bluuebunny/crossref_metadata_2025 huggingface dataset:
A New Method to Determine Crack Shape and Size in Solder Joints
Authors: Reinikainen T. , Wulff F. W. , Kolbe W. , Ahrens T.
Published: February 2008

In this paper, a method based on image analysis is introduced to produce a 3-dimensional picture of a solder joint. This 3-D image gives a detailed view of the solder contour and the fatigue cracks inside the solder. The practical example is a surface mount 1206 ceramic capacitor which has been soldered on a FR4 board with Sn62Pb36Bi2 solder. The assembly has been through severe temperature cycling. The 3-D image is created by first making several (≈15) micrographs of the cross-section of the joint until the whole joint has been examined. The photopositives are then scanned with a black and white scanner. To each of the scanned micrographs the contours of component metallisation, copper pad and solder are marked manually using an image processing program. Another program has been developed to extract the marked contours from the scanned images and to place them on a CAD-file with the correct distance between them. With CAD the 3-D image of the contours can be viewed from different view points and also the fatigue crack inside the joint can be seen and measured.
Optimal Chip Layout on a Printed Circuit Board Using Design Sensitivity Analysis of Subdomain Configuration
Authors: Joo Seo Jin , Kwak Byung Man
Published: February 2008

A chip layout problem is formulated as a new class of shape optimal design called a subdomain optimization, where the chips correspond to subdomains whose configuration and location are to be decided. Shape design sensitivity analysis for a perturbed subdomain is made based on the concept of material derivative and adjoint system. Introducing a suitable category of design velocity fields, the change of the configuration is adequately describable. Sensitivities and optimal positions of chips on a printed circuit board are obtained and their accuracy discussed.
Comparison of Two-Phase Electronic Cooling Using Free Jets and Sprays
Authors: Estes Kurt A. , Mudawar Issam
Published: February 2008

The performances of free jets and sprays were compared experimentally in cooling a 12.7 × 12.7 mm2 chip in order to ascertain the effects of key parameters on cooling performance and to develop correlations for critical heat flux (CHF) which are applicable to dielectric coolants. Increasing liquid flow rate and subcooling increased CHF for both cooling schemes. At high subcooling, comparable CHF values were attained with both for equal flow rates. However, spray cooling produced much greater CHF at low subcooling than did jet cooling. This phenomenon was found to be closely related to the hydrodynamic structure of the liquid film deposited upon the chip surface. In jet cooling, the film (wall jet), being anchored to the surface only at the impingement zone, was separated from the surface during vigorous boiling due to the momentum of vapor normal to the surface. The individual spray drops were more effective at securing liquid film contact with the surface at low subcooling, which delayed CHF relative to jet cooling with the same flow rate. This paper also discusses practical concerns associated with implementation of each cooling scheme including system reliability and the risk associated with premature CHF during chip power transients.
PCB Layout Design Using a Genetic Algorithm
Authors: Jain Sakait , Gea Hae Chang
Published: February 2008

This paper presents an approach to find the optimal design layout of chips on a circuit board in a manner that minimizes the area covered on the board and the connections between the various chips. In addition, there are no major heat sources next to each other and certain physical constraints are satisfied while finding a layout design. In this approach, the whole circuit board area is divided into a finite number of cells for mapping it into a Genetic Algorithm (GA) chromosome. The mutation and crossover operators have been modified and are applied in conjunction with connectivity analysis for the chips to reduce the creation of a lot of faulty generations. Examples of GA based chip layout are presented to show how each of the objectives are attained separately followed by examples to arrive at layouts using multiple objectives.
Fine Pitch Stencil Printing Process Modeling and Optimization
Authors: Li Y. , Mahajan R. L. , Nikmanesh N.
Published: February 2008

In this paper, we present a statistical-neural network modeling approach to process optimization of fine pitch stencil printing for solder paste deposition on pads of printed circuit boards (PCB). The overall objective was to determine the optimum settings of the design parameters that would result in minimum solder paste height variation for the new board designs with 20-mil, 25-mil, and 50-mil pitch pad patterns. As a first step, a Taguchi orthogonal array, L27, was designed to capture the main effects of the six important printing machinery parameters and the PCBs pad conditions. Some of their interactions were also included. Fifty-four experimental runs (two per setting) were conducted. These data were then used to construct neural network models relating the desired quality characteristics to the input design parameters. Our modular approach was used to select the appropriate architecture for these models. These models in conjunction with the gradient descent algorithm enabled us to determine the optimum settings for minimum solder paste height variation. Confirming experiments on the production line validated the optimum settings predicted by the model. In addition to the combination of all the three pad patterns, i.e., 20, 25, and 50 mil pitch pads, we also built neural network models for individual and dual combinations of the three pad patterns. The simulations indicate different optimum settings for different pad pattern combinations.
Single-Phase and Boiling Cooling of Small Pin Fin Arrays by Multiple Nozzle Jet Impingement
Authors: Copeland David
Published: February 2008

