Showing papers in "Journal of Electronic Packaging in 2003"

Thermal Spreading Resistance of Eccentric Heat Sources on Rectangular Flux Channels

Abstract: A general solution, based on the separation of variables method for thermal spreading resistances of eccentric heat sources on a rectangular flux channel is presented. Solutions are obtained for both isotropic and compound flux channels. The general solution can also be used to model any number of discrete heat sources on a compound or isotropic flux channel using superposition. Several special cases involving single and multiple heat sources are presented. @DOI: 10.1115/1.1568125#

Pumpless Loop for Narrow Channel and Micro-Channel Boiling

Abstract: A compact cooling system is examined which capitalizes upon fluid density differences between two vertical, parallel, interconnected tubes to achieve a pumpless cooling loop. A heat-dissipating device is incorporated into a boiler at the bottom of the hot tube. The large density differences between the two tubes produces a substantial nonequilibrium in hydrostatic pressure, drawing liquid downwards through the cold tube as a two-phase mixture is released upwards in the hot tube. Cooling with this pumpless loop is fundamentally different from, and far superior to, pool boiling thermosyphons because of the former’s ability to separate the path of replenishment liquid from that of the released vapor. Experiments were performed to explore the effects of boiler gap (separation distance between the boiling surface and opposite insulating wall) on cooling performance and critical heat flux (CHF) for water and FC-72. The gap, which is the primary measure of boiler miniaturization, was varied from 0.051 to 21.46 mm. For large gaps, CHF showed insignificant dependence on the gap for both fluids. However, small gaps produced CHF variations that were both drastic and which followed opposite trends for the two fluids. Decreasing the gap below 3.56 mm produced a substantial rise in CHF for FC-72. For water, CHF was fairly insensitive down to 0.51 mm, below which it began to decrease sharply. These trends are shown to be closely related to the small surface tension and contact angle of FC-72 producing very small bubbles which can easily pass through narrow gaps in FC-72, while much larger bubbles in water obstruct liquid replenishment in narrow gaps. A numerical model is constructed to determine how the gap influences the various components of pressure drop, velocities, coolant flow rate, and hence system response to heat input. @DOI: 10.1115/1.1602708#

A Damage Mechanics-Based Fatigue Life Prediction Model for Solder Joints

Abstract: A thermomechanical fatigue life prediction model based on the theory of damage mechanics is presented. The damage evolution, corresponding to the material degradation under cyclic thermomechanical loading, is quantified thermodynamic framework. The damage, as an internal state variable, is coupled with unified viscoplastic constitutive model to characterize the response of solder alloys. The damage-coupled viscoplastic model with kinematic and isotropic hardening is implemented in ABAQUS finite element package to simulate the cyclic softening behavior of solder joints. Several computational simulations of uniaxial monotonic tensile and cyclic shear tests are conducted to validate the model with experimental results. The behavior of an actual ball grid array (BGA) package under thermal fatigue loading is also simulated and compared with experimental results. @DOI: 10.1115/1.1536171# With the increasing use of surface mount bonding technology in microelectronics industry, the reliability concerns for solder joints are increasing exponentially. Eutectic solder alloys are most commonly used bonding materials in electronic packaging, which provide electrical and thermal interconnection, as well as mechanical support. The temperature fluctuations due to device internal heat dissipation and ambient temperature changes, along with the coefficient of thermal expansion ~CTE! mismatch between the soldered layers, result in thermo-mechanical fatigue of the solder joints. Progressive damage in solder balls eventually leads to device failure. Fatigue life prediction of solder joints is critical to the reliability assessment of electronic packaging. Standard state of practice in the electronic industry for the number of cycles to failure prediction is based on using empirical relations, such as Coffin-Manson approach. Typically, using the CTE differential between the bonded components, the maximum elastic and plastic strain in the solder joint is calculated. Most of the time, using the plastic strain value, Coffin-Manson curves are used to predict the fatigue life of solder joints. Usually this approach yields very conservative results for BGA packages, Zhao et al. @1#. Recently, numerous physics-of failure based models have been developed for the evaluation of reliability of solder alloys under thermo-mechanical fatigue loading, such as Busso

