Showing papers in "Journal of Electronic Packaging in 1999"

Effect of Temperature and Strain Rate on Mechanical Properties of 63Sn/37Pb Solder Alloy

Abstract: In this study, tensile tests of 63Sn/37Pb solder were carried out at various strain rates from 10 s to 10 s over a wide temperature range from – 40 C to 125 C to study the effect of strain rate and testing temperature on the mechanical properties in a systematic manner. Based on these experimental data, a set of empirical formulae was derived by a statistical method to describe the effect of temperature and strain rate in a quantitative manner and explain the variation in the mechanical properties published in other reports. It is concluded that the empirical formulae can be used to characterize the mechanical properties of 63Sn/37Pb over a wide range of temperatures and strain rates.

Three-Dimensional Versus Two-Dimensional Finite Element Modeling of Flip-Chip Packages

Abstract: In this study, both two-dimensional and three-dimensional finite element analyses were used to study the stress distribution in and deflection of the flip chip assembly under thermal loading. It is found that the three-dimensional results compared favorably with experimental measurements, while the two-dimensional results consistently overestimate both stresses and deflection. Among the two-dimensional models, the plane stress assumption seems to yield results closer to the full three-dimensional predictions. Furthermore, three-dimensional models were used to investigate the effect of printed wiring board size on the overall deflection of the flip-chip assembly. This size effect of the printed wiring board has significant implications on the design of multi-chip modules. The results indicate that a square array placement pattern is preferable to a staggered array for multiple chip modules in order to reduce mechanical interaction between chips. For square arrays, such mechanical interaction between chips can be neglected when the minimum distance between adjacent chips is more than 2 times the chip size.

Thermoelastic properties of plain weave composites for multilayer circuit board applications

Abstract: The thermoelastic properties of several common woven glass/epoxy substrate materials were characterized in both the warp and fill directions. Five common commercially pressed consisted of either one or two plies of C-staged woven glass epoxy substrate sandwiched between 1 ounce copper cladding. After the copper was removed from the cores, samples were cut for either mechanical property characterization or microstructural analysis with the test axis lying along either the warp or fill direction. The crimp angle and relative fiber volume fraction of each fabric was first determined from photomicrographs of polished cross-sections. Next, Young's modulus was measured via standard tension tests at room temperature. The storage and loss moduli were then measured as function of temperature using dynamic mechanical analysis (DMA). Finally, the coefficients of thermal expansion were determined using constant force thermal mechanical analysis (TMA) measurements. All of the substrates showed significant differences in microstructure and material properties between the warp and fill directions. Most of the laminates had a much lower crimp angle in the warp direction, which resulted in a higher modulus and lower coefficient of thermal expansion than the fill direction. Of the cores investigated, the properties of 3313 were the most balanced.

Flow-Field Prediction in Submerged and Confined Jet Impingement Using the Reynolds Stress Model

Abstract: The flow field of a normally impinging, axisymmetric, confined and submerged liquid jet is predicted using the Reynolds Stress Model in the commercial finite-volume code FLUENT. The results are compared with experimental measurements and flow visualizations and are used to describe the position of the recirculating toroid in the outflow region which is characteristic of the confined flow field. Changes in the features of the recirculation pattern due to changes in Reynolds number, nozzle diameter, and nozzle-to-target plate spacing are documented. Results are presented for nozzle diameters of 3.18 and 6.35 mm, at jet Reynolds numbers in the range of 2000 to 23,000, and nozzle-to-target plate spacings of 2, 3, and 4 jet diameters. Up to three interacting vortical structures are predicted in the confinement region at the smaller Reynolds numbers. The center of the primary recirculation pattern moves away from the centerline of the jet with an increase in Reynolds number, nozzle diameter, and nozzle-to-target plate spacing. The computed flow patterns were found to be in very good qualitative agreement with experiments. The radial location of the center of the primary toroid was predicted to within ±40 percent and ±3 percent of the experimental position for Re = 2000–4000 and Re = 8500–23000, respectively. The magnitude of the centerline velocity of the jet after the nozzle exit was computed with an average error of 6 percent. Reasons for the differences between the numerical predictions at Re = 2000–4000 and experiments are discussed. Predictions of the flow field using the standard high-Reynolds number k-e and renormalization group theory (RNG) k-e models are shown to be inferior to Reynolds stress model predictions.

Damage Mechanics of Surface Mount Technology Solder Joints Under Concurrent Thermal and Dynamic Loading

