Showing papers in "IEEE Transactions on Components and Packaging Technologies in 2000"

A precise numerical prediction of effective dielectric constant for polymer-ceramic composite based on effective-medium theory

Abstract: Nanostructure polymer-ceramic composite with high dielectric constant (/spl epsiv//sub /spl tau///spl sim/90) has been developed for embedded capacitor application. This polymer-ceramic system consists of lead magnesium niobate-lead titanate (PMN-PT) ceramic particle and modified high-dielectric constant low-viscosity epoxy resin. In order to obtain precise prediction of effective dielectric constant of this composite, an empirical prediction model based on self-consistent theory is proposed. The electrical polarization mechanism and interaction between epoxy resin and ceramic filler has been studied. This model can establish the relevant constitutional parameters of polymer-ceramic composite materials such as particle shape, composition, and connectivity that determine the dielectric properties of the composite. This model is simpler, uses fewer parameters and its prediction compares better with experiment (error <10%). The precision and simplicity of the model can be exploited for predictions of the properties and design of nanostructure ferroelectric polymer-ceramic composites. The effective-medium theory (EMT) has been proved a good tool to predict effective properties of nanocomposites.

Electronic part life cycle concepts and obsolescence forecasting

Abstract: Obsolescence of electronic parts is a major contributor to the life cycle cost of long-field life systems such as avionics. A methodology to forecast life cycles of electronic parts is presented, in which both years to obsolescence and life cycle stages are predicted. The methodology embeds both market and technology factors based on the dynamic assessment of sales data. The predictions enabled from the models developed in this paper allow engineers to effectively manage the introduction and on-going use of long field-life products based on the projected life cycle of the parts incorporated into the products. Application of the methodology to integrated circuits is discussed and obsolescence predictions for dynamic random access memories (DRAMs) are demonstrated. The goal is to significantly reduce design iterations, inventory expenses, sustainment costs, and overall life cycle product costs.

Enhancement of air jet impingement heat transfer using pin-fin heat sinks

Abstract: The enhancement of heat transfer from a discrete heat source in confined air jet impingement was experimentally investigated. A variety of pin-fin heat sinks were mounted on the heat source and the resulting enhancement studied, Average heat transfer coefficients are presented for a range of jet Reynolds numbers (8000/spl les/Re/spl les/45000) and orifice diameters (12.7/spl les/d/spl les/38.1 mm). A total fin effectiveness was computed for the pinned heat sinks relative to the unpinned ones, and was in the range of 2.4 to 9.2; the highest value was obtained for the largest nozzle diameter. Heat transfer rates from the bare heat source were increased by a factor of 7.5 to 72 due to the introduction of the heat sinks. Results for the average heat transfer coefficient were correlated in terms of Reynolds number, fluid properties and geometric parameters of the heat sinks.

An improved methodology for determining temperature dependent moduli of underfill encapsulants

Abstract: Finite element analyses (FEAs) have been widely used to preventively predict the reliability issues of flip-chip (FC) packages. The validity of the simulation results strongly depends on the inputs of the involved material properties. For FC packages Young's modulus-temperature relationship is a critical material property in predicting of the package reliability during -55/spl deg/C to 125/spl deg/C thermal cycling. Traditional tensile tests can obtain the modulus at selected temperatures, but are tedious, expensive, and unable to accurately predict the Young's modulus-temperature relationship within a wide temperature range. Thus, this paper is targeted to provide a simple but relatively accurate methodology to obtain the Young's modulus-temperature relationship. In this paper, three commercial silica filled underfill materials were studied. A simple specimen (based on ASTM D638M) preparation method was established using a Teflon mold. A dynamic-mechanical analyzer (DMA) was used to obtain the stress-strain relationship under controlled force mode, storage and loss modulus under multi-frequency mode, and stress relaxation under stress relaxation mode. A simple viscoelastic model was used and an empirical methodology for obtaining Young's modulus-temperature relationship was established.

