Studies With Transient Compact Models of Packages and Heat Sinks

This paper deals with the application of dynamic compact thermal models of packages. First an algorithm and a methodology is presented, that was developed for the inclusion of compact electrical RC thermal models of packages into field solvers, to enable fast board level simulation with compact models of packages. Application examples demonstrate the advantages of the method. In the second part of the paper a method is presented for the generation of nonlinear compact models. Simulation experiments comparing linear and nonlinear compact models show that for small temperature excursions the use of linear models is acceptable, but in case of larger than 80°C temperature increases the use of linear models results in an about 20–30% regular error for the usual package structures.

8CH AWG Multi/Demultiplexer With MEMS Variable Optical Attenuator

An integrated arrayed waveguide grating multi/demultiplexer (AWG) with a micro-electro-mechanical systems (MEMS) based variable optical attenuator (VOA) is reported. The device consists of an AWG based on silica and a MEMS-VOA chip. The MEMS chip includes 100 μm × 100 μm polysilicon shutter plates coated with gold and electrostatic comb-drive actuators. The MEMS chip is interposed in a trench located in the middle of the I/O waveguides of the AWG to tune the optical transmitting power intensity through the waveguides continuously. The MEMS-VOA shutters have more than a 10 μm displacement. Using those shutters, 30 dB optical contrast from 5 dB at the transmit state to 35 dB at the isolation state is achieved. The obtained attenuation contrast is greater than that of a conventional waveguide-based Mach-Zehnder interferometer VOA and sufficient to adjust and equalize the optical signal power in the wavelength division multiplexing (WDM) network systems.

Characterizing Bulk Thermal Conductivity and Interface Contact Resistance Effects of Thermal Interface Materials in Electronic Cooling Applications

Thermal Interface Materials (TIMs) are used as thermally conducting media to carry away the heat dissipated by an energy source (e.g. active circuitry on a silicon die). Thermal properties of these interface materials, specified on vendor datasheets, are obtained under conditions that rarely, if at all, represent real life environment. As such, they do not accurately portray the material thermal performance during a field operation. Furthermore, a thermal engineer has no a priori knowledge of how large, in addition to the bulk thermal resistance, the interface contact resistances are, and, hence, how much each influences the cooling strategy. In view of these issues, there exists a need for these materials/interfaces to be characterized experimentally through a series of controlled tests before starting on a thermal design. In this study we present one such characterization for a candidate thermal interface material used in an electronic cooling application. In a controlled test environment, package junction-to-case, Rjc, resistance measurements were obtained for various bondline thicknesses (BLTs) of an interface material over a range of die sizes. These measurements were then curve-fitted to obtain numerical models for the measured thermal resistance for a given die size. Based on the BLT and the associated thermal resistance, the bulk thermal conductivity of the TIM and the interface contact resistance were determined, using the approach described in the paper. The results of this study permit sensitivity analyses of BLT and its effect on thermal performance for future applications, and provide the ability to extrapolate the results obtained for the given die size to a different die size. The suggested methodology presents a readily adaptable approach for the characterization of TIMs and interface/contact resistances in the industry.

Deformation and Damage in Solder During Fast Cyclic Loading

Experimental and numerical studies on fast cyclic loading of eutectic tin-lead solder and relevant micromechanical issues are presented. High-frequency twin-lap shear tests on solder joints show cracking inside the solder but often connecting the intruded tips of the intermetallic. Finite element modeling was carried out to study the effect of intermetallic morphology. Without the influence of local phase coarsening, the intrusion of intermetallic into the solder alloy is seen to trigger strain localization which promotes failure. The effect of local phase coarsening was also studied numerically, taking into account the individual phase arrangement. A coarser phase structure always shows a faster accumulation of local plastic strain, leading to early failure. Such results, in agreement with typical thermomechanical fatigue features, cannot be obtained from the traditional argument of strength vs. microstructural size. Modeling of the entire lap-shear specimen was also conducted for the purpose of quantifying the deformation behavior. The exact geometry of solder is found to play a dominant role in affecting the shear response.

Thermodynamic Efficiency of Porous Glass Electroosmotic Pumps

This paper presents an analytical and experimental study of electroosmotic (EO) pumps designed to be integrated with two-phase microchannel heat exchangers with load capacities of order 100 W and greater. We have fabricated sintered glass EO pumps that provide maximum flow rates and pressure capacities 33 ml/min and 1.3 atm, respectively, at 100 V applied potentials. We have developed an analytical model to solve for electroosmotic flow rate, total pump current, and thermodynamic efficiency as a function of pump pressure load for these porous-structure EO pumps. The model uses a symmetric electrolyte approximation valid for the high zeta potential regime and numerically solves the Poisson-Boltzmann equation for charge distribution in the idealized pore geometry. The model also incorporates an approximate ionic-strength-dependent zeta potential formulation. The effects of pressure and flow rate on thermodynamic efficiency are also analyzed theoretically and compared to our measurements.