Experimental measurements of multiple nozzle submerged jet array impingement single-phase and boiling heat transfer were made using FC-72 and 1 cm square copper pin fin arrays, having equal width and spacing of 0.1 and 0.2 mm, with aspect ratios from 1 to 5. Arrays of 25 and 100 nozzles were used, with diameters of 0.25 to 1.0 mm providing nozzle area from 5 to 20 mm2 (5 to 20% of the heat source base area). Flow rates of 2.5 to 10 cm3/s (0.15 to 0.6 l/min) were studied, with nozzle velocities from 0.125 to 2 m/s. Single nozzles and smooth surfaces were also evaluated for comparison. Single-phase heat transfer coefficients (based on planform area) from 2.4 to 49.3 kW/m2 K were measured, while critical heat flux varied from 45 to 395 W/cm2. Correlations of the single-phase heat transfer coefficient and critical heat flux as functions of pin fin dimensions, number of nozzles, nozzle area and liquid flow rate are provided.
Heat Transfer From an Array of Strips to Fluorinert Coolant in a Mixed Impinging-Jet/Channel-Flow Configuration
Authors: Nakayama Wataru , Behnia Masud , Mishima Hiroaki , Sun Hua
Published: February 2008

Cooling of an array of parallel foil strips stretched on the wall of a narrow rectangular channel to the flow of fluorinert FX3250 is studied experimentally. The coolant is injected in planar jet normal to the center strip, which splits into two streams to cool the other strips. The data for each strip are correlated in the form of NuL/Pr0.4 as a function of ReL. The correlations compare favorably to the data of Schafer et al. (1991) who conducted experiments using water in similar geometric configurations.
An Empirical Crack Propagation Model and its Applications for Solder Joints
Authors: Hu Jimmy M.
Published: February 2008

This paper investigates a strain based crack propagation model, and discusses the application of fracture mechanics approach in the reliability validation of leadless solder joints. The model includes the creep effect at different temperatures and hold times, and correlates well the thermal cycling test of 90 Pb/10 Sn joints. By apply this model, an engineering method to develop the inspection criteria in accelerated reliability validation tests of leadless solder joints is proposed.
Effect of Microstructure Size on Deformation Kinetics and Thermo-Mechanical Fatigue of 63Sn37Pb Solder Joints
Authors: Guo Zhenfeng , Conrad Hans
Published: February 2008

An increased cooling rate following reflow produced a decrease in the microstructure size including the size of the colonies, the Pb-rich dendrites and the eutectic phases. The decrease in microstructure size gave an increased creep rate at room temperature and a decrease in the creep stress exponent. Also, thermo-mechanical fatigue crack growth rate decreased and fatigue life increased with the decrease in microstructure size. These effects of the increased cooling rate were attributed to an increase in boundary sliding which could occur from the increase in boundary area with decrease in microstructure size.
Stress Analysis of Printed Circuit Boards With Highly Populated Solder Joints and Components: A Micromechanics Approach
Authors: Hu K. X. , Huang Y. , Yeh C. P. , Wyatt K. W.
Published: February 2008

The single most difficult aspect for thermo-mechanical analysis at the board level lies in to an accurate accounting for interactions among boards and small features such as solder joints and secondary components. It is the large number of small features populated in a close neighborhood that proliferates the computational intensity. This paper presents an approach to stress analysis for boards with highly populated small features (solder joints, for example). To this end, a generalized self-consistent method, utilizing an energy balance framework and a three-phase composite model, is developed to obtain the effective properties at board level. The stress distribution inside joints and components are obtained through a back substitution. The solutions presented are mostly in the closed-form and require a minimum computational effort. The results obtained by present approach are compared with those by finite element analysis. The numerical calculations show that the proposed micromechanics approach can provide reasonably accurate solutions for highly populated printed circuit boards.
Computer Simulation of Solder Bridging Phenomena
Authors: Singler T. J. , Zhang Xu , Brakke K. A.
Published: February 2008

Solder bridging is investigated under the assumption that liquid solder bridges are equilibrium capillary surfaces and that the principal factor that determines whether a bridge will freeze to form a permanent short is its configurational stability. A computational parametric bridge stability study is conducted to determine the response of bridging to the system volume, the distance between pads, the contact angle between the liquid metal and resist surface and the relevant physicochemical properties of the liquid metal.
An Interfacial Delamination Analysis for Multichip Module Thin Film Interconnects
Authors: Hu K. X. , Yeh C. P. , Wu X. S. , Wyatt K.
Published: February 2008