Thermal dynamics and the time constant domain

Abstract: The time-constant spectrum representation is a useful description of the dynamic thermal behavior of packages, assemblies and microsystems. After presenting the idea of the time constant spectrum representation of microelectronic structures, the paper provides an algorithm that can be used for time constant spectrum calculation in thermal simulator programs. The obtained time-constant spectrum can be the basis of calculating the transient behavior pulse thermal resistance diagrams, and structure reconstruction. All these applications are presented in the paper with examples.

Effects of underfill material properties on the reliability of solder bumped flip chip on board with imperfect underfill encapsulants

Abstract: Three different types of underfill imperfections were considered; i.e., (1) interfacial delamination between the underfill encapsulant and the solder mask on the PCB (crack initiated at the tip of underfill fillet), (2) interfacial delamination between the chip and the underfill encapsulant (crack initiated at the chip corner), and (3) the same as (2) but without the underfill fillet. Five different combinations of coefficient of thermal expansion (CTE) and Young's modulus with the aforementioned delaminations were investigated. A fracture mechanics approach was employed for computational analysis. The strain energy release rate at the crack tip and the maximum accumulated equivalent plastic strain in the solder bumps of all cases were evaluated as indices of reliability. Besides, mechanical shear tests were performed to characterize the shear strength at the underfill-solder mask interface and the underfill-chip passivation interface. The main objective of the present study is to achieve a better understanding in the thermo-mechanical behavior of flip chip on board (FCOB) assemblies with imperfect underfill encapsulants.

Design optimization of an integrated liquid-cooled IGBT power module using CFD technique

Abstract: This paper presents a novel approach to optimize pin array design of an integrated, liquid-cooled, insulated gate bipolar transistor (IGBT) power module. With the aid of a computational fluid dynamics (CFD) code, the fluid field and heat transfer inside the module were analyzed, and several design options on pin arrays were examined. For IGBT die circuitry, the uniformity of temperature distribution among dies is as critical as the magnitude of the die temperature. A noticeable variation in temperature among dies can accelerate the thermal runaway and reduce the reliability of the devices. With geometrically-optimized-pin designs located both upstream and downstream of the channel, a total power dissipation of 1200 W was achieved. The maximum junction temperature was maintained at 100/spl deg/C and the maximum variation among dies was controlled within 1/spl deg/C. The results from this study indicated that the device junction temperatures were not only reduced in magnitude but were equalized as well. In addition, the maximum power dissipation of the module was enhanced. Comparison with other direct- (pool boiling) and indirect- (cold plate) liquid cooling techniques was also discussed.

Solder metallization interdiffusion in microelectronic interconnects

Abstract: We investigated the growth of intermetallic compounds in Cu/Ni/Au/PbSn solder joints. The substrates that we investigated had been Au plated by one of two different techniques. The Au finish thicknesses ranged from 0.25 to 2.6 /spl mu/m. After solder renew, structural examinations using optical and electron microscopy of cross-sectioned solder joints revealed the growth of Ni/sub 3/Sn/sub 4/ at the solder/Ni interface after reflow. Solder joints with thicker layers of Au annealed in Ar gas at a temperature of 150/spl deg/C for up to 450 h, displayed an appreciable growth of Au/sub 0.5/Ni/sub 0.5/Sn/sub 4/ at the Ni/sub 3/Sn/sub 4//solder interface. Previous investigators correlated growth of a Au-Sn alloy with the degradation of the mechanical properties of the solder joint. The determination of the stoichiometry of the Au/sub 0.5/Ni/sub 0.5/Sn/sub 4/ phase provides some understanding of why this phase grew at the Ni/sub 3/Sn/sub 4//solder interface, as Sn, Au and Ni are all readily available at this interface. The growth of this ternary alloy is also consistent with trends observed in the kinetics of formation of solder alloys.