Packaging Induced Die Stresses—Effect of Chip Anisotropy and Time-Dependent Behavior of a Molding Compound

Abstract: This paper investigates the effect of the anisotropic behavior of the die and the time- and temperature-dependent behavior of epoxy molding compound on the packaging induced stresses for a quad flat package. Finite element (FE) simulations using isotropic and anisotropic properties of the die are carried out, respectively, and the results are compared. Creep experiments were performed at different temperatures ranging from 265°C to 230°C to obtain the long-term master curves and the related shift factors for the creep compliance of the molding compound. FE models which incorporate the viscoelastic constitutive relation of the material are constructed to simulate the thermo-mechanical stresses caused by the packaging processes. The influences of both the chip anisotropy and the viscoelastic behavior of the molding compound on the packaging induced stresses are discussed. @DOI: 10.1115/1.1604153# At present, thermo-mechanical reliability of integrated circuit ~IC! packages is still one of the major concerns in the electronic industry. Critical stress levels may be induced in the package constituents during the thermal processing due to mainly the mismatch in thermal-expansion coefficients of the materials. The prediction of these packaging induced thermo-mechanical stress levels can only be true if reliable material properties for each constituent are taken into account in the modeling. In most of the publications addressing the thermo-mechanical behavior of IC packages, the silicon die was modeled as temperature independent and isotropic. However, given the nature of the silicon material, which is a crystal, the assumption of isotropy is not true. In fact, due to the orientation of the silicon crystal, a diamondlike crystallographic structure, anisotropic material behavior is to be expected. Ultrasound measurements ~20/40 MHz! were used to obtain the stiffness values in the different directions for the silicon crystal @1‐3#. It was reported that the stiffness values at temperature 273 K range from 170 GPa in the @110# plane to 130 GPa in the @100# plane @2#. These values show that an isotropic approach of the silicon material may not be valid. The temperature dependence of the stiffness values was found to be negligible, for instance the in plane value at 473 K is only 0.5% lower. Previous research work also showed that the temperature independence of the thermal-expansion coefficient ~CTE! for the silicon crystal is valid within a certain temperature range @3‐6 #. Thermosetting resins, like other polymeric materials, have strong time- and temperature-dependent mechanical properties even if they are filled with a high percentage of filler. The creep and relaxation of the packaging material during packaging processes and/or testing will cause a redistribution of stress and strain levels in the chip. However, for the reason of simplicity, in most of the thermo-mechanical packaging simulations the viscoelasticity of the molding compound is totally or partially neglected. As a consequence, the predicted stress levels and its evolution during packaging processes and/or testing may not be representative for the reality. In Refs. @7‐9# it was reported that by using a viscoelastic model for the molding compound, the predicted stresses and deformations are closer to the real situations. The present study focuses on the investigation of the influence of chip anisotropy and the time- and temperature-dependent behavior of a molding compound on the stress levels during packaging processes and testing conditions. In the finite element ~FE! simulations, the anisotropic and isotropic properties of the silicon die are used, respectively, and a comparison of their results is made. Creep experiments were performed at different temperatures ranging from 265°C to 230°C to obtain the long-term master curves and the related shift factors for the creep compliance of the molding compound. FEM models, representing a quad flat package ~QFP! package, in which the viscoelastic model of the material is implemented, are constructed and used to simulate the thermo-mechanical stresses caused by the packaging processes.

Response surface modeling for nonlinear packaging stresses

Abstract: The present study focuses on the development of reliable response surface models (RSM's) for the major packaging processes of a typical electronic package. The major objective is to optimize the product/process designs against the possible failure mode of vertical die cracks. First, the finite element mode (FEM)-based physics of failure models are developed and the reliability of the predicted stress levels was verified by experiments. In the development of reliable thermo-mechanical simulation models, both the process (time and temperature) dependent material nonlinearity and geometric nonlinearity are taken into account. Afterwards, RSM's covering the whole specified geometric design spaces are constructed. Finally, these RSM's are used to predict, evaluate, optimize, and eventually qualify the thermo-mechanical behavior of this electronic package against the actual design requirements prior to major physical prototyping and manufacturing investments.

