Showing papers in "IEEE Transactions on Components and Packaging Technologies in 2003"

Effects of Cu/Al intermetallic compound (IMC) on copper wire and aluminum pad bondability

Abstract: Copper wire bonding is an alternative interconnection technology that serves as a viable, and cost saving alternative to gold wire bonding. Its excellent mechanical and electrical characteristics attract the high-speed, power management devices and fine-pitch applications. Copper wire bonding can be a potentially alternative interconnection technology along with flip chip interconnection. However, the growth of Cu/Al intermetallic compound (IMC) at the copper wire and aluminum interface can induce a mechanical failure and increase a potential contact resistance. In this study, the copper wire bonded chip samples were annealed at the temperature range from 150/spl deg/C to 300/spl deg/C for 2 to 250 h, respectively. The formation of Cu/Al IMC was observed and the activation energy of Cu/Al IMC growth was obtained from an Arrhenius plot (ln (growth rate) versus 1/T). The obtained activation energy was 26Kcal/mol and the behavior of IMC growth was very sensitive to the annealing temperature. To investigate the effects of IMC formation on the copper wire bondability on Al pad, ball shear tests were performed on annealed samples. For as-bonded samples, ball shear strength ranged from 240-260gf, and ball shear strength changed as a function of annealing times. For annealed samples, fracture mode changed from adhesive failure at Cu/Al interface to IMC layer or Cu wire itself. The IMC growth and the diffusion rate of aluminum and copper were closely related to failure mode changes. Micro-XRD was performed on fractured pads and balls to identify the phases of IMC and their effects on the ball bonding strength. From XRD results, it was confirmed that the major IMC was /spl gamma/-Cu/sub 9/Al/sub 4/ and it provided a strong bondability.

Microjet cooling devices for thermal management of electronics

Abstract: This research is an effort to demonstrate the applicability of miniaturized synthetic jet (microjet) technology to the area of thermal management of microelectronic devices. Synthetic jets are jets which are formed from entrainment and expulsion of the fluid in which they are embedded. Design issues of microjet cooling devices are discussed along with characterization of excitation elements and the turbulent synthetic jets produced thereby. Geometrical parameters of the microjet cooling devices were empirically optimized with regards to cooling performance. The cooling performance of the optimized microjets was compared with previous theoretical and empirical studies of conventional jet impingement. The cooling performance of the microjet devices has been investigated in an open environment as well as in a vented and closed case environment. In such experiments, the synthetic jet impinges normal to the surface of a packaged thermal test die, comprising a heater and a diode-based temperature sensor. This test assembly allows simultaneous heat generation and temperature sensing of the package, thereby enabling assessment of the performance of the synthetic jet. Using microjet cooling devices, a thermal resistance of 30.1/spl deg/C/W has been achieved (when unforced cooling is used, thermal resistance is 59.6/spl deg/C/W) when the test chip is located at 15mm spacing from the jet exit plane. In order to more directly compare and scale the cooling results, a preliminary study on heat transfer correlations of the microjet cooling device has been performed. Finally, a comparison of the performance of the microjet cooler with standard cooling fans is given.

A life consumption monitoring methodology for electronic systems

Abstract: Failures in electronic products are often attributable to various combinations, intensities, and durations of environmental loads, such as temperature, humidity, vibration, and radiation. For many of the failure mechanisms in electronic products, there are models that relate environmental loads to the time to failure of the product. Thus, by monitoring the environment of a product over its life cycle, it may be possible to determine the amount of damage induced by various loads and predict when the product might fail. This paper describes the development of a physics-of-failure-based methodology for determining the damage or life consumption in a product. As a demonstration of the methodology, a data recorder has been used to monitor the temperature and vibration loads on a printed circuit board placed under the hood of a car. The data collected by the recorder has been used to determine the life consumption in the solder joints of the printed circuit board due to temperature and vibration loading. The calculated remaining life has then been compared with temperature cycling test results on the board to assess the validity of the approach.