Analysis of interfacial delamination for multichip module thin-film interconnects (MCM/TFI) is the primary objective of this paper. An interface crack model is integrated with finite-element analysis to allow for accurate numerical evaluation of the magnitude and phase angle of the complex stress intensity factor. Under the assumption of quasi-static delamination growth, the fate of an interfacial delamination after inception of propagation is determined. It is established that whether an interfacial delamination will continue to grow or become arrested depends on the functional behavior of the energy release rate and loading phase angle over the history of delamination growth. This functional behavior is numerically obtained for a typical MCM/TFI structure with delamination along die and via base, subjected to thermal loading condition. The effect of delamination interactions on the structural reliability is also investigated. It is observed that the delamination along via wall and polymer thin film can provide a benevolent mechanism to relieve thermal constraints, leading to via stress relaxation.
A Model Study of Thermal Stress-Induced Voiding in Electronic Packages
Authors: Huang Y. , Hu K. X. , Yeh C. P. , Li N.-Y. , Hwang K. C.
Published: February 2008

Thin film metallizations are one of the most important interconnects in large-scale integrated circuits. They are covered by substrates and passivation films. Large hydrostatic (mean) tension develops due to the constraint and thermal mismatch, and voiding is identified as the failure mechanism. This phenomenon of rapid nucleation and growth of voids is called cavitation instability and it can lead to the failure of ductile components in electronic packages such as metallizations. A micromechanics model is developed to provide the critical mean stress level that will trigger the cavitation instability. It is found that this critical mean stress level the cavitation stress, not only depends on the material properties but also is very sensitive to defects in the material. For example, the cavitation stress decreases drastically as the void volume fraction increases. The stress-based design criterion for ductile components in electronic packages should then be: (1) Von Mises effective stress < yield stress; and (2) mean stress < cavitation stress, which is particularly important to the constrained ductile components in electronic packages such as vias and conductive adhesives. An analytical expression of cavitation stress for elastic-perfectly plastic solids is obtained, and numerical results for elastic-power law hardening solids are presented.
Transient Modeling of the Thermal Behavior of Outdoor Electronic Cabinets
Authors: Zimmerman Eric B. , Colwell Gene T.
Published: February 2008

This paper presents an investigation into the various factors which influence the peak average inside air temperature of outdoor telephone switching cabinets. The primary goal is to determine which factors are important and to develop a correlation which will predict peak average air temperatures inside the cabinet under a variety of situations. A network of lumped thermal capacitances and resistances is used to model the transient thermal behavior of the electronics enclosure. Energy balances for each element are developed creating a system of ordinary differential equations which are solved using a fourth order Runge-Kutta method. The numerical model is verified by comparison to experimental data. A search of parameters which affect the thermal behavior of telephone switching cabinets is conducted. A sensitivity analysis is performed to determine the important terms. Parametric studies are conducted to develop a correlation which relates the internal cabinet air temperature to the important dimensionless parameters.
Efficient Heat Transfer Approximation for the Chip-on-Substrate Problem
Authors: Fisher Timothy S. , Zell F. A. , Sikka Kamal K. , Torrance Kenneth E.
Published: February 2008

An analytically based approximate solution is presented for the thermal resistance of an axisymmetric heat source mounted on a conductive substrate with bottom- and top-side convective cooling of the substrate. The approximation closely matches an exact solution for bottom-side convective cooling and reference finite element solutions for top-side and both-side cooling over broad ranges of substrate thickness (10−4 ≤ t* ≤ 104 and 10−2 ≤ t* ≤ 102), substrate outer radius (1 ≤ b* ≤ 100) and convective Blot numbers (10-4 to 102). With bottom-side cooling, a minimum in the thermal resistance can occur over a wide range of substrate thicknesses. The approximate solution possesses simplicity and ease of computation as compared to exact or computational solutions for many microelectronic applications.
System for Real-Time Measurement of Thermally Induced PWB/PWA
Warpage
Authors: Stiteler M. R. , Ume I. C.
Published: February 2008

An automated on-line warpage measurement system for printed wiring board assemblies (PWBAs) has been developed. The system is capable of simulating an infrared reflow soldering process and performing real-time PWBA warpage measurements using the shadow moire´ technique. The system can be used to characterize the warpage behavior of virtually any PWBA during infrared soldering processes as well as during operational conditions. Using this system, warpage of PWB test vehicles was measured during simulated infrared reflow soldering. The measurement results and the measurement system will be presented. The measured warpage varied significantly during reflow soldering from that observed both before and after reflow. These results help us to understand how the board deforms at every stage of the reflow process.
Thermal Performance of Pin-Fin Fan-Sink Assemblies
Authors: Wirtz R. A. , Sohal R. , Wang H.
Published: February 2008

Experiments are reported on the thermal performance of model fan-sink assemblies consisting of a small axial flow fan which impinges air on a square array of pin-fins. Cylindrical, square, and diamond shape cross section pin-fins are considered. The pin-fin heat transfer coefficient is found to be maximum immediately under the fan blades and minimum below the fan hub and near the corners of the array. The overall heat sink thermal resistance, R, decreases with an increase in either applied pressure rise or fan power and fin height. At fixed applied pressure rise, R is minimized when the fin pitch-to-diameter ratiois maximum. At fixed fan power, R is minimized when the pitch-to-diameter ratio is reduced toward unity. Finally, cylindrical pin-fins give the best overall fan-sink performance.
A Comparison of Computational and Experimental Results for Flow and
Heat Transfer From an Array of Heated Blocks
Authors: Anderson A. M.
Published: February 2008