Thermal Resistances of Circular Source on Finite Circular Cylinder With Side and End Cooling

Abstract: General solution for thermal spreading and system resistances of a circular source on a finite circular cylinder with uniform side and end cooling is presented The solution is applicable for a general axisymmetric heat flux distribution which reduces to three important distributions including isoflux and equivalent isothermal flux distributions The dimensionless system resistance depends on four dimensionless system parameters It is shown that several special cases presented by many researchers arise directly from the general solution Tabulated values and correlation equations are presented for several cases where the system resistance depends on one system parameter only When the cylinder sides are adiabatic, the system resistance is equal to the one-dimensional resistance plus the spreading resistance When the cylinder is very long and side cooling is small, the general relationship reduces to the case of an extended surface (pin fin) with end cooling and spreading resistance at the base The special case of an equivalent isothermal circular source on a very thin infinite circular disk is presented @DOI: 101115/11568124#

Choosing Working Fluid for Two-Phase Thermosyphon Systems for Cooling of Electronics

Abstract: The heat fluxes from electronic components are steadily increasing and have now, in some applications, reached levels where air-cooling is no longer sufficient. One alternative solution, which has ...

Time- and Temperature-Dependent Thermo-Mechanical Modeling of a Packaging Molding Compound and its Effect on Packaging Process Stresses

Abstract: For reliable virtual thermo-mechanical prototyping of electronic packages appropriate descriptions of the mechanical behavior of the constituent materials are essential In many packages molding compounds are used for encapsulation and underfill to provide environmental protection and/or to improve the package thermal mechanical reliability Therefore, among others, the availability of appropriate constitutive models for various epoxy-molding compounds is one of the requirements for computational prototyping As there is a large variability of available molding compounds, it is essential to be able to experimentally establish the model parameters in an efficient manner Because of the implied simplicity, linear visco-elastic models combined with the time-temperature superposition theory are mostly used in thermo-mechanical simulations Among the various experimental possibilities to efficiently establish the model parameter functions, in the present paper the use of unidirectional creep testing is worked out for a chosen molding compound Here isothermal one-day creep experiments at different temperatures (ranging below and above the glass transition temperature of the compound) are performed The tensile creep compliance and the time-dependent Poisson’s ratio of the material at different temperatures are successfully used to construct visco-elastic master curves As the Poisson’s ratio shows a significant change during a creep or relaxation test, its effect in partly constraint situations (as in packages) will be evident Therefore it is not reliable to approximate this variable using a constant value Further, the visco-elastic model of the material is implemented in a finite element program and verified by means of a shear stress relaxation experiment and a creep experiment both under nonisothermal conditions Moreover, the effect of the creep behavior of the molding compound on the packaging process stress field and its evolution is investigated Substantial cost saving was realized by package design optimization based on the reliable prediction of the packaging process stresses @DOI: 101115/11604156#

Optimal Internal Structure of Volumes Cooled by Single-Phase Forced and Natural Convection

Abstract: Design is the permanent struggle for better and better global system performance, under imposed constraints. This paper reviews a body of electronics cooling results that illustrate how the physical structure of the system—its architecture, construction— springs out of the pursuit of objective under constraints. The same ‘constructal’ principle accounts for the generation of geometric form in other flow systems, engineered and natural @1#. They are part of a more general trend in which a greater appreciation of geometric form, scales and similitude is leading us to more powerful ~unifying! correlations of heat and fluid flow results developed originally for individual configurations. Professor Yovanovich has played a pioneering role in this direction @2‐4 #. Consider the main issues while designing a package of electronic components that must fit inside a given volume—the global constraint. The objective of the design is to install as much circuitry as possible in this volume. In rough terms, this is equivalent to installing as much heat generation rate ~q! as possible, because electrical components generate heat. The highest temperatures in the package ~the hot spots! must not exceed a specified value, Tmax : this is also a global constraint, because the maximum temperature is a feature of the entire system. The exact location of the hot spots is not an issue. If the local temperature ~T! rises above this allowable ceiling, the electronic functioning of the local component is threatened. In sum, the thermal design is better when q is larger. In other words, the design is better when the global thermal conductance ratio q/(Tmax2T0) is larger, where T0 is the initial ~reference, sink! temperature of the coolant that absorbs q. Papers and reviews of the electronics cooling literature illustrate this design philosophy @5‐13#.