Optimization study of stacked micro-channel heat sinks for micro-electronic cooling

Abstract: With smaller inlet flow velocity, a micro-channel stack requires less pumping power to remove a certain rate of heat than a single-layered micro-channel, because it provides a larger heat transfer area. A simple thermal resistance network model was developed to evaluate the overall thermal performance of a stacked micro-channel heat sink. Based on this simple model, in this study, a single objective minimization of overall thermal resistance is carried out using genetic algorithms. The aspect ratio, fin thickness and the ratio of channel width to fin thickness are the variables to be optimized, subject to constraints of maximum pressure drop (4 bar) and maximum volumetric flow rate (1000 ml/min). During the optimization, the overall dimensions, number of layers and pumping power (product of pressure drop and flow rate) are fixed. The study indicates that reduction in thermal resistance can be achieved by optimizing the channel configuration. The effects of number of layers in the stack, pumping power per unit area, and the channel length are also investigated.

PCM thermal control unit for portable electronic devices: experimental and numerical studies

Abstract: This paper investigates the effectiveness of a thermal control unit (TCU) for portable electronic devices by performing experimental and numerical analyses. The TCU objective is to improve thermal management of electronic devices when their operating time is limited to a few hours. It is composed of an organic phase change material (PCM) and a thermal conductivity enhancer (TCE). To overcome the relatively low thermal conductivity of the PCM, a TCE is incorporated into the PCM to boost its conductivity. The TCU structure is complex, and modeling an electronic device with it requires time and effort. Hence, this research develops approximate, yet effective, solutions for modeling the TCU, which employ effective thermo-physical properties. The TCU component properties are averaged and a single TCU material is considered. This approach is evaluated by comparing the numerical predictions with the experimental results. The numerical model is then used to study the effect of important parameters that are experimentally expensive to examine, such as the PCM latent heat, Stefan number, and heat source power. It is shown that the TCU can provide a reliable solution to portable electronic devices, which avoids overheating and thermally-induced fatigue, as well as a solution which satisfies the ergonomic requirement.

Hygroscopic swelling and sorption characteristics of epoxy molding compounds used in electronic packaging

Abstract: Moisture induced swelling and sorption characteristics of four types of epoxy molding compounds used in the packaging of semiconductor devices were experimentally investigated. The hygroscopic strain with respect to moisture content was found to be linear except at the initial stages of desorption where anomalous trends were observed. The swelling coefficient values obtained from the "stable" swelling regions were found to range from 0.3 to 0.6 (%L/%M) at 85/spl deg/C for the four types of molding compounds tested. It was also found that the swelling coefficient increased with temperature for all four types of molding compounds tested. The significance impact of hygroscopic mismatch strain was investigated by comparing the strains induced due to mismatch of the coefficient of hygroscopic swelling and the coefficient of thermal expansion for each type of molding compound when attached to a copper lead frame. The hygroscopic and thermal mismatch strains were compared using the swelling coefficient and CTE values for each type of molding compound and adjacent material (i.e., copper lead frame). Hygroscopic mismatch strains were found to be highly significant relative to the thermal mismatch strains, and they should be accounted for in the reliability modeling of packages subjected to accelerated testing. The effect of hygroscopic mismatch strains is often ignored in the reliability tests and models. In this study it is found that the hygroscopic strains can be comparable to, if not higher than, thermal mismatch strains.

Compact thermal models for electronic systems

Abstract: Compact thermal models have been constructed for different levels in electronic systems with a variable degree of success. In this work first steps toward constructing a unifying theory are presented for linear systems giving general constraints on the form of the compact model that will ensure an adequate description of thermal systems. The theory is also used to study the completeness of the set of boundary conditions used to derive and/or validate thermal compact models.

Heat rejection limits of air cooled plane fin heat sinks for computer cooling

Abstract: Current desktop computers typically use fan-heat sinks for cooling the CPU, referred to as active heat sinks. This work seeks to determine the heat rejection limits for such fan-heat sinks, within specific fan and heat sink space limits. A fixed volume, 80 /spl times/ 60 /spl times/ 50 mm is chosen as the limiting dimensions, which includes the fan volume. The present work addresses plane fin heat sinks, on which a typical 60 mm fan is mounted. Both duct flow and impinging flow are considered. Analytically based models are used to predict the optimum geometry (minimum convection resistance) for plane fins with duct and impinging flow configurations. Also assessed are the effects of increased fan speed (up to 25%) and heat sink base size (33% increase) on air-cooling limits in duct and impinging flow. Tests on fan-heat sinks are done to validate the predictions. Optimization is also done for an enhanced (offset-strip) fin geometry in duct flow. The plane fin is found to outperform the enhanced geometry.