Advanced cooling system using miniature heat pipes in mobile PC

Abstract: This paper describes various cooling solutions using heat pipes for cooling a notebook PC. These are: 1) heat pipe with heat spreader plate; 2) hybrid system-i.e., heat pipe with heat sink and fan; and 3) hinged heat pipe system. For heat input of less than 12 W, the thermal resistance measured between the surface of the CPU to ambient was obtained as follows: greater than 8/spl deg/C/W for system 1) and 4-6/spl deg/C/W for systems 2) and 3). For the CPU having specification of surface temperature of 95/spl deg/C and 40/spl deg/C ambient, then system 1) can be dissipated by about 6 W, whereas systems 2) and 3) can handle 13 W. Experimental results of these three systems are included and discussed in this paper.

Analytic modeling, optimization, and realization of cooling devices in silicon technology

Abstract: A novel cooling device fully built in silicon technology is presented. The new concept developed in this work consists of micromachining the bottom side of the circuit wafer in order to embed heat sinking microchannels directly into the silicon material. These microchannels are then sealed, by a direct wafer bonding procedure, with another silicon wafer where microchannels and inlet-outlet nozzles are micromachined too. A cooling fluid (water) is then forced through the array of channel to convey heat outside the chip. Such a configuration presents advantages to provide a significant reduction of the cooler overall dimensions, to reduce the number of the involved materials and to be compatible with integrated circuit fabrication procedures, In this study analytical tools were used in order to get a global evaluation of all the thermal resistances characteristic of such devices. Using these adequate analytic models with appropriate approximations, a global optimization procedure was then applied and led to the definition of he optimum dimensions of the silicon micro heat sink. The realization procedure was then carried out in a clean room environment. First experimental characterization results obtained from the earlier prototypes demonstrated that the thermal properties of this silicon-based cooling device are satisfactory and can be reasonably compared to those of commercially available copper micro heat sinking components.

Development of conductive adhesives for solder replacement

Abstract: Develops conductive adhesives with stable contact resistance and desirable impact performance. Effects of purity of the resins and moisture absorption on contact resistance are investigated. Several different additives (oxygen scavengers and corrosion inhibitors) on contact resistance stability during elevated temperature and humidity aging are studied, and effective additives are identified. Then, several rubber-modified epoxy resins and two synthesized epoxide-terminated polyurethane resins are introduced into ECA formulations to determine their effects on impact strength. The loss factor, tan /spl delta/, of each formulation is measured using a dynamic mechanical analyzer (DMA) and impact strength is evaluated using the National Center for Manufacturing Science (NCMS) standard drop test procedure. Finally, high performance conductive adhesives are formulated by combining the modified resins and the effective additives. It is found that 1) purity of the resins and moisture absorption of the formulation affect the contact resistance stability of an ECA; 2) the oxygen scavengers and corrosion inhibitors can delay contact resistance shift; 3) one of the corrosion inhibitors is very effective in stabilizing the contact resistance; 4) some rubber-modified epoxy resins and the epoxide-terminated polyurethane resins can provide the conductive adhesives with superior impact performance; and 5) conductive adhesives with stable contact resistance and desirable impact performance are developed.

A benchmark study of computational fluid dynamics predictive accuracy for component-printed circuit board heat transfer

Abstract: The application of computational fluid dynamics (CFD) analysis for the thermal design of electronic systems has the potential to enable accurate solutions to be generated and quickly assessed. With the use of validated numerical models, numerical analysis can also be used to provide useful insights into heat transfer processes which could otherwise be difficult to characterize experimentally. However, the capabilities of the CFD tool need to be carefully evaluated so as to provide a degree of confidence in prediction accuracy, thereby minimizing the need to qualify thermal designs. Such an evaluation is presented in this paper, which represents the culmination of a benchmark study by Rodgers et al. [1999]. This overall study assesses the predictive accuracy of a commercial CFD code for both natural and forced convection heat transfer of single- and multicomponent printed circuit boards (PCBs). Benchmark criteria were based on both component junction temperature and component-PCB surface temperature profiles. In the context of the overall study, this paper brings these analyses together to provide a more comprehensive assessment of CFD predictive accuracy for component junction temperature. Additionally the validated numerical models are used to further investigate the sensitivity of component heat transfer to convective environment, both natural and forced, component position relative to the PCBs leading edge, impact of upstream aerodynamic disturbance, and the representation of PCB FR4 thermal conductivity. The significance of the listed variables is quantified by analyzing predicted component energy balances. Qualitative descriptions of the fluid flow fields obtained using a novel paint film evaporation technique are also provided in this study. Both analyses yield new insights of the heat transfer processes involved and sources of numerical error.