This paper summarizes computational results for flow and heat transfer over an array ofidealized electronic components and compares them to experimental data. The numerical modeling was performed using a commercial finite control volume computer code (Flotherm1, by Flomerics) and the results are compared to a set of experimental data. The experimental model consists of a uniform array of eight rows by six columns of solid aluminum blocks (9.5 mm high × 46.5 mm wide × 37.5 mm long) mounted on an adiabatic wall of a channel in forced convection flow. Four channel heights (H/B = 1.5–4.6) and a range of inlet velocities (3.0 to 8.1 m/s) were modelled. The flow was modeled as turbulent flow using the κ-ε turbulence model. Data for the adiabatic heat transfer coefficient had, the superposition kernel function g*, and the channel pressure drop ΔP are compared. The computational results for had are in excellent agreement with the experimental data (within about five percent on average). The computationalresults for g* predict the correct trends (roll off with downstream distance, channel height dependence, and velocity independence). However, values are as much as 50 percent higher than the experimental results which means the computational model under-predicts the amount of cross channel mixing. Computational results for ΔP compare reasonably well (within 20 percent on average).
Numerical Prediction of Flow and Heat Transfer in an Impingement Heat
Sink
Authors: Sathe S. , Kelkar K. M. , Karki K. C. , Tai C. , Lamb C. , Patankar S. V.
Published: February 2008

Forced flow of air over extended surfaces offers a simple, reliable, and effective heat removal mechanism and is often employed in electronic equipment. The IBM 4381 heat sink, used in production IBM computers, utilizes this cooling technique. This heat sink consists of a ceramic substrate on which fins made of an aluminum-copper alloy are arranged in a regular array. Cooling air enters the fin array from a nozzle. Extensive experiments have been carried out to characterize the performance of this heat sink at the Advanced Thermal Engineering Laboratory at IBM Endicott. This paper presents computational analysis of the three-dimensional flow and heat transfer in this device for two different air flow rates through the nozzle. The heat dissipated by the electronic components is conducted into the fins through the ceramic base. In the present study the ceramic base is assumed to be subjected to a uniform heat flux at the bottom. The computational method incorporates a special block-correction procedure to enable iterative solution of conjugate heat transfer in the presence of large differences in thermal conductivities of the air and the fin material. The results of computations reproduce the flow pattern in the fin array that is observed experimentally. The part of the ceramic base directly below the nozzle is well cooled with the temperatures gradually increasing from the center towards the corner. The predicted pressure drop and most of the local temperatures at the base and the tip of the fins agree well with the experimental observations. This study illustrates the utility of computational flow analysis in the analysis and design of electronic cooling techniques.
Optimal Three-Dimensional Placement of Heat Generating Electronic Components
Authors: Campbell M. I. , Amon C. H. , Cagan J.
Published: February 2008

This work introduces an algorithm that uses simulated annealing to perform electronic component layout while incorporating constraints related to thermal performance. A hierarchical heat transfer analysis is developed which is used in conjunction with the simulated annealing algorithm to produce final layout configurations that are densely packed and operate within specified temperature ranges. Examples of three-dimensional component placement test cases are presented including an application to embedded wearable computers.
Effect of Load-Mix on Fatigue Crack Growth in 63Sn-37Pb Solder Joints
Authors: Yao D. , Shang J. K.
Published: February 2008

Fatigue crack growth at the interface between Sn-Pb solder and Cu was examined under various mixed-mode conditions. The load-mix, in terms of the ratio of Mode-II to Mode-I stress intensities, was systematically changed by varying the thickness ratio of Cu layers in the flexural-peel specimens made from Sn-Pb/Cu joints. Fatigue crack growth experiments were conducted using a sinusoidal waveform at a frequency of 5 Hz and a load-ratio of zero. Fatigue crack growth rates were measured as a function of the total strain energy release rates for a given load-mix. Fatigue crack growth resistance was found to increase with the load-mix, with the effect of the load-mix being more pronounced in the near-threshold regime. The effect of the load-mix on fatigue crack growth is shown to result from shear-enhanced frictional sliding of fatigue crack surfaces.
Thermal Stresses in Layered Electronic Assemblies
Authors: Jiang Z. Q. , Huang Y. , Chandra A.
Published: February 2008

Thermal stresses in layered electronic assemblies are one of the causes of the mechanical failure of electronic packages. A simple but accurate method of estimating these thermal stresses is needed for the design of these packages. A simple approach based on beam theory exists, but it suffers from nonequilibrium of the peeling stress distribution. An improved method that overcomes this drawback is proposed here. For layered electronics with thin adhesives, simple analytical expressions are obtained for interfacial shear stress and peeling stress, as well as for other stress components. The finite element method is used to verify these solutions. It shows excellent agreement between the finite element results and these simple solutions, especially when the moduli of adhesive layers are significantly lower than the moduli of the other layers. This method provides an accurate estimate of thermal stresses for use in package design involving thin and compliant interface or adhesive layers.
The Effects of Strain Rate and Thickness on the Response of Thin Layers of Solder Loaded in Pure Shear
Authors: Gilat A. , Krishna K.
Published: February 2008