Thermal management of BioMEMS: temperature control for ceramic-based PCR and DNA detection devices

Abstract: Integrated microfluidic devices for amplification and detection of biological samples that employ closed-loop temperature monitoring and control have been demonstrated within a multilayer low temperature co-fired ceramics (LTCC) platform. Devices designed within this platform demonstrate a high level of integration including integrated microfluidic channels, thick-film screen-printed Ag-Pd heaters, surface mounted temperature sensors, and air-gaps for thermal isolation. In addition, thermal-fluidic finite element models have been developed using CFDRC ACE+ software which allows for optimization of such parameters as heater input power, fluid flow rate, sensor placement, and air-gap size and placement. Two examples of devices that make use of these concepts are provided. The first is a continuous flow polymerase chain reaction (PCR) device that requires three thermally isolated zones of 94/spl deg/C, 65/spl deg/C, and 72/spl deg/C, and the second is an electronic DNA detection chip which requires hybridization at 35/spl deg/C. Both devices contain integrated heaters and surface mount silicon transistors which function as temperature sensors. Closed loop feedback control is provided by an external PI controller that monitors the temperature dependant I-V relationship of the sensor and adjusts heater power accordingly. Experimental data confirms that better than /spl plusmn/0.5/spl deg/C can be maintained for these devices irrespective of changing ambient conditions. In addition, good matching with model predictions has been achieved, thus providing a powerful design tool for thermal-fluidic microsystems.

An Arnoldi based thermal network reduction method for electro-thermal analysis

Abstract: In this paper we consider the problem of approximating the large discretized thermal network that models the heat conduction phenomenon in an electrical system by means of models of reduced state-space dimensions. To this aim we present an efficient and numerically stable Arnoldi type algorithm by which a multi-point moment matching approximant of the discretized thermal network is obtained and we apply it to the electro-thermal analysis of an operational transconductance amplifier.

Effects of solder joint shape and height on thermal fatigue lifetime

Abstract: Solder joint thermal fatigue failure is a major concern for area array technologies such as flip chip and ball grid array technologies. Solder joint geometry is an important factor influencing thermal fatigue lifetime. In this paper, the effects of solder joint shape and height on thermal fatigue lifetime are studied. Solder joint fatigue lifetime was evaluated using accelerated temperate cycling and adhesion test. Scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), scanning acoustic microscopy (nondestructive evaluation) and optical microscopy were utilized to examine the integrity of the joint and to detect cracks and other defects before and during accelerated fatigue tests. Our accelerated temperature cycling test clearly shows that solder joint fatigue failure process consists of three phases: crack initiation, crack propagation and catastrophic failure. Experimental results indicated that both hourglass shape and great standoff height could improve solder joint fatigue lifetime, with standoff height being the more effective factor. Experimental data suggested that shape is the dominant factor affecting crack initiation time while standoff height is the major factor influencing crack propagation time.

Thermal performance of aluminum-foam heat sinks by forced air cooling

Abstract: Experiments have been carried out to investigate the heat transfer characteristics of an aluminum-foam heat sink placed on a heat source in a channel. The thermal performance of aluminum-foam heat sinks is evaluated in terms of the Nusselt number and thermal resistance of the heat sinks. The pore density of the aluminum-foam heat sinks and the Reynolds number are varied in the range of parameters: 10, 20, 40 pores per inch (PPI) and 710/spl les/Re/spl les/2900, respectively. It is found that thermal resistance is substantially reduced by employing an aluminum-foam heat sink with low pore density due to the relatively intense airflow through the heat sink. The aluminum-foam heat sink may provide 28% or higher thermal performance than a conventional parallel-plate heat sink of the same size. Further, the aluminum-foam heat sinks can dramatically reduce the overall mass of electronics-cooling devices owing to high porosity.

Maximum thermal spreading resistance for rectangular sources and plates with nonunity aspect ratios

Abstract: An exact three-dimensional (3-D) solution is derived for the steady-state heat conduction equation with the source on an otherwise adiabatic surface and Newtonian cooling on the opposing surface. Dimensionless solutions are provided for maximum and source-averaged thermal spreading resistances. The solution for unit-source contours in a plane perpendicular to the source plane is also included. Maximum thermal spreading resistance plots are provided for several source-to-plate edge length ratios and source aspect ratios (plate maintained square). The Newtonian cooling effects are incorporated via a Biot number. Examples illustrate application of the theory.