Scale effects on fluid flow and heat transfer in microchannels

Abstract: Many investigators have recently experimentally established that fluid flow and heat transfer characteristics in microchannels, used to cool electronic chips, deviate from those of normally sized channels. Deviations increase as the channel size decreases. Many hypotheses were advanced to explain some of the observed deviations. However, they were usually in contradiction with other observed deviations. Hence, no conclusive explanation has been given so far for these phenomena. In this work, observed deviations will be listed, different hypotheses advanced to interpret these deviations will he critically revised. Finally, a new hypothesis will be advanced based on the increased role of surface roughness on flow and heat transfer in microchannels. Simple models will be built capable of reproducing observed trends for both friction and heat transfer coefficient in the laminar region, as well as giving an improved prediction for the laminar turbulent transition.

Flip chip underfill flow characteristics and prediction

Abstract: This paper presents recent results on underfill flow characterization. The flow properties of a number of commercial and experimental underfills were recorded and analyzed using quartz test chips with specially designed bump patterns (e.g., peripheral, full array, and mixed designs). Each was bonded onto an organic laminate substrate to form a flip chip package. Underfill was then applied to the packages and flow time, filler settling, and air entrapment were evaluated. Good flow can be described in terms of three measurable parameters, namely, viscosity, contact angle, and more importantly, filler size and distribution. Viscosity and contact angle are commonly used in Hele Shaw and Washburn models. However, these models do not take filler properties into consideration. In general, underfills with particles less than 5 /spl mu/m exhibited faster and more uniform flow fronts than materials with larger particles. The best flowing materials worked well with standoff heights between 50 and 75 /spl mu/m, while the poorer flowing materials showed streaking, voiding, and fingering at these heights. At gaps of 25 /spl mu/m, however, nearly all the materials exhibited pronounced and reproducible streaking.

Adhesion measurement for electronic packaging applications using double cantilever beam method

Abstract: Multilayers and interfaces are ubiquitous in microelectronics devices, interconnect and packaging structures. As the interface integrity becomes the major concern of performance, yield, and reliability, the need to evaluate the fracture and delamination behavior of various interfaces increases. This work focused on quantifying interfacial adhesion performance of a typical electronics packaging structure, flip-chip-on-organic-substrate. A series of experiments and analyzes were conducted to investigate the adhesion and fracture behaviors of the underfill/silicon and underfill/organic substrate interfaces. The experimental techniques for the interfacial fracture experiments were developed to produce the double-cantilever-beam (DCB) specimens and to establish a reproducible testing protocol. To extract the interfacial fracture energies, a closed-form solution was developed based on a beam-on-elastic-foundation model. A two-dimensional elastoplastic finite element analysis (FEA) model was also implemented to examine effects of mode-mixity, thermal/residual stresses, and underfill plasticity. The techniques allow for reproducible determination of underfill/printed circuit board (PCB) and underfill/silicon chip interfacial adhesion strength. The developed techniques are also readily applicable to evaluate interfacial adhesion performance for many other similar electronic packaging systems. This provides capabilities in optimizing material selections and process conditions to improve interfacial adhesion performance, Additionally, the interfacial fracture energy measured with high accuracy can provide a basis for realistic modeling of thermo-mechanical reliability of electronic components.

Low current arc modes of short length and time: a review

Abstract: Until the middle of the 20th century the electric arc was known solely as a steady, self sustaining gas discharge phenomenon requiring certain minimum values of current and voltage, depending on the electrode material and the gas environment. Special investigations on material transfer in contacts switching low direct currents, however, revealed step by step that there were certain till then unknown unstable arc modes appearing in extremely short electrode gaps only but requiring minimum current and voltage values distinctly lower than those of the classical steady arc discharge in air. Low current break arcs have to pass one or more of these transient modes whether or not they may eventually attain the steady gas arc mode.