A new configuration for testing thin layers of solder is introduced and employed to study the effects of strain rate and thickness on the mechanical response of eutectic Sn-Pb solder. The solder in the test is loaded under a well defined state of pure shear stress. The stress and deformation in the solder are measured very accurately to produce a reliable stress-strain curve. The results show that both the stress needed for plastic deformation and ductility increase with increasing strain rate.
The Nature of Centroidal Locus in Misaligned Flip-Chip Solder Joints
Authors: Subbarayan G. , Deshpande A.
Published: February 2008

The self-alignment mechanism of molten flip-chip solder joints is being increasingly used in passive alignment of optoelectronic devices. For these applications, three-dimensional models of misaligned solder joints are necessary to understand the effect of solder joint design parameters on self alignment. To reduce the complexity of fully three-dimensional models, intuitively reasonable assumptions are often made in their theoretical development. Two such assumptions for misaligned flip-chip solder joints with circular pads are that the locus of centroids is a straight line and that the cross sections are circular in shape. In the present paper, the limits of validity of these two assumptions are explored. In general, if either the top and bottom pad radii are identical, or if there is no misalignment between the pads, then the centroidal locus is a straight line and the cross sections are circular. The extent of deviation from straight line centroidal locus or circular cross section depends on the ratio of the top and bottom pad radii and on the extent of misalignment between the pads. For a misalignment equal to 20 percent of the solder joint height and a joint with 90 percent pad diameter ratio, the deviation from straight line locus is 7 percent and the deviation from circularity is less than 1 percent. However, as the pad ratio is decreased to 50 percent, and as the misalignment is increased to 100 percent, the deviation in centroidal locus increases to 43 percent and the deviation from circularity increases to 33 percent. Thus, straight line locus and circular cross sections are reasonable assumptions for flip-chip solder joints provided the pad diameter ratio and misalignment are small.
Nonlinear Analysis of Full-Matrix and Perimeter Plastic Ball Grid Array Solder Joints
Authors: Jung W. , Lau J. H. , Pao Y.-H.
Published: February 2008

The application of Ball Grid Array (BGA) technology in electronic packaging on high I/O plastic and ceramic packages has grown significantly during the past few years. Although PBGA (plastic BGA) has several advantages over fine-pitch Quad Flat Pack (QFP) in terms of smaller package area, higher I/Os, lower switching noise, large pitch, higher assembly yield, and improved robustness in manufacturing process, potential package reliability problems can still occur, e.g., excessive solder joint deformation induced by substrate warpage, moisture ingression (popcorn effect), large variation in solder ball size, voiding as a result of flux entrapment and improper pad/solder mask design (Marrs and Olachea, 1994; Solberg, 1994; Freyman and Petrucci, 1995; Lau, 1995; Donlin, 1996; Lasky et al., 1996; Munroe et al., 1996). Regardless of its improved thermal fatigue performance over the past few years through an extensive amount of research, the BGA solder joint may still pose a reliability issue under harsh environment, e.g., automotive underhood, larger package size, or higher temperature and temperature gradient due to increase in power dissipation of the package. Numerous studies in BGA solder joint deformation and reliability under thermal and mechanical loadings can be found in the literature, e.g., Borgesen et al. (1993), Choi et al. (1993), Guo et al. (1993), Ju et al. (1994), Lau et al. (1994) Lau (1995), and Heinrich et al. (1995). Also, reliability prediction models have been developed by, e.g., Darveaux et al. (1995) and Darveaux (1996). The present study focuses on the application of a detailed nonlinear finite element analysis (FEA) to studying the thermal cyclic response of solder joints in two particular BGA packages, full-matrix and perimeter. Both time-independent plasticity and time-dependent effect, i.e., creep and relaxation, are considered in the constitutive equations of solder joint to evaluate the discrepancy in the results of life prediction. The critical solder joint is identified, and the locations that are most susceptible to fatigue failure in the critical joint are discussed. Some limitations in computation and reliability prediction are also discussed.
Gull-Wing Solder Joint Fatigue Life Sensitivity Evaluation
Authors: Wong T. E. , Kachatorian L. A. , Tierney B. D.
Published: February 2008