Failure mechanism study of anisotropic conductive film (ACF) packages

Abstract: Anisotropic conductive film (ACF) consists of an adhesive polymer matrix with dispersed conductive particles. In flip-chip technology, ACF has been used in place of solder and underfill for chip attachment to glass or organic substrates. The filler particles establish the electrical contacts between the interconnecting areas. ACF flip-chip bonding provides finer pitch, higher package density, reduced package size and improved lead-free compatibility. Nevertheless, the interconnection is different from traditional solder joints, the integrity and durability of the ACF interconnects have major concerns. Failures in anisotropic conductive film (ACF) parts have been reported after temperature cycling, moisture preconditioning and autoclave. The failures have not been well understood and have been attributed to a wide variety of causes. This paper investigates the failure mechanism of ACF using finite element simulation. From a failure-initiation point of view, the response of ACF packages to environmental (temperature and humidity) exposure is very different from standard underfilled packages. These differences cause the ACF package to fail in different ways from an underfilled package. Simulation results have shown that moisture-induced ACF swelling and delamination is the major cause of ACF failure. With moisture absorption, the loading condition at the interface is tensile-dominant, which corresponds to lower interface toughness (or fracture resistance). This condition is more prone to interface delamination. Therefore, the reliability of ACF packages is highly dependent on the ACF materials. The paper suggests a new approach toward material selection for reliable ACF packages. This approach has very good correlation with experimental results and reliability testing of various ACF materials.

Least-energy optimization of forced convection plate-fin heat sinks

Abstract: A coefficient of performance (COP/sub T/) analysis for plate fin heat sinks in forced convection is presented and shown to provide a viable technique for combining least-material optimization with the entropy minimization methodology. The COP/sub T/ metric relates the heat sink cooling capability to the invested fan pumping work and the thermodynamic work required to manufacture and assemble the heat sink. The proposed optimization methodology maximizes the forced convection cooling that can be achieved by a heat sink occupying a specified volume, with a fixed energy investment and entropy generation rate. In addition, the study identifies the presence of an optimal resource allocation ratio, providing the most favorable distribution of existing energy resources, between heat sink manufacturing and operation, over a fixed product life cycle.

Silicon heat pipes used as thermal spreaders

Abstract: An increase in power densities in electronic devices is a direct consequence of their miniaturization and performance improvements. We propose the use of flat miniature heat pipes with micro capillary grooves to spread heat flux across a heat sink. Models of the structure were developed to calculate heat transfer limitations and temperature drops. A brass/water prototype was fabricated to demonstrate the feasibility of heat spreading using this type of heat pipe. Simulation and experimental results obtained with the prototype are described. The dissipated power reached 110 W/cm/sup 2/ without heat transfer limitations. The results are then extended to the design of this type of heat pipe in silicon. Thermal performance was calculated. Simulation, experimental results and the fabrication process are presented.

Smart pumpless loop for micro-channel electronic cooling using flat and enhanced surfaces

Abstract: Two-phase cooling of a square simulated electronic device surface of 21.3 mm side was successfully carried out without the need for a pump. This smart, passive cooling system incorporates a self-enhancing and self-sustaining mechanism, wherein the system inherently enhances its cooling capacity by increasing the velocity of the two-phase mixture along the boiling surface when an increase in heat flux is sensed. Other practical attributes of this pumpless loop are small liquid inventory requirements and absence of the incipient boiling temperature drop. It is shown small surface tension and contact angle render dielectric coolants such as FC-72 ideally suited for flow in narrow gaps. These unique properties are responsible for very small bubble size, precluding any appreciable blockage of the replenishment liquid flow even in narrow gaps. Critical heat flux (CHF) was found to generally increase with decreasing boiler gap. CHF for flat, micro-channel (with 0.2 mm rectangular fins) and mini-channel (with 1.98 mm rectangular fins) surfaces was 4.5, 5.9, and 5.7 times greater than for pool boiling from a flat surface for corresponding gaps. A pressure drop model was formulated to predict coolant mass flow rate, boiling surface inlet and exit velocities, and pressure drop components throughout the loop. The model predictions illustrate the pumpless loop's self-sustaining and self-enhancing attributes, and relate CHF trends to those of the two-phase mixture acceleration along the boiling surface.