Reliability investigations on conductive adhesive joints with emphasis on the mechanics of the conduction mechanism

Abstract: The isotropic conductive adhesive (ICA) mounting technology is of growing interest, but reliability concerns are still preventing its broad application. Reports on environmental testing results are related to both high temperature storage and thermal cycling. Additionally, the influence of moisture has been investigated for both pressure cooker test and humidity storage with exposure times up to several weeks. In an ICA, the conductive particles are embedded in a polymeric matrix material, where they can form conductive paths. This mechanical part of the conductive mechanism was studied in more detail using a finite element (FE-) model, because only a little information is available on this subject. A joint of a chip resistor on an organic board was selected for the model. The conductive adhesive is not treated as a homogeneous material, but split into the polymeric matrix material and idealized conductive particles. A temperature dependent viscoelastic constitutive description has been used to model the epoxy behavior. Additionally, moisture diffusion analyzes of the adhesive joints were conducted. The contacting pressure of the particles is shown to depend on cure shrinkage, temperature changes, and moisture swelling effects.

Electronic part life cycle

Abstract: The life cycle mismatch problem requires that engineers be cognizant of which parts will be available and obsolete when the product is to be manufactured. Next generation parts with improved performance characteristics must be anticipated in the design, to ensure that circuit timing, noise margins, and EMI nonconformances do not suddenly arise at the product level. An expensive part today may not be so at the time of manufacture, but assembly processes may have to be upgraded in order to use tomorrow's parts. If the product requires a long application life, then an open architecture, or a parts obsolescence strategy, such as preventive redesign, lifetime buy, aftermarket purchases or part substitution may be required. So, what is the life cycle of a part? Most electronic parts pass through several life cycle stages corresponding to changes in part sales. A representative life cycle curve of units shipped per time is given, which depicts the six common life cycle part stages: introduction, growth, maturity, decline, phase-out and discontinuance. The article summarizes the characteristics over the stages of the part life cycle, An overview of each stage is presented.

Fluxless process of fabricating In-Au joints on copper substrates

Abstract: Based on the oxidation-free fluxless bonding technology, we have developed a bonding process to manufacture In-Au joints on copper substrates. 4 mm/spl times/4 mm Si blank dice and 6 mm/spl times/6 mm copper substrates are used. The dice are deposited with indium-rich Au/In/Cr multilayer structure in a single high vacuum cycle to prevent oxidation. Right after deposition, the outer Au layer interacts with the In layer to form AuIn/sub 2/ intermetallic compound. This compound is quite stable and thus can protect the In layer against oxygen penetration when it is exposed to ambient. On the other hand, it can easily be dissolved by the molten In during the bonding process. The substrate is deposited with Cr and Au. The dice are bonded to the substrates at 180/spl deg/C in inert environment. Nearly void-free joints have been obtained as examined by a 75 MHz Scanning Acoustic Microscope (SAM). Cross sections of several samples are studied using SEM and EDX to identify the microstructure and composition of the joints. Shear test has been performed according to MIL-STD-883C. All the well-bonded devices meet the shear test force requirement. Despite the large mismatch on the thermal expansion coefficient between silicon and copper, no die cracking is observed on the 30 samples produced. To assess further endurance, two samples underwent thermal cycling test between -50 and 120/spl deg/C for 20 cycles, SAM examination indicates that the joints incur little degradation after the test. This bonding method requires neither flux nor scrubbing action. It is thus particularly attractive for bonding devices that cannot be exposed to flux.

