bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermomechanical Mechanism for Delamination of Polymer Coatings From Optical Fibers
Authors: King W. W. , Aloisio C. J.
Published: February 2008

Sometimes the polymer coating on an optical fiber is observed to have separated from the fiber over a small portion of the interface. Irregularities on the capstans and sheaves of draw, rewind, coloring, and cabling machines can initiate such delaminations. Subsequent growth would not be anticipated under the condition of radial compressive stress that might be expected for a coating shrinking over a relatively rigid fiber as the composite cools during manufacture. Compressive stress is indeed found at the interface when a single-layer coating is used. However, for a two-layer system, having a high-modulus secondary over a low-modulus primary (for improved protection against microbending), the different rates of thermal expansion can lead to radial tension at the silica/primary interface, and this tension can “drive” the growth of delaminations. A principal result of this study is that the analysis predicts the primary coating, although rubbery, to be approximately in a state of uniform hydrostatic tension. This tensile stress is of substantial magnitude because of constraints imposed by the relatively stiff secondary coating and by the fiber. The existence of significant radial tension at the fiber surface is consistent with experimental observations of induced delaminations, which are seen to grow long after cessation of external disturbances.
Solid Model Based Preprocessor to CFD Code for Applications to Electronic Cooling Systems
Authors: Agonafer D. , Vimba A.
Published: February 2008

The use of a solid model based Computer Aided Design (CAD) tool as a preprocessor to a finite control-volume based Computational Fluid Dynamics (CFD) code is presented. Preprocessing includes geometry description, grid generation, definition of material properties, application of boundary conditions, and definition of solution control parameters. The CAD based preprocessor, as opposed to traditional finite control-volume preprocessors, provides the above capabilities in a powerful graphic environment. Using a solid model based CAD tool, work is reduced, and visualization is enhanced employing the capabilities of the three-dimensional solid modeler. In addition, a technique which categorizes control volumes into groups comprising the solid and fluid portions of the problem domain is presented. At the completion of preprocessing, a model appropriate as input to a CFD code is generated. This model is then solved using the CFD program. The process is shown in a tutorial form by considering a two-dimensional turbulent flow problem in an electronic card on board package. Although the methodology shown in this paper focuses on specific CFD and Solid Model programs, the concept can readily be applied to other CFD and/or Solid Model programs.
Moisture Migration and Cracking in Plastic Quad Flat Packages (PQFPs)
Authors: Taylor S. A. , Chen K. , Mahajan R.
Published: February 2008

Moisture migration in PQFPs (plastic quad flat packages) was investigated analytically and experimentally. Coupons made of the molding compound material were exposed to humid air for an extended time to determine its equilibrium moisture composition as a function of environmental moisture level. An IR (infrared) heating test was designed to simulate the high-temperature heat reflow process when PQFPs are soldered on computer boards. The C-SAM (C-mode Scanning Acoustic Microscopy) technique was employed to measure the delaminated areas. All packages failed after IR heating when the moisture content in PQFPs was higher than 60 percent of the equilibrium composition.
Uniform Channel Micro Heat Exchangers
Authors: Yin X. , Bau H. H.
Published: February 2008

The performance characteristics of micro heat exchangers consisting of straight channels fabricated in a silicon wafer and operating with liquid nitrogen are evaluated theoretically. The three-dimensional conjugate temperature and flow fields are calculated numerically. The thermal resistance is computed as a function of various geometric parameters. The optimal dimensions which minimize the heat exchanger’s thermal resistance are identified. Additionally, a simple, approximate, analytical model, adequate when the ratio of the solid and liquid thermal conductivities is large, is presented and used to predict the heat exchanger’s optimal dimensions. The approximate model’s predictions are compared with the numerical results.
Combined Pressure and Subcooling Effects on Pool Boiling From a PPGA Chip Package
Authors: Watwe A. A. , Bar-Cohen A. , McNeil A.
Published: February 2008

This study presents a detailed experimental investigation of the combined effects of pressure and subcooling on nucleate pool boiling and critical heat flux (CHF) for degassed fluorocarbon FC-72 boiling on a plastic pin-grid-array (PPGA) chip package. In these experiments pressure was varied between 101.3 and 303.9 kPa and the subcooling ranged from 0 to 65°C. As expected, lower wall superheats resulted from increases in pressure, while subcooling had a minimal effect on fully developed pool boiling. However, the superheat reductions and CHF enhancements were found to be smaller than those predicted by existing models. The CHF for saturated liquid conditions increased by nearly 17 percent for an increase in pressure from 101.3 to 202.7 kPa. In experiments with both FC-72 and FC-87 further increases in pressure did not produce any significant increase in CHF. At a pressure of 101.3 kPa a subcooling of 30°C increased CHF on horizontal upward-facing chips by approximately 50 percent, as compared to 70 percent on vertically oriented packages. The enhancement in CHF due to subcooling decreased rapidly with increasing pressure, and the data showed that the influence of pressure and subcooling on CHF is not additive. A correlation to predict pool boiling CHF under the combined effects of pressure and subcooling is proposed.
Maximizing Solder Joint Reliability Through Optimal Shape Design
Authors: Deshpande A. M. , Subbarayan G. , Mahajan R. L.
Published: February 2008