A Taguchi design of experiment approach was applied to thermostructural analyses of a gull-wing solder joint assembly. This approach uses a minimum number of finite element analyses to evaluate the impact of solder joint assembly parameters on fatigue life of the assembly. To avoid costly complex modeling efforts for each parametric case study, a commercially available program, MSC/PATRAN’s PATRAN Command Language, was used to automatically create finite element models of a two-dimensional gull-wing solder joint assembly based on nine parameters. Modeling time was dramatically reduced from days to a few minutes for each detailed lead/solder model. Two sets of parametric studies were conducted to evaluate the impact of variation of the six parameters. The analysis results indicate that lead ankle radius is the most critical parameter affecting solder joint total fatigue life, and lead and minimum solder thicknesses are the next most critical ones. Therefore, to effectively improve the solder joint fatigue life margin, it is recommended to: (1) increase the minimum solder thickness; (2) use thinner lead; and (3) use a larger lead ankle radius, even though this may require reducing lead shin length. By implementing only the last recommendation to modify the current solder joint assembly, the fatigue life margin in this design could, in general, be improved by 27 percent or more.
Deformation Mechanism of Two-Phase Solder Column Interconnections Under Highly Accelerated Thermal Cycling Condition: An Experimental Study
Authors: Han B.
Published: February 2008

Thermomechanical behavior of solder interconnections of a ceramic column grid array package assembly is characterized by high sensitivity moire´ interferometry. The in situ thermal deformations of the most distant column from the axis-of-symmetry are documented for (a) an initial isothermal loading of ΔT = −60° and (2) subsequent accelerated thermal cycles of (20°C)−(125°C)−(−40°C)−(20°C). The deformed shape of the column obtained from the displacement fields clearly delineates the effect of the eutectic solder fillet on the column deformation accumulated during accelerated thermal cycling. A deformation mechanism is suggested to explain the results, which is confirmed by the results from the actual accelerated thermal cycle test of the same assembly. The distributions of inelastic strains accumulated during thermal cycles are determined along the sides of the column. The results emphasize the importance of the solder fillet height for optimum reliability. The effect of specimen preparation required for the moire´ experiment on the column deformation is also discussed to validate the experimental data.
Improved Analytical Estimate of Global CTE Mismatch Displacement in Areal-Array Solder Joints
Authors: Heinrich S. M. , Shakya S. , Lee P. S.
Published: February 2008

An analytical expression is derived for determining the maximum solder joint shearing displacement occurring in an a real-array interconnect under global CTE mismatch loading. The result may be viewed as a “load correction factor” to be applied to the commonly used estimate which is based on the free thermal expansion of component and substrate. The new expression for the correction factor includes the following parameters: (a) dimensions and material properties of component and substrate; (b) array size and population; (c) material properties of solder; and (d) geometric parameters of the individual joints. The theoretical result is based on modeling the assembly as two circular elastic disks connected by a shear-type “elastic foundation” whose distributed shear stiffness is related to the joint/array characteristics. The analytical expression and the graphical aids presented herein may provide convenient alternatives to performing time-consuming and expensive finite element “macro-analyses” on the assembly for the purpose of specifying boundary conditions for a subsequent “micro-analysis” on a single joint.
Fatigue Life Predictions for Thermally Loaded Solder Joints Using a Volume-Weighted Averaging Technique
Authors: Akay H. U. , Paydar N. H. , Bilgic A.
Published: February 2008

Fatigue lives of thermally loaded solder joints are predicted using the finite element method. An appropriate constitutive relation to model the time-dependent inelastic deformation of the near-eutectic solder is implemented into a commercial finite element code, and the stress-strain responses of different electronic assemblies under the applied temperature cycles are calculated. The finite element analysis results are coupled with a newly developed approach for fatigue life predictions by using a volume-weighted averaging technique instead of an approach based on the maximum stress and strain locations in the solder joint. Volume-weighted average stress and strain results of three electronic assemblies are related to the corresponding experimental fatigue data through least-squares curve-fitting analyses for determination of the empirical coefficients of two fatigue life prediction criteria. The coefficients thus determined predict the mean cycles-to-failure value of the solder joints. Among the two prediction criteria, the strain range criterion uses the inelastic shear strain range and the total strain energy criterion uses the total inelastic strain energy calculated over a stabilized loading cycle. The obtained coefficients of the two fatigue criteria are applied to the finite element analysis results of two additional cases obtained from the literature. Good predictions are achieved using the total strain energy criterion, however, the strain range criterion underestimated the fatigue life. It is concluded that the strain information alone is not sufficient to model the fatigue behavior but a combination of stress and strain information is required, as in the case of total inelastic strain energy. The superiority of the volume-weighted averaging technique over the maximum stress and strain location approach is discussed.
0.4 μm Diameter Nickel-Filament Silicone-Matrix Resilient
Composites for Electromagnetic Interference Shielding
Authors: Shui Xiaoping , Chung D. D. L.
Published: February 2008

Resilient silicone-matrix composites containing 7–13 volume percent nickel filaments (0.4 μm diameter) exhibited 74–93 dB electromagnetic interference (EMI) shielding effectiveness at 1–2 GHz and 1 × 10−1 – 2 × 10−2 Ω cm DC volume electrical resistivity. The high shielding effectiveness is due to the small diameter of the nickel filaments. The composites are useful for EMI shielding gaskets and cable jackets.
Wire-Sweep Study Using an Industrial Semiconductor-Chip-Encapsulation Operation
Authors: Han S. , Wang K. K. , Crouthamel D. L.
Published: February 2008