Residual Stresses in Multilayer Ceramic Capacitors: Measurement and Computation

Abstract: In this paper, we present a combined experimental and computational study of the thermomechanical reliability of multilayer ceramic capacitors (MLCC’s). We focus on residual stresses introduced into the components during the cooling down step of the sintering process. The technique of microindentation turned out to be a useful method to measure the stresses locally. The computations were done with three-dimensional finite element simulations. We find that the cooling step introduces compressive in-plane stresses in the ceramic layers. There is reasonably good overall agreement between the residual stresses obtained from the indentation experiments and the numerical simulations. Some discrepancies do exist, though, for measurements on cross-sectioned MLCC’s. Possible reasons for the differences are discussed.@DOI: 10.1115/1.1604151# The basic structure of a multilayer ceramic capacitor ~MLCC! consists of alternating thin layers of dielectric ceramic material and metal electrodes, as shown schematically in Fig. 1. Two metal end terminations function as electrical contacts and are used to solder the MLCC on a printed circuit board. The trend for these components is a reduction in overall size and in layer thickness, accompanied by an increase in the number of layers. The reliability of multilayer ceramic capacitors is related directly to their thermomechanical integrity; for a review see Ref. @1#. Therefore each MLCC must meet certain standard requirements with respect to its thermomechanical integrity, and these requirements become more and more stringent. Tests such as the flex test, the temperature cycling test, the temperature shock test or resistance to solder heat ~RSH! test are used to assess the thermomechanical reliability of the components. The behavior of the components during these tests is determined by many factors. First, material parameters such as the elastic modulus, the fracture toughness, and the thermal expansion coefficient play a role. Second, residual stresses present in the components due to processing are important. Finally, it is obvious that the stress distribution generated in the components during testing is a key factor. The present study focuses on residual stresses introduced into the MLCC during the cooling traject after sintering of the component. The manufacturing of MLCC’s consists of many process steps, see, e.g., Ref. @2#. One important step is sintering. At this stage, the component consists only of a stack of ceramic and metal layers, so the end terminations visible in Fig. 1 have not been applied yet ~this happens at a later stage in manufacturing!. This geometry is called a ‘‘brick.’’ The brick is sintered at a temperature of about 1300°C, during which the ceramic material densifies and obtains properties critical for a proper functioning of the final MLCC. After sintering the component is cooled down to room temperature. During the cooling down, the metal electrodes solidify. Moreover, both the metal and the ceramic layers decrease in volume. However, due to the differences in thermal expansion between metal and ceramic, residual stresses are introduced after the metal has solidified. These resididual stress may play a crucial role in the mechanical integrity of the final components; see, e.g., Ref. @1#. We have carried out full three-dimensional finite element simulations of this process step to compute these stresses. The material properties needed as input parameters have been measured with various experimental techniques. At the same time, we have carried out dedicated experiments to measure the residual stresses to be able to validate the simulation results. The technique we have used is microindentation. 2 Description of the Experiments

Packaging of Optical MEMS Devices

Abstract: Recently, optical MEMS devices have gained considerable attention in the telecommunications industry -- particularly in the optical networking and switching arenas. Since optical MEMS are micro-systems which rely on high precision optics, electronics and mechanics working in close concert, these emerging devices pose some unique packaging challenges yet to be addressed by the general packaging industry. Optical MEMS packages often are required to provide both optical and electrical access, hermeticity, mechanical strength, dimensional stability and long-term reliability. Hermetic optical access necessitates the use of metallized and anti- reflection coated windows, and ever-increasing electrical I/O count has prompted the use of higher density substrate/package technologies. Taking these requirements into consideration, we explore three ceramic packaging technologies, namely High Temperature Co-fired Ceramic (HTCC), Low Temperature Co-fired Ceramic (LTCC) and thin-film ceramic technologies. In this paper, we describe some optical MEMS packages designed using these three technologies and discuss their substrate designs, package materials, ease of integration and assembly.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.