The automated search techniques from the field of numerical optimization provide tools that enable optimal design of electronic packages in general, and solder joints in particular. However, there is considerable difficulty in using these procedures for solder joints since the estimation of fatigue life is computationally very expensive. In this paper, global approximation schemes based on designed experiments, linear regression models, and artificial neural network models are developed to approximate the fatigue life as a function of solder joint design parameters. Since these approximate surfaces are inexpensive to evaluate, their use with the numerical optimization techniques leads to a computationally efficient method for optimizing electronic packages. The developed techniques are demonstrated using the 225 I/O Plastic Ball Grid Array (PBGA) package, manufactured by Motorola, Inc. An exact optimization of the solder joints (without approximations) is also carried out and used as a basis for comparing the accuracy and efficiency of the developed methods.
A Nonlinear Multi-Domain Thermomechanical Stress Analysis Method for Surface-Mount Solder Joints—Part II: Viscoplastic Analysis
Authors: Ling S. , Dasgupta A.
Published: February 2008

This is part II of a two-part paper presented by the authors for thermomechanical stress analysis of surface mount interconnects. A generalized multi-domain Rayleigh Ritz (MDRR) stress analysis technique has been developed to obtain the stress and strain fields in surface-mount solder joints under cyclic thermal loading conditions. The methodology was first proposed in Part I by the authors and results were presented for elastic-plastic loading (Ling et al., 1996). This paper extends the analysis for viscoplastic material properties. The solder joint domain is discretized selectively into colonies of nested sub-domains at locations where high stress concentrations are expected. Potential energy stored in the solder domain and in the attached lead and Printed Wiring Board (PWB) is calculated based on an assumed displacement field. Minimization of this potential energy provides a unique solution for the displacement field, consequently, stress and strain distribution. The MDRR technique was demonstrated to provide reasonable accuracy for elastic deformation (Ling and Dasgupta, 1995) and for time-independent elastic-plastic deformation (Ling and Dasgupta, 1996) for solder joints under cyclic thermal loading conditions. A piecewise linear incremental loading technique is used to solve the nonlinear elastic-plastic problem. The focus in the current paper is primarily on time-dependent viscoplastic deformation of the solder joints. Full field elastic, plastic, and viscoplastic analyses are performed, and the stress, strain hysteresis loops are obtained. Results are presented for a J-lead solder joint as an illustrative example.
Stress Analysis of Surface-Mount Interconnections Due to Vibrational Loading
Authors: Darbha K. , Ling S. , Dasgupta A.
Published: February 2008

Recently, accelerated testing of surface mount interconnects under combined temperature and vibration environments has been recognized to be a necessary activity to ensure enhanced test-time compression. Successful use of vibration stresses requires a clear understanding of the correlation between vibrational damage and thermomechanical damage in surface mount solder joints. Hence, fatigue due to vibrational loading is important and accurate quantitative models are required to model effects due to vibrational fatigue. The proposed analysis in this paper contributes towards development of such quantitative models. This paper presents an approximate method to analyze stresses in surface mount solder joints subjected to vibration loading, using a generalized multidomain Rayleigh-Ritz approach (Ling and Dasgupta, 1995). The advantage of this approach is in its computational efficiency, compared to general-purpose finite element methods. Ling developed this approach in the context of thermomechanical stress analysis of solder joints. In this paper, the technique is modified and adapted for analyzing stresses caused by out-of-plane flexural dynamic modes of the printed wiring boards (PWBs). The analysis uses a two-step procedure where the local PWB curvatures are first estimated and the resulting deformations in the solder interconnect are then determined. The input boundary conditions for the first step are the bending moments in the PWB due to random vibrations. The stiffness of the interconnect assembly is then predicted using an energy method and curved-beam analysis. The bending moment and the computed stiffness of the interconnect assembly are then used to predict the local curvature of the PWB under any given surface-mount component by using an eigenfunction technique developed by Suhir (Suhir, 1988). In the second step of the analysis, the local curvature of the PWB is used as a boundary condition to predict the state of deformations, stresses, and strains in the solder joint using a modified version of the multidomain Rayleigh-Ritz approach. The overall method is applied to a specific example (J-lead solder joint) for illustrative purposes, and compared to finite element predictions for validation.
Warpage and Interfacial Stress Distribution in a Single-Level Integrated Module (SLIM)
Authors: Dunne R. C. , Sitaraman S. K.
Published: February 2008