Strategy for designing accelerated aging tests to evaluate IGBT power modules lifetime in real operation mode

Abstract: Accelerated aging tests are widely used in electronics industry to validate the technological choices of packaging, especially when significant lifetime is required. The thermal coefficient of expansion mismatch between the soldered materials results in crack propagation in the solder joints, which irreversibly increases the thermal resistance of the assembly and progressively leads to the failure of the module. Accelerated testing is very expensive and can last a long time, so that such experiments must be optimized. In addition, thermomechanical behavior of the assembly under realistic operating conditions for long times can be studied in shorter times thanks to finite element simulations. Non-linear finite element simulations have been carried out to make a correlation between accelerated aging tests and real operation. Stress-strain history under representative thermal loading has been determined in both cases. The energy dissipated in solder joints has been calculated and has been used as an estimator of the damage in the solder joint. Finally, thermal fatigue experiments with representative samples have allowed validating the previous results. This study is a step toward the understanding of the correlation between accelerated testing and actual operating conditions.

Molecular basis of the interphase dielectric properties of microelectronic and optoelectronic packaging materials

Abstract: High frequency microelectronic and optoelectronic device packaging requires the use of substrate and encapsulation materials having a low dielectric constant, low dielectric loss and high volume resistivity. Most packaging materials are polymer-ceramic composites. A clear understanding of the broadband dielectric properties of composite materials is thus of great current importance for the effective development of high frequency packaging materials and optimized package design. Toward this goal, a general framework for understanding the dielectric properties of packaging materials was recently developed in which the dielectric constant of polymer-ceramic composite materials is characterized by the electrical properties of the polymer phase, the filler phase and an interphase region within the composite system. However, for this framework to be a viable tool for tailoring the dielectric properties of packaging materials, one must understand the dielectric properties of the polymer-filler interphase region, which represents a region of polymer surrounding and bonded to the surface of each filler particle having unique dielectric and physical characteristics. This work presents a model to explain and predict the dielectric properties of the composite interphase region based on dipole polarization theory.

An overview of experimental methodologies and their applications for die strength measurement

Abstract: As the trends in semiconductor packages continue toward a decrease in overall package size and an increase in functionality and performance requirements, they bring challenges of processing, handling, and understanding smaller components and, in particular, thinner dies. In the meantime, high reliability remains a critical necessity. It is necessary to be able to appropriately characterize thinned dies in terms of their mechanical integrity and, equally important, in terms of the processes used to produce them. In practice, die strength can be adversely affected during various manufacturing processes, such as thinning and singulation. A realistic understanding of the significance of processing on die strength is gained through the study of the actual, processed component. This work outlines three methodologies that enable the measurement of die strength and demonstrates their application in three studies. Characterization of die damage, experimentation, and failure analysis are coupled to gain understanding of die strength with respect to processing conditions. The approaches demonstrated ultimately show the use of such information toward quantifying die strength, developing design criteria, selecting wafer processes, and optimizing processes.

Thermal transport properties of gold-covered thin-film silicon dioxide

Abstract: Due to continued miniaturization, the performance and reliability of electronic devices composed of multiple thin layers of material are highly dependent on effective thermal management. Since the thermal properties of thin films, such as SiO/sub 2/, can vary considerably from bulk values, the determination of those properties (as well as the interface resistance between SiO/sub 2/ and adjacent layers) is critical for the purposes of design. In this work, a transient thermo-reflectance system has been employed to measure the thermal characteristics of thin-film SiO/sub 2/ layers. Results show that for layers of SiO/sub 2/ in the range of 100-1000 /spl Aring/, the intrinsic thermal conductivity (TC) is independent of thickness and smaller than the traditionally reported value of bulk silicon dioxide (1.4 W/m-K). The intrinsic value was measured to be around 90% (1.27 W/m-k) and 75% (1.05 W/m-k) of the latter bulk value for thermally grown (TG) and ion beam sputtered (IBS) oxides, respectively. The thermal interface resistances of TG and IBS SiO/sub 2/ films were measured at 1.68 /spl times/ 10/sup -8/ m/sup 2/-K/W and 2.58 /spl times/ 10/sup -8/ m/sup 2/-K/W, respectively. If a chromium film of around 100 /spl Aring/ is deposited between the gold and SiO/sub 2/ layers, the interface thermal resistance improves to 0.78 /spl times/ 10/sup -8/ m/sup 2/-K/W for TG films and 1.15 /spl times/ 10/sup -8/ m/sup 2/-K/W for IBS films. Thus, the effective thermal resistance of SiO/sub 2/ thin-films (i.e., with interface effects) is up to one order of magnitude smaller than the values reported for bulk SiO/sub 2/.