In this study, the wire-sweep problem has been studied by performing experiments using a commercial-grade epoxy molding compound, a real chip assembly, and an industrial encapsulation process. After encapsulating the chip, the deformed wire shape inside the plastic package has been determined by X-ray scanning. A procedure for the wire-sweep calculation during encapsulation process has been developed. The wire sweep values have been calculated using this procedure with material properties measured from experiments. The calculated wire-sweep values are compared with experimental values measured at different mold temperatures, fill times, and cavities. In most cases, the calculated values are in good agreement with the experimental values.
Z-Axis Anisotropic Electrically Conducting Polymer-Matrix Composite Film
Authors: Li L. , Chung D. D. L.
Published: February 2008

A z-axis anisotropic electrically conducting polymer-matrix composite film was developed. It comprised 25 volume percent copper coated polyether sulfone particles and a polyimidesiloxane matrix. Each particle protruded from both sides of the film and provided a conducting path along the z-axis. A z-axis pressure of 40 kPa resulted in a z-axis electrical resistivity of 2 Ω · cm for the overall film (i.e., 0.5 Ω · cm for a conducting path); subsequent pressure removal caused the resistivity to rise to 7 Ω · cm only. The film exhibited negligible stress relaxation and a low modulus of 1.67 GPa.
The Impact of Interfacial Adhesion on PTH and Via Stress State
Authors: Subbarayan G. , Ramakrishna K. , Sammakia B. G.
Published: February 2008

In this paper the impact of the state of barrel-laminate adhesion on the PTH and via stresses is investigated. A representative PTH and a representative blind via in a printed wiring board are used as demonstration vehicles. Transient heat conduction and nonlinear elastic-plastic-contact finite element analyses are carried out on PTHs and vias under the extreme conditions of perfect and zero barrel-laminate adhesion. In general, the maximum equivalent plastic strains in PTHs and vias with strong barrel-laminate adhesion are not large enough to fracture the plating. Fracture is possible at or near the innerplane junction when barrel-laminate adhesion is lost. The possibility of plating fracture in a blind via is strongly dependent on the tolerances in manufacturing process parameters such as drill depth and laminate thickness.
Thermomechanical Response of Materials and Interfaces in Electronic Packaging: Part II—Unified Constitutive Models, Validation, and Design
Authors: Desai C. S. , Chia J. , Kundu T. , Prince J. L.
Published: February 2008

The constitutive modeling approach based on the disturbed state concept (DSC) described in Part I, provides a unified basis for the characterization of thermomechanical response of materials and joints in electronic chip-substrate systems. Using the material constants given in Part I, the DSC model predictions, obtained by integrating the incremental constitutive equations, are shown here to provide satisfactory backpredietions of stress-strain, fatigue, and failure responses of typical solder materials. The DSC also provides a simple criterion based on the critical disturbance to identify cyclic fatigue failure. Model predictions show good correlation with those from previous models. It is also shown how the DSC model can be used for design applications. Overall, based on Papers I and II, it can be stated that the DSC can provide a new and powerful means to characterize the thermomechanical behavior of materials and joints in a number of problems in electronic packaging.
A Finite-Element and Experimental Analysis of Stress Distribution in Various Shear Tests for Solder Joints
Authors: Reinikainen T. , Poech M. , Krumm M. , Kivilahti J.
Published: February 2008

Solder alloys are commonly tested with shear tests to study their mechanical properties or low-cycle fatigue performance. In this work, the suitability of various shear tests for quantitative solder-joint testing is investigated by means of the finite element method. The stress state and stress distribution in the following well known geometries are studied: the double-lap test, the ring and plug test, the losipescu test, and two single-lap tests. A new test geometry, the grooved-lap test, is introduced and compared to the conventional tests. The results of simulations with an elastic material model in plane-strain indicate that considerable differences in the purity of the state of shear (rε = −ε1/ε3) as well as in the stress distribution in the joint exist among the shear tests. However, simulations with a nonlinear material model show that stress inhomogenities are smoothed by the plastic and creep deformation occurring in the joint. Optical measurements of the deformation of real single-lap and grooved-lap joints show that the single-lap joint rotates slightly during creep, whereas in the grooved-lap joint no rotation can be detected. This confirms the simulation results that in the single-lap test the initially nonuniform stress distribution changes during creep, and in the grooved-lap test the uniform stress distribution remains constant through the test.
Thermomechanical Finite Element Analysis of Problems in Electronic Packaging Using the Disturbed State Concept: Part 2—Verification and Application
Authors: Basaran C. , Desai C. S. , Kundu T.
Published: February 2008

The finite element procedure with the unified disturbed state modeling concept presented in Part I, Basaran et al. (1998), is verified here with respect to laboratory test results for Pb40/Sn60 eutectic solder alloy. This solder alloy is a commonly used interconnection material for surface mount technology packages. It is demonstrated that the proposed procedure provides highly satisfactory correlation with the observed laboratory behavior of materials and with test results for a chip-substrate system simulated in the laboratory.
Analysis of Tombstoning Phenomenon During Reflow
Authors: Qi Q. , Mahajan R. L.
Published: February 2008