The thermomechanical behavior of a new multilayered substrate with embedded passive components is the focus of this study. Such structures are typically susceptible to localized interfacial stresses near the free edge, which might initiate delamination and subsequent failure of the electronic component. The effect of some key parameters—base layer material, interlayer dielectric material, base layer thickness, and temperature range—on substrate warpage and interfacial shear and peel stress distribution is presented. Material and design recommendations for improved thermomechanical response are suggested based on this initial parametric study.
Thermal and Mechanical Evaluations of a Cost-Effective Plastic Ball Grid Array Package
Authors: Lau J. H. , Chen K.-L.
Published: February 2008

The temperature distribution and thermal resistance of a facedown PBGA (Plastic Ball Grid Array) package assembled on a FR-4 epoxy glass PCB (Printed Circuit Board) are presented. By varying the thickness of the copper heat spreader and organic substrate, an optimum PBGA package is designed. The effect of power and ground planes in the PCB and the size of PCB on the thermal performance of the PBGA is also given. Furthermore, the warpage (deflection) of the package under thermal loading is predicted.
Transient Response of Microchannel Heat Sinks in a Silicon Wafer
Authors: Rujano J. R. , Rahman M. M.
Published: February 2008

The transient conjugated heat transfer in forced convection for simultaneously developing laminar flow inside a microchannel heat sink is studied by solving the steady momentum equation and the transient energy equation. A parametric study is performed to understand the effects of channel depth and width, Reynolds number, spacing between channels, and solid to fluid thermal conductivity ratio. Silicon as well as indium phosphide are used as wafer’s material. Step and pulsed variations of the heat load are analyzed. Results show that the time required for the heat transfer to reach steady state condition is longer for the system with larger channel depth or spacing and smaller channel width or Reynolds number. Characteristic results for the fluid mean temperature at the exit, solid maximum temperature, local Nusselt number, and local heat flux are presented graphically as functions of position and time.
Prediction of Solder Geometry for an Axisymmetric Through-Hole Joint
Authors: Elkouh A. F. , Ramasubramanian N. , Hsu T. F. , Nigro N. J. , Heinrich S. M. , Lee P. S. , Shangguan D.
Published: February 2008

Solutions for axisymmetric profiles of solder joints formed between a cylindrical pin and a printed circuit board (PCB) are presented. The dimensionless differential equation governing the formation of the solder joints is developed and then solved numerically for the cases of single upright joints, single inverted joints, and through-hole joints. Results are presented in terms of the following dimensionless parameters: bond number, solder volume, board thickness, and tinning radius. The dimensionless approach makes the results from this study suitable for use in a broader range of applications.
Residual Stress Analysis of Bimaterial Strips Under Multiple Thermal Loading
Authors: Tierney J. W. , Eischen J. W.
Published: February 2008

The residual stress distribution in bimaterial beams induced by multiple thermal loadings has been investigated. Three models for the nonlinear stress-strain material behavior were considered: bilinear elastic-plastic, power law elastic-plastic, and power law purely plastic. The equations governing equilibrium, compatibility of strain, and stress-strain for the bimaterial configuration make up a system of nonlinear algebraic equations which is solved numerically. The elastic-plastic power law model leads to stress discontinuity in the layers. The other two models have been verified with a finite element analysis. Several examples are included using materials common to the microelectronics industry.
Development of a Two Matrix Model for Thermal Analysis of a Multichip Module
Authors: Butterbaugh M. A.
Published: February 2008

The customary one-dimensional thermal resistance model used for single chip modules is not well suited for use with multichip modules. In this paper, it is proposed to model a multichip module (MCM) using a two matrix approach. An external matrix of influence coefficients is used in conjunction with cap temperatures measured above each chip site to determine individual chip powers within the MCM. These chip powers are then used with a matrix of internal influence coefficients to determine chip temperature rise relative to the cap temperature. The proposed matrices are determined using a “thermal test module” or “computational model” where chip power, chip temperature, and cap temperature can be explicitly measured. This dual matrix model is then available for use with production modules to determine chip powers and temperatures from a measurement of cap temperatures above each chip-site. The proposed model is demonstrated for a four chip MCM using data from numerical simulations. A sample of test runs using the model predicts the chip powers and temperatures to within 3.5 and 5.5 percent, respectively, of the values from the numerical model. Module physical properties are also parametrically varied to determine their effect on matrix elements. The matrix model is also proposed as a design tool to augment more numerically intensive methods of thermal analysis.
Shearing Deformation in Partial Areal Arrays: Analytical Results
Authors: Heinrich S. M. , Shakya S. , Lee P. S.
Published: February 2008