Determining the impact resistance of electrically conductive adhesives using a falling wedge test

Abstract: This study was conducted to determine the impact resistance of electrically conductive adhesives (ECAs). A novel falling wedge test that was used to quantitatively characterize the impact resistance of conductive adhesives at a material level has been described, and some important findings obtained from the falling wedge test are presented. This unique impact resistance testing method is not only a substitution of the conventional drop test which has several severe drawbacks, but also provides some useful information for screening adhesives at the materials level and helping formulate new conductive adhesives with improved impact performance. Three model conductive adhesives were studied in this work and the impact fracture energies of the adhesive materials were measured utilizing the falling wedge test. The effect of test temperature on the fracture behavior of ECAs was examined and the correlation between the impact resistance and damping property of the conductive adhesive was also investigated. This study suggests that 1) the falling wedge test is able to discriminate between adhesives and this technique is capable of screening adhesives for bonding purposes; 2) the viscoelastic energy has played an important role in the fracture behavior of the conductive adhesives. As a measure of the internal friction, the loss factor tan/spl delta/ is found to be a good indicator of a conductive adhesive's ability to withstand impact loading.

A review of selected thermal management solutions for military electronic systems

Abstract: Thermal management of electronics is vital to the successful design, manufacture, and tactical operation of a variety of military electronic systems. Designs employ all modes of heat transfer including: conduction, natural and forced convection, aerodynamic heating, radiation, and two-phase heat transfer. A variety of heat sinks and heat exchange devices are employed, including the use of cold plates, electronic chassis coldwalls, compact heat exchangers, air-cycle and vapor-cycle refrigeration systems, phase change materials, thermoelectric devices, and heat pipes. This paper describes several military electronic systems on a variety of platforms and discusses the thermal management issues involved in the design of the thermal control systems. Specific examples are employed in the paper to emphasize the variety of thermal management problems encountered and the solution techniques employed.

An effective methodology for thermal characterization of electronic packaging

Abstract: An effective and novel methodology that integrates infrared (IR) thermography measurement and a three-dimensional (3-D) finite element (FE) model is proposed for thermal characterization of packages in a steady state under a natural convection environment based on JEDEC specification . To perform surface temperature measurement using an IR thermometer, a black paint coating is applied on the surface of packages so as to calibrate the surface radiation. The associated emissivity is approximately assessed using a simple calibration experiment, and an appropriate thickness of the coating is determined. By using a typical 100-lead Thin Quad Flat package (TQFP) as the test vehicle, the proposed methodology is benchmarked by a thermal test die measurement in terms of the junction-to-ambient (J/A) thermal resistance and the chip junction temperature. To demonstrate the accuracy of the benchmarked data from the thermal test die measurement, a corresponding uncertainty analysis is performed. It is found that the worst possible uncertainty in the measured power, based on the specific power supply, is about 0.005 W and that of chip junction temperature measurement is about 0.78/spl deg/C. Additional studies are performed to evaluate the feasibility of the correlation models for convective heat transfer coefficients on typical TQFP packages. It turns out that for a small device such as the TQFP package, these correlation models are fairly reliable.

Formation and growth kinetics of interfacial intermetallics in Pb-free solder joint

Abstract: The formation and growth kinetics of the intermetallic compound (IMC) between lead-free solder and Cu substrate in the surface mount process were studied. Optical and scanning electron microscope (OM and SEM) were used to measure the thickness of intermetallic layers and observe the microstructure evolution of solder joint. The IMC phases were identified by energy dispersive X-ray (EDX) and X-ray diffraction (XRD). The grain size of the intermetallic compound /spl eta/-phase Cu/sub 6/Sn/sub 5/ was observed to increase with the increase in peak temperature hold time. The results show that a /spl eta/-phase Cu/sub 6/Sn/sub 5/ IMC layer is formed at solder-Cu substrate interface at a very short time. The growth of intermetallics formation was found not to follow the Fick's law that predicts the mean total thickness increases linearly with the square root of the time. It deviates the Fick's law at the early stage of the growth process and then approaches the parabolic law. To explore the growth kinetics, the IMC growth mechanism is suggested and a lagging diffusion model is presented for predicting the intermetallic compound layer growth. Comparison between the model and experimental results demonstrates that the proposed phase-lag model captures the growth history of IMC layers quite well.