A dynamic model is established to investigate tombstoning phenomenon of passive SMT components during reflow. This model consists of three dynamic equations for the motion of the component and four nonlinear algebraic equations for area conservation, constant curvatures for solder fillets, and solder height-length relations. The formulation neglects the gravity effect in the solder. It is shown that formation of tombstoning depends on a combination of parameters such as component geometry, pad sizes, gap size between pads, molten solder volume and properties, and contact angles. For some values of these parameters encountered in practice, tombstoning can take place. For a given component, this model may be used to improve solder pad and stencil designs to avoid tombstoning.
Orifice and Impingement Flow Fields in Confined Jet Impingement
Authors: Morris G. K. , Garimella S. V.
Published: February 2008

The flow fields in the orifice and the confinement region of a normally impinging, axisymmetric, confined, and submerged liquid jet were computationally investigated. Numerical predictions were made for orifice diameters of 3.18 and 6.35 mm at several orifice-to-target plate spacings, with turbulent jet Reynolds number ranging from 8500 to 23,000. The commercial finite-volume code FLUENT was used to solve the flow fields using a modified k–ε model based on renormalization group theory. The predicted characteristics of the separation region at the entrance of the orifice agree with experiments in the literature. The pressure drop across the orifice was predicted to within 5 percent of proposed empirical correlations based on published experimental data. The computed flow patterns in the confinement region of the impinging jet were in good qualitative agreement with flow visualizations; however, a secondary recirculation zone observed in experiments was not predicted by the models. The results presented for the flow (and pressure drop) in the orifice, as well as that in the confined outflow region, influence heat transfer on the impingement surface and are important considerations in electronics packaging design.
Natural Convection Cooling of Multiple Heat Sources in Parallel Open-Top Cavities Filled With a Fluorinert Liquid
Authors: Behnia M. , Dehghan A. A. , Mishima H. , Nakayama W.
Published: February 2008

Natural convection immersion cooling of discrete heat sources in a series of parallel interacting open-top cavities filled with a fluorinert liquid (FC–72) has been numerically studied. A series of open-top slots which are confined by conductive vertical walls with two heat sources on one side are considered. One of the slots is modeled and simulated. The effect of the separation between the heat sources on the flow and heat transfer characteristics of the wall and the effect of strength of the lower heat source (which location is upstream of the other one) on the flow and heat transfer of the upper heat source are considered. The wall thermal conductivity considered ranges from adiabatic to alumina-ceramic. The results of bakelite and alumina-ceramic are shown, which are commonly used as wiring boards in electronic equipment. It is found that conduction in the wall is very important and enhances the heat transfer performance.
Photothermal Reliability Testing of a Multilayer Coating—Substrate Assembly: A Theoretical Approach
Authors: Elperin T. , Rudin G.
Published: February 2008

The paper presents the theoretical principles underlying the method for photothermal reliability testing of a multilayer coating-substrate assembly. In this method, the assembly is irradiated by the pump laser beam which causes heating and formation of thermal stresses in an assembly. The irradiated surface is monitored by a weak probe beam of another laser to detect the buckling of the coating. The two-dimensional distributions of temperature and thermal stresses in an assembly heated by the pump laser beam are obtained from the solution of the thermal elasticity problem. The developed mathematical procedure is based on the expansion of the Laplace and Hankel transforms of temperature and displacement distributions in series of a small parameter, which is of the order of the ratio of a coating thickness to a pump beam radius. The explicit expressions for the stress tensor components in the layers of a coating and the probe beam deflection angle are derived. Calculations show that the dependence of the deflection angle on a Fourier number is nonmonotone and attains a maximum at some value of the Fourier number. The latter property of the dependence of the deflection angle on the Fourier number can be used for detecting subsurface structural defects in an assembly.
Natural Convection Air Cooling of a Discrete Heat Source on a Conducting Board in a Shallow Horizontal Enclosure
Authors: Ortega A. , Lall B. S.
Published: February 2008

Experiments were performed to measure the heat transfer coefficient on the surface of a square flush heat source mounted at the center of an FR-4 plate in a small horizontal enclosure. The plate area was six times larger than the heat source area. Four cases were considered: the plate facing upwards and downwards, and the backside either insulated or convecting. The heat transfer coefficients exhibited distinct behavior at high aspect ratio in which the dominant length scales were related to the source. At intermediate aspect ratios, the length scales of both the source and the enclosure were relevant, and at small aspect ratios a conduction limit was observed. The heat transfer coefficients at high aspect ratios exceeded the prior correlations by 14 percent for upward facing isolated plates when the ratio of heat source area to perimeter was used as the significant length scale, but the dependence on Ra1/4 was consistent. For the downward facing case, the data exceeded the values for a uniformly heated isolated plate by 68 percent. Classical correlations for shallow differentially heated horizontal enclosures were not satisfactory in describing the dependence on enclosure height.