Simple, closed-form expressions, based on elasticity theory, are derived for determining the location and magnitude of the maximum shearing displacement in a partial areal array of solder joints. Both uniform and nonuniform thermal loadings are considered, as is the heterogeneity of the component, which often arises due to different values of coefficient of thermal expansion (CTE) and elastic properties among the module’s constituent materials. The model is based on the following assumptions: (a) the square geometry of the array and component may be replaced with an equivalent axisymmetric geometry; and (b) the stiffness of the solder joints is negligible with respect to that of the component and substrate. The “soft joint” assumption corresponds to low-modulus solders or to thermal excursions occurring at high temperatures or low frequencies, for which significant stress relaxation occurs in the solder. For arrays exhibiting higher stiffness characteristics, the model yields conservative estimates of shearing displacement. Results indicate that, unlike homogeneous-component models under uniform temperature changes, the critical joints are not necessarily at the outer corners of the array. Other candidate locations predicted by the model (and observed in experimental and numerical studies) include the inner corner joints and any joints positioned beneath the die corners. The analytical results, also presented graphically, are found to depend on only three dimensionless parameters: the ratio of inner to outer array dimension, the ratio of die size to outer array dimension, and a “mismatch parameter,” which depends on the material, geometry, and loading characteristics of the problem. The results can be used to quickly determine the location and magnitude of peak shearing displacement in the array, possibly minimizing or eliminating the need to perform expensive and time-consuming finite element macroanalyses on entire assemblies involving hundreds of joints. Thus, the analyst may proceed directly to a detailed finite element microanalysis of the critical joint for fatigue life estimation, using the calculated shearing displacement as a required boundary condition in the finite element model.
A Finite Element Procedure of a Cyclic Thermoviscoplasticity Model for Solder and Copper Interconnects
Authors: Fu C. , McDowell D. L. , Ume I. C.
Published: February 2008

A finite element procedure using a semi-implicit time-integration scheme has been developed for a cyclic thermoviscoplastic constitutive model for Pb-Sn solder and OFHC copper, two common metallic constituents in electronic packaging applications. The scheme has been implemented in the commercial finite element (FE) code ABAQUS (1995) via the user-defined material subroutine, UMAT. Several single-element simulations are conducted to compare with previous test results, which include monotonic tensile tests, creep tests, and a two-step ratchetting test for 62Sn36Pb2Ag solder; a nonproportional axial-torsional test and a thermomechanical fatigue (TMF) test for OFHC copper. At the constitutive level, we also provide an adaptive time stepping algorithm, which can be used to improve the overall computation efficiency and accuracy especially in large-scale FE analyses. We also compare the computational efforts of fully backward Euler and the proposed methods. The implementation of the FE procedure provides a guideline to apply user-defined material constitutive relations in FE analyses and to perform more sophisticated thermomechanical simulations. Such work can facilitate enhanced understanding thermomechanical reliability issue of solder and copper interconnects in electronic packaging applications.
Towards an Integrated Approach for Analysis and Design of Wafer Slicing by a Wire Saw
Authors: Sahoo R. K. , Prasad V. , Kao I. , Talbott J. , Gupta K. P.
Published: February 2008

Crystalline and polycrystalline ingots of silicon and other materials need to be cut into thin wafers for microelectronics, photovoltaics, and many other applications. For slicing process to be cost-effective, the kerf loss should be minimum and the surface finish should be of a high quality. Wire saw can meet these demands and is considered to be a potentially better technology than the inner diameter (ID) saw. An initial study of the current technology shows that the wire saw cutting is a poorly understood process, and no model exists for simulation, design, and control of this process. The wire saw slicing process can be well modeled as a cutting process, where the initial fracture occurs because of the stress distribution between the two surfaces subjected to compressive loading and sliding friction. A preliminary analysis is carried out using a standard finite element method to develop a better understanding of this process and to determine possible ways of improving the process design. The results of vibration (modal) and thermal stress analyses show that an accurate prediction of the effects of process parameters would help in improving the wire saw design. Similarly, a proper feedback control algorithm would enable a better control of the process by using on-line information on wire tension and stiffness, temperature, and other relevant quantities. A methodology for systematic approach to analysis and design of an advanced wire saw process is also outlined.
The Effect of Stencil Printing Optimization on Reliability of CBGA and PBGA Solder Joints
Authors: Li Y. , Mahajan R. L. , Subbarayan G.
Published: February 2008

As a follow-up and conclusion to previous work in stencil printing process modeling and optimization (Li et al., 1996), we investigate the effect of stencil printing optimization on the reliability of the ceramic and plastic ball grid arrays. For ceramic ball grid arrays, the eutectic solder fillet shape is calculated using a series of simple mathematical equations. The thermal strain distributions within the solder joints after two cycles of accelerated thermal cycling test are estimated using three-dimensional finite element models. The modified Coffin-Manson relationship is applied to calculate the mean fatigue lives of the solder joints. The results reveal that an optimized stencil printing process significantly reduces variation in the fatigue life of ceramic ball grid arrays. The results also show that the fatigue life of ceramic ball grid arrays is very sensitive to the card-side solder volume. The maximum strain region shifts from the card-side eutectic solder to the module side as the card-side eutectic solder volume increases. This shift in maximum strain suggests that there exists an optimum ratio between the card-side solder volume and the module-side solder volume for the reliability of a given ceramic ball grid array design. The implications of this for the package developers and users are discussed. The calculations indicate that the fatigue life of plastic ball grid arrays is almost insensitive to the card-side solder volume.
CBGA Solder Fillet Shape Prediction and Design Optimization
Authors: Li Y. , Mahajan R. L.
Published: February 2008