Transient thermal behavior of high power diode laser arrays

Abstract: Reliability and lifetime of high power laser arrays are governed by their thermal properties. Thus the understanding of the thermal behavior such as thermal transients as well as the optimization of laser chips and mounting are key features for obtaining improved devices. We present numerical simulations of the active layer temperature employing the finite element method (FEM). Both continuous wave (cw) operation and thermal transients are modeled within a unified theoretical concept, which basically connects a balanced equation model that provides information on the temperature dependence of the loss mechanisms, such as spontaneous emission, Auger and surface recombination with a 2 dimensional FEM model. For a given laser array architecture we calculated the effect of the introduction of different heat spreader materials such as copper, silicon and diamond, Furthermore, different array designs such as broad area devices and stripe arrays having different output power (cw: 1-10 W) are numerically described. These results are compared with experimental data on the averaged temperature of the optically active layer. The temperature values are determined from the spectral shift of the emission spectrum of the array at a certain time windows after applying the operation current. Both experimental and theoretical results are compared from the 10 ns to the cw range. Thus both the theoretical description concept as well as the parameter set used for the calculation are carefully tested. Remarkable agreement between calculated and measured thermal transients was found. Additionally, the very divergent temporal behavior of special array structures was verified coincidentally by theory and experiment.

Integrated temperature microsensors for characterization and optimization of thermosonic ball bonding process

Abstract: A novel ball bond process optimization method based on the thermal response of an integrated aluminum microsensor is reported. The in situ temperature during ball bonding is measured and analyzed. The ultrasonic period shows distinct stages corresponding to scrubbing of the ball on the pad, intermetallic bond growth, and ball deformation by ultrasonic softening. A peak of the signal indicates the end of interconnection growth. This can be used for bond time optimization. When optimizing bonding force, the sensor signal correlates with ball shear strength. Using this method, bonding force process windows can be determined by on-line measurements. A test measurement shows that at a chip temperature of 34/spl deg/C, the bonding force optimized by the microsensor method is 260 mN whereas it is 252 mN when using conventional shear testing for optimization. In summary, the method produces a wealth of new insights in transient thermal phenomena of the ball bonding process and promises to simplify the evaluation of process windows.

Combined effects of sub-cooling and operating pressure on the performance of a two-chamber thermosyphon

Abstract: The heat dissipation rates at the chip level are projected to reach the 50-100 W/cm/sup 2/ mark for some future high performance electronic systems. Liquid cooling with phase change has been demonstrated to be a very efficient technique for thermal management of such high heat dissipation rates. Past work on liquid immersion cooling using fluorocarbons has shown the advantage of using enhanced structures to reduce boiling incipience excursion and raise the critical heat flux (CHF). Thermosyphons, employing these enhanced structures are an alternative to liquid immersion and are suitable for point cooling applications, where very compact evaporators are needed. This study investigates the combined effect of sub-cooling and pressure on the performance of an enhanced microstructure based thermosyphon, which has shown very high heat transfer rates (up to 100 W/cm/sup 2/ with a wall superheat of 27.8/spl deg/C). The pressure levels tested were partial vacuum (40-101.3 kPa), atmospheric pressure (101.3 kPa) and high pressure (101.3-370 kPa). The experiments were initiated at room temperature, and hence the sub-cooling corresponded to the difference in the liquid saturation temperature at the starting system pressure and room temperature. The results show a reduction in wall superheat values at higher pressures, at a given heat flux. The performance of the system was evaluated by defining a surface-to-ambient resistance. Results show that a partial vacuum at all heat fluxes results in better performance compared to higher pressures. The combined effect of pressure and sub-cooling was also tested for a compact evaporator and the results obtained were similar to the baseline case (larger evaporator).

A study of contact resistance of conductive adhesives based on anhydride-cured epoxy systems

