunication filter for decades. Due to its simple interdigitated transducer (IDT) layout and geometry-dependent frequency, the SAW filter operates at the designed frequency and its working bandwidth could be designed to fulfill specific applications. Researchers also use SAW devices for sensing the mass or pressure in air. Furthermore, SAW device can be employed in liquid environments. The main focus of this paper is to present a Love mode device for liquid sensing. The Love mode device included a shear-horizontal surface acoustic wave (SH-SAW) delay-line configuration with a photoresist waveguide, which was deposited on split-electrode IDT and reflectors. The substrate was ST-cut quartz, and the SH-SAW propagated between the waveguide and the piezoelectric substrate. Using the Love mode device, we monitored the frequency shift corresponding to a water drop. We demonstrate that the insertion loss level is not critical for S-parameter transmission signal readout. The signal quality within the resonant narrowband is very important for water sensing. In this study, two types of SH-SAW devices were fabricated and tested; SH-SAW resonator and SH-SAW delay-line. We also demonstrate single and split electrodes electrode configurations to generate acoustic waves. Four different waveguide thickness values were tested to prove the benefit of thick polymer waveguide. This research also offers a standard method to fabricate SAW on ST-quartz for liquid application. In the future, we plan to integrate the Love mode device with a cell-culturing chamber to obtain a biosensor.