In this paper, we first present a mathematical method that can be used to predict the eutectic solder fillet shape for ceramic ball grid array joints. An underlying assumption is that the solder fillets on both the module and the card sides can be represented as arcs. The fillets’ profiles are then calculated for the factors affecting the shape including solder volume, pad size, solder ball size, the wetting angle between eutectic solder and solder ball, and the gap between solder ball and pad. The second part of the paper focuses on design for reliability and investigates the effect of the interactions between the card-side and the module-side solder fillets on CBGA solder joint reliability. To this end, a central composite design of experiment is set up to systematically vary the pad size and the eutectic solder volume on both the module and the card sides. For each of the design settings, the proposed mathematical method is used to calculate the solder fillet shape. Using ABAQUS and the modified Coffin-Manson relationship, the mean fatigue life is predicted. The implications of the simulations are discussed. In addition, a response surface model is presented to find the optimum settings for maximum reliability. Finally, a comparison is made for the fatigue life predictions obtained using the proposed mathematical method and the linear solder fillet assumption.
Transient and Steady-State Thermal Design Evaluation of a Carbon Monoxide Gas Sensor Using CFD Tool
Authors: Lee T.-Y. Tom , Sharma R. , Peyre-Lavigne A.
Published: February 2008

This paper summarizes thermal modeling work performed on the Motorola Carbon Monoxide (CO) chemical sensor. Gas sensors need low cost reliable packages, good thermal operation, and low power consumption. The goal is to provide a validated thermal model of a gas sensor and its package and to develop a sensor design capability with reduced design cycle time. Due to the complex structure of the sensor package, a computational fluid dynamics (CFD) tool was used to analyze the heat transfer and fluid flow within the package. Based on the validated model, parametric studies on filter location and package orientation were performed. In order to minimize the influence of humidity, the sensor is toggled between high and low temperatures by applying 5 volts for 5 s of heating, and 1 volt for 10 s of cooling. Transient thermal analysis was also performed to predict the temperature response of various components. A detailed description of the thermal model and its results are described in the paper.
Two Test Specimens for Determining the Interfacial Fracture Toughness in Flip-Chip Assemblies
Authors: Yan X. , Agarwal R. K.
Published: February 2008

Two test specimens are developed to measure interfacial fracture toughness in flip-chip assemblies. The specimens consist of three layers: silicon chip, underfill, and circuit board. Two symmetric edge cracks are embedded along the interface, either between the chip and the underfill or between the underfill and the circuit board. The specimens are subjected to four-point-bend loading and critical loads are obtained. Analytical solutions for energy release rate have been derived for these two specimens and used to obtain the toughness from the measured critical loads. These specimens have been used to evaluate material combinations of chip passivation, underfill and solder mask for desired interfacial strength.
Real-Time Monitoring and Simulation of Thermal Deformation in Plastic Package
Authors: Zhu J. , Zou D. , Liu S.
Published: February 2008

Thermally-induced failure is a major reliability issue for electronic packaging. Due to the highly nonlinear behaviors and thermal mismatch of packaging materials, an electronic package exhibits uneven thermal deformation in the whole temperature range during thermal cycling. This behavior will affect the buildup of thermal strain/stress within the package, which may affect the reliability of the package. Therefore, a real-time method is needed to monitor the thermal deformation of packages during the thermal cycling. In this study, a real-time moire´ interferometry technique coupled with a thermal vacuum chamber is used to monitor the thermal deformation of a plastic package. A grating is transferred onto the cross section of the sample at room temperature. The fringe patterns are recorded by a CCD camera system and are compared with the displacement contours obtained by nonlinear finite element simulation. High temperature moire´ results up to 200°C are reported here. The comparison between the moire´ fringe patterns and finite element results shows a good agreement. The results also show that the real-time moire´ interferometry technique is an effective way to monitor the thermal deformation of electronic packaging and is a powerful validation method for finite element analysis.
Investigation of the Lead-On-Chip Package’s Reliability
Authors: Tsao P.-H. , Chang L. C. , Chen T. C. , Haung C. , Chen C. Z.
Published: February 2008

An assembly test chip (ATC), consisting of varieties of test structures, had been utilized to monitor the package integrity of SOJ lead-on-chip (LOC) packages after various reliability tests. Two different types of epoxy molding compounds, namely biphenyl and EOCN epoxies, were chosen to investigate their effects on package’s reliability. After the reliability tests, silicon chip crack was observed in three test samples due to the existence of a large chip backside chipping (˜120 μm). Qualitative study about this failure mechanism was carried out by a finite element analysis and it was found that, due to the higher flexural modulus and CTE of the biphenyl epoxy, the thermally induced stresses developed in the chip encapsulated by this epoxy during reliability tests were more likely to cause the crack propagation in the silicon chip than those induced by using EOCN type epoxy.
Development in Optical Methods for Reliability Analysis in Electronic Packaging Applications
Authors: Guo Y. , Liu S.
Published: February 2008