Abstract: Electrically conductive adhesives (ECAs) are an environmentally friendly alternative to tin/lead (Sn/Pb) solders in electronics packaging applications. However, current conductive technology is still in its infancy and limitations do exist. One of the critical reliability issues is that contact resistance of silver flake-filled ECAs on nonnoble metals increases in elevated temperature and humidity environments. The main objective of this study is to investigate the contact resistance behaviors of a class of conductive adhesives, which are based on anhydride-cured epoxy systems. Curing profiles, moisture pickup, and shifts of contact resistance of the ECAs on a nonnoble metal, tin/lead (Sn/Pb), during aging are investigated. Also, two corrosion inhibitors are employed to stabilize the contact resistance. The effects of these corrosion inhibitors on contact resistance are compared. It is found that: (1) this class of ECAs shows low moisture absorption, (2) the contact resistance of the ECAs on Sn/Pb decreases first and then increases slowly during 85/spl deg/C/85% relative humidity (RH) aging, (3) one of corrosion inhibitors is very effective to stabilize contact resistance of these ECAs on Sn/Pb, and (4) the corrosion inhibitor stabilizes contact resistance through adsorption on Sn/Pb surfaces. From this study, it can be concluded that ECAs based on anhydride cured epoxy systems are promising formulations for electronics packaging applications.

Study on effect of carbon black on behavior of conductive polymer composites with positive temperature coefficient

Abstract: Conductive polymer composites showing large positive temperature coefficient (PTC) are made of semi-crystalline polymer as an insulator and a conducting filler, whose concentration is close to the critical volume fraction. In this study, the resistivity and PTC behavior of high density polyethylene (HDPE) filled with different carbon blacks were studied. Among those composites, N660 carbon black filled PE showed the greatest PTC behavior. Carbon black with large particle size, small surface area and small amount of aggregated structure leads to large amplitude of PTC transition (defined as the ratio of maximum resistivity to the resistivity at 25/spl deg/C). The great PTC behavior is due to some microscopic mechanism under the macroscopic thermal expansion of polymer matrix during melting of polymer crystal.

Effect of bump height on the strain variation during the thermal cycling test of ACA flip-chip joints

Abstract: Flip chip joining using anisotropically conductive adhesive (ACA) has become a very attractive technique for electronics packaging. Many factors can influence the reliability of the ACA flip-chip joint. Bump height, is one of these factors. In this work, the strain development during the thermal cycling test of flip-chip joining with different bump heights was studied. The effect of bump height is significant in the interface between the bumps and the pads. Bigger volume area of high strain is found for higher bump in the interface between the bumps and the pads. Our calculations show that there is practically no effect of the bump height on the strain variation in the bumps and in the pads.

An investigation of thermal enhancement on flip chip plastic BGA packages using CFD tool

Abstract: This paper demonstrates the advantage of applying Predictive Engineering in the thermal assessment of a 279 inputs/outputs (I/Os), six-layer, depopulated array flip chip PBGA package. Thermal simulation was conducted using a computational fluid dynamics (CFD) tool to analyze the heat transfer and fluid flow in a free convection environment. This study first describes the modeling techniques on a multilayer substrate, thermal vias, solder bumps, and printed circuit board (PCB). For a flip chip package without any thermal enhancement, more than 90% of the total power was conducted from the front surface of the die through the solder ball interconnects to the substrate, then to the board. To enhance the thermal performance of the package, the heat transfer area from the backside of the die needs to increase dramatically. Several thermal enhancing techniques were examined. These methods included a copper heat spreader with various thicknesses and with thermal pads, metallic lid, overmolded with and without a heat spreader, and with heat sink. An aluminum lid and a heat sink gave the best improvement; followed by a heat spreader with thermal pads. Both methods reduced thermal resistance by an average of 50%. Detailed analyses on heat flow projections are discussed.

Compact thermal models of packages used in conduction cooled applications

Abstract: Results of an extensive study aimed at developing boundary condition independent compact steady-state thermal models of a variety of electronic packages used in conduction cooled applications are presented. Formal mathematical principles were used to establish a nonredundant set of thermal boundary conditions representing board edge and backside cooling with variable board and underfill conductivity. A Design of Experiments approach was employed to reduce the total number of boundary conditions to four, allowing the generation of boundary condition independent CTM's. Two general network topologies, incorporating both simple star-shaped and more complex, shunted networks were developed. To extract the CTM parameters, the thermal networks were optimized using a genetic algorithm-based approach allowing constrained nonlinear global optimization in a standard spreadsheet environment. Comparisons of the accuracy of models from simple to complex are presented for two types of generic parts. It was found that optimized star-shaped CTM's accurately predict junction temperatures, but usually give insufficient accuracy for the heat flows leaving via the package prime lumped surfaces. The inclusion of a floating node allows sufficient degree of freedom to correctly redistribute the heat flows between the "outlet" nodes of the networks. Using the optimization technique, CTM's were derived for thirty parts representing thirteen package families. For most of the packages only network topologies that included a floating node and surface-to-surface links provided satisfactory accuracy. With three different network configurations, for which examples are presented, it was possible to capture the thermal behavior of all the package families investigated.