A Numerical Analysis on the System Impedance in a Fan Cooling System

To seek the fan operating point on a cooling system with fans, it is very important to determine the system impedance curve and it has been usually examined with the fan tester based on ASHRAE standard and AMCA standard. This leads to a large investment in time and cost, because it could not be executed until the system is made actually. Therefore it is necessary to predict the system impedance curve through numerical analysis so that we could reduce the measurement time and effort. This paper presents how the system impedance curve (pressure drop curve) is computed by CFD in substitute for experiment. In reverse order to the experimental principle of the fan tester, pressure difference was adopted first as inlet and outlet boundary conditions of the system and then flow rate was calculated. After determining the system impedance curve, it was compared with experimental results. Also the computational domain of the system was investigated to minimize computational time.

Multiphysics Design and Analysis Simulations for Power Electronic Device Wirebonds

Multiscale multiphysics simulations were performed to analyze wirebonds for power electronic devices. Modern power-electronic devices can be subjected to extreme electrical and thermal conditions. Fully coupled electro-thermo-mechanical simulations were performed utilizing CFDRC’s CFD-ACE+ multiphysics simulation software and scripting capabilities. Use of such integrated multiscale multiphysics simulation and design tools in the design process can cut cost, shorten product development cycle time, and result in optimal designs. The parametrically designed multiscale multiphysics simulations performed allowed for a streamlined parametric analysis of the electrical, thermal, and mechanical effects on the wirebond geometry, bonding sites and power electronic device geometry. Multiscale analysis allowed for full device thermo-mechanical analysis as well as detailed analysis of wirebond structures. The multiscale simulations were parametrically scripted allowing for parametric simulations of the device and wirebond geometry as well as all other simulation variables. Analysis of heat dissipation from heat generated in the power-electronic device and through Joule heating were analyzed. The multiphysics analysis allowed for investigation of the location and magnitude of stress concentrations in the wirebond and device. These stress concentrations are not only investigated for the deformed wirebond itself, but additionally at the wirebond bonding sites and contacts. Changes in the wirebond geometry and bonding geometry, easily changed through the parametrically designed simulation scripts, allows for investigation of various wirebond geometries and operating conditions.

A Novel Electronics Cooling System Using Water Heat Pipes Under Freezing Conditions

A novel electronics cooling system that uses water heat pipes under an ambient temperature range from −30°C to 40°C has been developed. The system consists of several water heat pipes, air-cooled fins, and a metal block. The heat pipes are separated into two groups according to the thermal resistance of their fins. One set of heat pipes, which have fins with higher thermal resistance, operates under an ambient temperature range from −30°C to 40°C. The other set, which have lower resistance, operates from 0°C to 40°C. A prediction model based on the frozen-startup limitation of a single heat pipe was first devised and experimentally verified. Then, a prediction model for the whole-system was formulated according to the former model. The whole-system model was used to design a prototype cooling system, and it was confirmed that the prototype has a suitable cooling performance for an environmentally friendly electronics cooling system.

High-Density Through-Wafer Electrical Interconnects in Pyrex Substrate Integrated With Micromirror Array

A method for the construction of high density (2.4 mm−2) vertical leads through a pyrex substrate is presented. The pyrex substrate behaves as a TCE (Thermal Coefficient of Expansion) matched interposer that permits anodic bonding of silicon micromirrors on one side and flip-chip bumping of multiplexing electronic chips on its opposite side. Electrical leads consist of 250±25 μm-diameter holes formed by AJM machining and coated with evaporated Au yielding via resistances of 0.5–0.7 Ω. The via holes are sealed with a new spin-cast polyimide tenting process that enables the subsequent patterning of multiple levels of metal using conventional lithographic techniques.

Reliability Evaluation of Flip Chip Using Stress Intensity Factors of an Interface Crack

Anisotropic Conductive Adhesive Film (ACF) has been used for electronic assemblies such as the connection between a Liquid Crystal Display (LCD) panel and a flexible print circuit board (FPC). ACF is expected to be a key technology for flip chip packaging and chip size packaging. The goal of our work is to provide an optimum design scheme to achieve the best combination of electrical performance and mechanical reliability for electronic packages using the ACF. This study presents an evaluation technology for the delamination of the ACF connections. We utilized the stress intensity factors of an interface crack between jointed dissimilar materials. The evaluation technology presented herein was found to provide reliability of an electronic package using the ACF connection during the solder reflow process.