The mechanical analysis of electronic packaging has significant impact on the system reliability of an electronic device, and, therefore, is critical in packaging design and manufacturing. The main concerns of the mechanical analysis are the thermal strain/stress, interfacial failures, and the fatigue life of components and interconnections. As electronic technology advances, electronic packages are becoming smaller and smaller. Strain/stress concentrations are frequently localized in very tiny zones with high magnitudes. Determinations of thermal strain/stress and predictions of failure and reliability become more and more difficult. Recently, advanced optical techniques have been developed for electronic packaging analysis and have accomplished many tasks that could not be accomplished by conventional experimental methods. Optical techniques have been used for experimental stress/strain analysis, on-line inspections, simulation validations, and hybrid methods in many packaging areas. In this paper, recent developments in optical techniques are discussed. Several techniques are demonstrated by applications that are directly related to new electronic product development. The examples show the results and impact of using these optical techniques in the electronics industry to assist in product designs, qualifications, reliability improvements, and cycle time reductions.
Optimization of Wire Loop Height for a Cavity Down Plastic Pin Grid Array Package
Authors: Chaudhry I. , Barez F.
Published: February 2008

A study has been conducted to resolve wire neck break problem in a cavity-down plastic pin grid array (PPGA) packages with a specific range of parameters when subjected to temperature cycle (−55°C/+125°C). In most cases, a weak or broken neck of the wire was observed after 300 cycles of temperature cycling. The objective of this study is to determine an optimum wire loop height so that the package can pass a 1000 temperature cycles. Results of a simulation study, performed by other researchers, using a finite element model (FEM) were utilized. Their work considered the effect of temperature cycling on PPGA packages identical to those in this report. Several possible factors that can contribute to this failure mechanism were analyzed, and stresses in the wires were evaluated. The simulation results were verified by running an experiment on actual parts. The parts were subjected to temperature cycling, and data was gathered at different test points. The experimental results obtained did concur with simulation results which suggested that the area just above the ball experienced a significant level of thermal stresses, and such stresses could be reduced by determining an optimum loop height.
Installation Effects on Air Temperatures Within Outdoor Electronic Cabinets
Authors: Zimmerman E. B. , Colwell G. T.
Published: February 2008

A numerical model is used to assess the effect of shading by adjacent buildings on the peak average inside air temperature of an outdoor electronics cabinet. The numerical model was verified by comparing numerical predictions to experimental results obtained from a fully loaded telephone switching cabinet. The cabinet is assumed to be located in either Seattle, WA or Phoenix, AZ. Results indicate that careful selection of cabinet location around a building has the potential (at minimal cost) for providing a significant improvement in the thermal behavior of the cabinet, thus reducing the chances of thermal induced failures.
Avionics Passive Cooling With Microencapsulated Phase Change Materials
Authors: Fossett A. J. , Maguire M. T. , Kudirka A. A. , Mills F. E. , Brown D. A.
Published: February 2008

Analysis for an avionics application typical of remotely located, intermittently operated avionics on aircraft and missiles show that a large weight reduction (about 9:1) can be obtained by using recently developed microencapsulated phase change materials technology instead of a solid aluminum plate for a passive heat sink. Tests with a configuration based on the typical avionics application used for analysis show good agreement with analysis. Use of microencapsulated rather than bulk phase change materials avoids a number of design problems previously encountered with application of such materials.
Optimization of Finned Heat Sinks for Impingement Cooling of Electronic Packages
Authors: Kondo Y. , Behnia M. , Nakayama W. , Matsushima H.
Published: February 2008

The study of optimization of finned heat sinks for impingement cooling of electronic components was undertaken. The procedure was based on a semiempirical zonal approach to the determination of thermal resistance as well as pressure drop. To test the validity of the model’s predictions, experiments and CFD (computational fluid dynamics) simulations were performed. The results provided support for the approach. The model enables cost-effective design calculations to be performed for the optimization of heat sinks. We performed such calculations to optimize an LSI heat sink in consideration of sixteen design parameters, including fin thickness, fin spacing, fin height, and flow-orifice dimensions. For the particular application considered in our study, the optimum fin thickness was found to be 0.15 mm. The characteristics and limitations of air cooling for such applications were investigated under various conditions.
Evaluation of Elasto-Plastic Interfacial Fracture Parameters in Solder-Copper Bimaterial Using Moire´ Interferometry
Authors: Krishnamoorthy H. , Tippur H. V.
Published: February 2008

An experimental investigation dealing with failure characterization of 63/37 solder-copper interfaces is presented. The method of moire´ interferometry is used for mapping elasto-plastic deformations in bimaterials subjected to predominantly tensile loading. A method for quantifying the fracture parameter—the J-integral—using full-field measurement of crack opening displacements has been developed. A linear relationship between crack tip opening displacements (CTOD) and the J-integral is demonstrated. The crack growth resistance curve and hence crack initiation toughness Jc value for the bimaterial is obtained. Full-field optical information has also suggested the possibility of using a simpler method for the J-integral estimation using a single strain gage for fracture testing of interfaces with large mismatch in this geometry.
In Situ Evaluation of Residual Stresses in an Organic Die-Attach Adhesive
Authors: Voloshin A. S. , Tsao P.-H. , Pearson R. A.
Published: February 2008