Bayesian surrogates for integrating numerical, analytical, and experimental data: application to inverse heat transfer in wearable computers

Abstract: Wearable computers are portable electronics worn on the body. The increasing thermal challenges facing these compact electronics systems have motivated new cooling strategies such as transient thermal management with thermal storage materials. The ability of building models to assess quickly the effect of different design parameters is critical for effectively incorporating innovative thermal strategies into new products. System models that enable design space exploration are built from different information sources such as numerical simulations, physical experiments, analytical solutions and heuristics. These models, called surrogates, are nonlinear, adaptive, and suitable for system responses where limited information is available and few realizations of experiments or numerical simulations are feasible. This paper applies a Bayesian surrogate framework to estimate values for unknown physical parameters of an embedded electronics system. Physical experiments and numerical simulations are performed on an embedded electronics prototype system of a wearable computer. Numerical models for the experimental prototype, which involve five and three unknown parameters, are implemented with and without thermal contact resistances. Through the use of orthogonal arrays and optimal sampling, an efficient exploration of the parameter space is performed to determine thermal conductivities, thermal contact resistances and heat transfer coefficients. Surrogate models are built that combine information obtained from numerical simulations, experimental model measurements and a thermal resistance network. The integration of several information sources reduces the number of large-scale numerical simulations needed to find reliable estimates of the system parameters. For the embedded electronics case, the use of prior information from the thermal resistance network model reduces significantly the computational effort required to investigate the solution space.

Experimental and numerical investigation into the influence of printed circuit board construction on component operating temperature in natural convection

Abstract: The steady state thermal performance of an isolated SO-8 package is experimentally characterized on five thermal test printed circuit boards (PCBs) and the results compared against corresponding numerical predictions. The study includes the low and high conductivity JEDEC standard, FR4 test PCBs and typical application boards. With each PCB displaying a different internal structure and effective thermal conductivity, this study highlights the sensitivity of component operating temperature to the PCB, provides benchmark data for validating PCB numerical modeling methodologies, and helps one assess the applicability of standard junction-to-ambient thermal resistance (/spl theta//sub JA/) data for design purposes on nonstandard PCBs. Measurements of junction temperature and component-PCB surface temperature distributions were used to identify the most appropriate modeling methodology for both the component and the PCB. Based on these results, a new PCB modeling methodology is proposed that conserves the need for modeling detail without compromising prediction accuracy.

Thermal and hydraulic behavior of plate fin and strip fin heat sinks under varying bypass conditions

Abstract: Tests have been conducted in a wind tunnel with nine heat sinks of three different types including plate fin heat sinks and strip fin heat sinks arranged in both inline and staggered arrays. For each type, tests were run with fin heights (H) of 10, 15, and 20 mm while the heat sink width (B) was kept constant and equal to 52.8 mm. The width of the wind tunnel duct (CB) was varied in such a way that results were obtained for B/CB=0.84, 0.53, and 0.33. The wind tunnel height (CH) was varied similarly, and data were recorded for H/CH=1,0.67, and 0.33 while the duct Reynolds number was varied between 2000 through 14000. An empirical bypass correlation has been fitted to the experimental data. Generally, the agreement between experimental data and correlation is within /spl plusmn/10% for the thermal resistance, and within /spl plusmn/20% for the pressure drop. From the experimental data, the fraction of the total airflow passing through the heat sink have been estimated and are compared to a simple physical bypass model.