Evolution of cooling technology for electrical, electronic, and microelectronic equipment

Abstract: Selected aspects of cooling technology for electrical apparatus and electronic devices are considered for the past 80 years. The emphasis is on the past 50 years. The technology has evolved to meet the challenges of microminiaturization, and heat transfer considerations are now an integral part of the design procedure for microelectronic systems.

An investigation into the effects of flux residues on properties of underfill materials for flip chip packages

Abstract: Flux is necessary to promote wetting between solder and metallized surfaces during flip chip bonding process. However, flux residues are unavoidable and may have detrimental effects on the properties of the underfill materials. In this study, the effects of flux residues from a commercial no-clean flux on two types of underfills were systematically investigated using a contact angle goniometer, thermal mechanical analyzer (TMA), thermogravimetric analyzer (TGA) and dynamic mechanical analyzer (DMA). It was found that the presence of flux residue reduced the glass transition temperatures (Tg) of both cured underfills studied. Although /spl alpha//sub 1/ (CTE below Tg) of cured underfills were slightly affected, /spl alpha//sub 2/ (CTE above Tg) increased significantly. The DMA tan/spl delta/ curves of flux residue contaminated samples were found to be broader than those of pure underfills. In addition, the rubbery storage moduli of underfills contaminated with flux residues also decreased significantly. The existence of flux residues was found to increase the moisture absorption of the cured underfills.

Computer and telecommunications equipment room cooling: a review of literature

Abstract: Due to technology compaction, the information technology (IT) industry has seen a large increase in power density and heat dissipation within the footprint of computer and telecommunications hardware. The heat dissipated in these systems is exhausted to the room and the room has to be maintained at acceptable temperatures for reliable operation of the equipment. Cooling computer and telecommunications equipment rooms is becoming a major challenge. This paper reviews the literature dealing with various aspects of cooling computer and telecommunications equipment rooms. Included are papers on experimental work analyzing cooling schemes, numerical modeling, energy saving schemes, natural convection room cooling, forced convection room cooling, cooling raised floor versus nonraised floor type installations, and other related areas.

Thermal compact models: an alternative approach

Abstract: Thermal compact models enable the prediction of junction temperatures of complicated components in a system level numerical simulation with a minimum of computational effort. A method for generating compact models has been proposed by the DELPHI consortium. Here a new approach is proposed that is no longer based on a statistical treatment using optimization but on analyzing the heat flux distributions that occur. Using this new approach a new method for generating compact models is suggested, which obtains models that are, in principle, equal to the models found using the DELPHI approach, but at virtually no computational effort. The analysis puts compact models on a firm mathematical and physical basis and proves that no linear model with a limited number of nodes can ever be truly boundary condition independent. Furthermore, it is shown that the shape of the distribution function of the heat flux over the external surfaces of the component determines both the number of nodes needed in the compact model and its accuracy.

Micro heat pipes in low temperature cofire ceramic (LTCC) substrates

Abstract: With projected power densities above 100 W/cm/sup 2/ for devices, new methods for thermal management from the heat generation at the die to heat removal to the ambient must be addressed. By integrating micro heat pipes directly within the ceramic substrate, effective thermal conductivity for spreading heat in both radial and axial directions was achieved. New materials and processes were developed to fabricate the unique components required to handle high thermal loads. Enhanced thermal vias to minimize the thermal impedance through the ceramic in the evaporator and condenser sections were developed, increasing the effective thermal conductivity from 2.63 to near 250 W/m-/spl deg/C. The use of an organic insert fabricated into the desired complex shape using rapid prototyping methods, coupled with the viscoelastic flow of the low temperature cofire ceramic (LTCC) during lamination, allowed complex shapes to be developed while ensuring uniform green tape density during lamination prior to tape firing. Large cavities, three-dimensional fine structures and porous wicks for capillary 3-D flow have been utilized to fabricate the heat pipes. Heat pipes and spreaders, utilizing water as the working fluid, have been demonstrated operating with power densities in excess of 300 W/cm/sup 2/.