Development of the residual stresses in an organic adhesive, alumina filled epoxy (EPO-TEK H65-175MP) during curing process has been studied in-situ. The effect of the adhesive’s thickness was evaluated by preparing samples and analyzing residual stresses. Samples were prepared by applying a layer of the epoxy in various thicknesses on the back side of the silicon die. The topology changes of the die’s surface during curing process and cooling of the epoxy were monitored by digital image analysis enhanced moire´ interferometry (DIAEMI). Residual stresses were calculated from the curvature changes of the surface of the die. The results show that the stress buildup is mainly caused by the mismatch in coefficient of thermal expansion between the adhesive and the die. Relaxation of the residual stress was found while the samples were kept in an open environment at room temperature. Such reduction in stress may be attributed to moisture absorption by the epoxy that results in expansion of the epoxy.
Adhesion and Reliability of Polymer/Inorganic Interfaces
Authors: Kook S.-Y. , Snodgrass J. M. , Kirtikar A. , Dauskardt R. H.
Published: February 2008

The reliability of microelectronic components is profoundly influenced by the interfacial fracture resistance (adhesion) and associated progressive debonding behavior. In this study we examine the interfacial fracture properties of representative polymer interfaces commonly found in microelectronic applications. Specifically, interface fracture mechanics techniques are described to characterize adhesion and progressive bebonding behavior of a polymer/metal interface under monotonic and cyclic fatigue loading conditions. Cyclic fatigue debond-growth rates were measured from ~10−11 to 10−6 m/cycle and found to display a power–law dependence on the applied strain energy release rate range, ΔG. Fracture toughness test results show that the interfaces typically exhibit resistance-curve behavior, with a plateau interface fracture resistance, Gss, strongly dependent on the interface morphology and the thickness of the polymer layer. The effect of a chemical adhesion promoter on the fracture energy of a polymer/silicon interface was also characterized. Micromechanisms controlling interfacial adhesion and progressive debonding are discussed in terms of the prevailing deformation mechanisms and related to interface structure and morphology.
Measurement of Interfacial Fracture Toughness Under Combined Mechanical and Thermal Stresses
Authors: Jiao J. , Gurumurthy C. K. , Kramer E. J. , Sha Y. , Hui C. Y. , Borgesen P.
Published: February 2008

The interfacial fracture resistance of a polyimide passivation/underfill interface was measured using an asymmetric double cantilever beam technique. A thin layer of polyimide was coated on a standard epoxy as one of the beams and an underfill was flowed over the polyimide film and cured to form the second beam. The relative contributions of the thermal and mechanical stress were systematically varied by varying the thickness ratio of the two beams. The results indicate that the effect of thermal residual stress must be taken into account in the calculation of critical strain energy release rate to obtain the interfacial fracture energy.
A Thermodynamic Framework for Damage Mechanics of Solder Joints
Authors: Basaran C. , Yan C.-Y.
Published: February 2008

Damage mechanics describes the degradation process that takes place in materials and structures. Traditionally, Coffin–Manson type empirical curves are used to determine the fatigue life. Damage mechanics allows us to determine the fatigue life without the need for empirical curves. The main problem in damage mechanics has always been a lack of universally agreed upon definition of a damage metric. In this paper a damage metric based on the second law of thermodynamics and statistical mechanics is presented. The proposed thermodynamic framework treats a solid body as a thermodynamic system and requires that the entropy production be nonnegative. Verification of the damage model has been performed by extensive comparisons with laboratory test data of low cycle fatigue of Pb40/Sn60 solder alloy.
Cooling Rate Effect on Post Cure Stresses in Molded Plastic IC Packages
Authors: Yi S. , Sze K. Y.
Published: February 2008

Residual stresses during cool down process after curing are evaluated on the basis of thermoviscoelastic finite element analyses with material properties dependent on both time and temperature. Temperature distributions in packages are predicted by solving the heat conduction equations. The effects of cool-down history on thermomechanical responses of a lead-on-chip (LOC), thin small outline package (TSOP) are investigated. Three linear cool down temperature histories are considered. Numerical results show that residual stresses in a LOC TSOP are significantly influenced by the manufacturing temperature history. Residual stresses in plastic packages are strongly time-temperature dependent due to the thermoviscoelastic behavior of molding compounds. Substantial residual stresses arise when a LOC TSOP has cooled to room temperature and rapid cool down permits small viscoelastic effects.
OMPAC Package C5 Reliability—Parametric Study
Authors: Nagaraj B.
Published: February 2008

Over molded pad array carrier (OMPAC) also called as ball grid array (BGA) has been used in products, such as hand held two way radio within Motorola. Now it has gained interest outside Motorola in companies like Compaq for portable computers and AT&T for the telephone systems. The reduced size, thickness, and increased I/O density at the board level are the attractive features of the OMPAC package over other competing package types. In Ref. [1, 2], life time of C5 solder interconnects under temperature cycle loading was reported. That study was based on nonlinear FEM simulation incorporating creep behavior of the solder. In the current study, influence of four key package design variables on C5 solder reliability is addressed using analyses methodology described in Ref. [1, 2]. The selected design variables are thickness of the substrate, the C5 pitch, solder ball diameter, and height to diameter (h/d) ratio of the C5 interconnect. For each design variable, three levels (low, middle, and high) are considered. The full study required a total of 81 simulation runs. However, statistical design of experiments were used and an L9 design was selected in the current study. The maximum permanent strain range for stabilized temperature cycle was calculated for all the nine case studies. In all the cases it occurs at the substrate interface in the solder ball just inside the edge of the die. The response (maximum total permanent strain range and thus maximum number of temperature cycles before failure) as a function of the four design variables are studied and the trends are: as the substrate thickness, C5 ball diameter and C5 (h/d) ratio increases and C5 pitch decreases, the maximum total permanent strain range decreases and thus cycles to failure increases. The order of importance of the design variables for the C5 reliability are (i) the C5 (h/d) ratio, (ii) the C5 ball diameter, (iii) the C5 pitch, and (iv) the substrate thickness. The C5 (h/d) is the key design variable. The maximum total permanent strain range decreases from 0.0055 for (h/d) ratio of 0.5 to 0.0045 for (h/d) ratio of 0.7. This in other words increases cycles to failure from 3000 cycles to 4400 cycles, an improvement of 47 percent.
An Investigation of PWB Layout by Genetic Algorithms to Maximize Fatigue Life
Authors: Scholand A. J. , Fulton R. E. , Bras B.
Published: February 2008

Thermal considerations in printed wiring board (PWB) assemblies are becoming increasingly important as packaging constraints shrink and power use escalates. In this paper, we provide a study on the potential for a genetic algorithm-driven PWB layout design tool to improve the thermal performance of such assemblies. As a case study, the thermomechanical fatigue of surface mounted leadless chip carriers on an FR4 epoxy board is used. We have found that by utilizing appropriate formula-based engineering approximations, the efficiency of parallel implementations of genetic algorithms in finding near-optimal and results makes this approach effective as an explorative “scouting” approach to identify promising board configurations for more computationally expensive evaluations such as finite element method.
Thermoelastic Properties of Plain Weave Composites for Multilayer Circuit Board Applications
Authors: Sottos N. R. , Ockers J. M. , Swindeman M.
Published: February 2008

The thermoelastic properties of woven glass/epoxy substrates for multilayer circuit board applications were investigated and the influence of fabric geometry assessed. The woven fabric geometry of several commercially pressed boards was carefully characterized using optical microscopy. The elastic moduli and Poisson’s ratios were then measured in uniaxial tension, while the CTEs of the boards were measured using thermal mechanical analysis (TMA) at temperatures above and below the glass transition. Experimental data were compared to predictions from two new analytical models. One of the models, which does not use classical lamination theory, results in a significant improvement for the prediction of Poisson’s ratio and CTEs. Finally, a parametric study was performed to demonstrate the effect of fiber crimp on the properties of the fabric.
Modeling Natural Convection From Horizontal Isothermal Annular Heat Sinks
Authors: Wang C.-S. , Yovanovich M. M. , Culham J. R.
Published: February 2008

A study is presented for laminar natural convection heat transfer from isothermal, vertical disks with horizontal support cylinders, as found in annular-fin heat sinks. A distinction is made between the external and internal surfaces of the heat sink, and, hence, the heat transfer mechanisms which dominate in each of these surfaces can be more easily applied. A boundary layer solution for laminar natural convection, a fully developed flow solution, and a diffusive limit are successfully applied to this complex geometry to form the present model. The model shows good agreement with present experimental data and previous correlations.
Overheating and Frequent Thermal Cycling of Outdoor Electronic Cabinets in Cold Climates
Authors: Zimmerman E. B. , Hegab H. , Colwell G. T.
Published: February 2008

Telephone companies use electronics to route calls between customers. The electronics are generally closely packed together and placed in steel containers outdoors. Forced air convection utilizing the outside ambient air is an effective means to cool outdoor electronic cabinets and is generally the system of choice given the relatively low cost and simplicity when compared to alternative cooling methods. Simple axial flow fans are typically turned on and off by a thermostat located inside the cabinet to keep the inside air temperature below a predetermined maximum. This simple cooling system is usually effective during summer operations. However, it may result in overheating and excessive thermal cycling in winter operations. Transient temperature data from experiments on a telecommunications cabinet is presented illustrating this problem. One possible solution to this problem is using continuously operating fans at low flow rates. This solution was arrived at through a combination of experimental testing and numerical simulation.