Abstract: In the electronic industry, the dominant failure mode for solder joints is assumed to be thermal cycling. When semiconductor devices are used in vibrating environment, such as automotive and military applications, dynamic stresses contribute to the failure mechanism of the solder joint, and can become the dominant failure mode. In this paper, a damage mechanics based unified constitutive model for Pb40/Sn60 solder joints has been developed to accurately predict the thermomechanical behavior of solder joints under concurrent thermal and dynamic loading. It is shown that simultaneous application of thermal and dynamic loads significantly shorten the fatigue life. Hence, damage induced in the solder joint by the vibrations have to be included, in fatigue life predictions to correctly predict the reliability of solder joints. The common practice of relating only thermal cycling induced inelastic strain to fatigue life can be inadequate to predict solder joint reliability. A series of parametric studies were conducted to show that contrary to popular opinion all dynamic loading induced strains are not elastic. Hence, vibrations can significantly affect the fatigue life and reliability of solder joints in spite of their small mass. Introduction Among all the technology options in the electronics industry, Surface Mounted Technology (SMT) is the fastest growing one. By providing more space, SMT increases the interconnection density. As a result, the reliability of surface mount components has become a critical issue. Pb-Sn solder joints in microelectron- ics packages are extensively used for connecting surface mount components to a variety of substrates. During its design life, the solder joint experiences a wide range of temperature variations, which are caused by heat dissi- pation from the circuits and power on and offs. Because of the CTE mismatch, different elongations and contractions take place in the components. Since these bonded assemblies cannot deform freely, they experience relative motions during which stresses are induced in the solder joint. As a result of the temperature cycles, the solder joint experi- ences plastic and creep deformations. These deformations cause the accumulation of damage in the solder material. Conse- quently, the solder joint starts to lose its effective intact area, which leads to change in electrical resistance of the device and finally failure. Even though it is widely believed that the dominant failure mode for solder joint is thermal fatigue, when semiconductor devices are used in vibrating environment dynamic stresses con- tribute to the failure mechanism and sometimes, as it is demon- strated in this paper, can become the dominant failure cause. The frequency of thermal cycles effects the shear strain rate and the number of cycles to failure of the solder joint. The frequency range for harmonic vibrations for most vehicle and equipment is 1 to 5000 Hz (Suhir and Lee, 1988). Therefore, damage in the solder joint due to vibration accumulates very fast. Hence high cycle fatigue behavior of solders under vibra- Contributed by the Electrical and Electronic Packaging Division for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received by the EEPD December 8, 1998; revision received August 5, 1998. Associate Technical Editor: J. Lau. tion loading must be considered in solder joint reliability stud- ies. Presently, in the microelectronics industry, all vibration in- duced stresses on solder joints are considered to be elastic. It is assumed that there is no contribution from dynamic strains to the low cycle fatigue life (Barker et al., 1990). In this paper it is shown that vibration effects cannot be classified categori- cally as elastic only and ignored in low cycle fatigue studies. The accurate prediction of solder joint reliability under coupled thermal and dynamic loading by means of numerical methods is highly desirable for their competitive, reliable, and low-cost design and manufacturing of microelectronics packaging. Presently, most reliability predictions in the industry are em- pirical and based on destructive testing such as highly acceler- ated stress test (HAST). Computer simulation of the thermome- chanical behavior of the solder joints can be used to understand the behavior at temperature and acceleration levels that are too difficult or too expensive to obtain in the laboratory. Numerical analysis procedure can also be used to perform parametric study on a new package without having to build a prototype for every possible configuration. This later capability would significantly reduce the cost of design for new types of packages. In this paper, a unified damage mechanics based constitutive model is developed, and then implemented in a nonlinear finite element analysis procedure. The purpose of the material model and the implementation is to study the contribution of thermal and vibration induced strains to the fatigue life of solder in- terconnects. The combined loading situation is simulated by superposing the damage due to the vibrational loads and thermal loads. The damage due to each load type acting individually is determined and then superposed to assess the overall fatigue life of the joint. This superposition rule was proposed by Barker et al. (1990). As a first order approximation, a linear superposi- tion rule is utilized (Miner's 1945) rule. Authors realize the limitations of Miner's rule for a highly nonlinear problem. But as a first step approximation to a very complex problem, it is widely used in the literature (Barker et al., 1990; Steinberg, 1988). Journal of Electronic Packaging Copyright © 1999 by ASME JUNE 1999, Vol. 121 / 61

Nonlinear Dynamic Analysis of Surface Mount Interconnects: Part I—Theory

Abstract: Solder joints are commonly used in surface mount technology microelectronics packaging. It is well known that the dominant failure mode for solder joints is thermal fatigue. When semiconductor devices are used in a vibrating environment, such as in automotive and military applications, dynamic stresses contribute to the failure mechanism and in certain circumstances they can become the dominant failure cause. In this paper a unified constitutive model for Pb40/Sn60 solder joints is developed and then implemented in a finite element dynamic analysis procedure. The purpose of the material model and the implementation is to study the contribution of vibration induced strains to the fatigue life of solder interconnects in low cycle and high cycle fatigue. The proposed material model, which is based on the disturbed state concept (DSC), is used for a dynamic analysis of a solder joint in the following paper, Part II, Basaran and Chandaroy (1998).

Natural Convection Cooling of Vertical Rectangular Channels in Air Considering Radiation and Wall Conduction

Abstract: Temperature distributions on the surfaces of vertical channels formed by parallel plates heated uniformly and symmetrically and cooled by conduction, radiation, and natural convection in air are determined numerically and experimentally. Effects of wall separation, thickness, thermal conductivity, and emissivity on the wall temperature distribution are determined. Both cases of controlled and uncontrolled channel edge leading and exit edge temperatures are examined. Optimum channel widths and correlations for the maximum wall temperature rise are offered for both the controlled and uncontrolled edge temperature conditions.

Thermomechanical Durability Analysis of Flip Chip Solder Interconnects: Part 1—Without Underfill

Abstract: The effect of underfill material on reliability of flip chip on board (FCOB) assemblies is investigated in this study by using two-dimensional and three-dimensional finite element simulations under thermal cycling stresses from −55°C to 80°C. Accelerated testing of FCOB conducted by the authors reveals that the presence of underfill can increase the fatigue durability of solder interconnects by two orders of magnitude. Similar data has been extensively reported in the literature. It is the intent of this paper to develop a generic and fundamental predictive model that explains this trend. While empirical models have been reported by other investigators based on experimental data, the main drawback is that many of these empirical models are not truly predictive, and can not be applied to different flip chip architectures using different underfills. In the proposed model, the energy-partitioning (EP) damage model is enhanced in order to capture the underlying mechanisms so that a predictive capability can be developed. A two-dimensional finite element model is developed for stress analysis. This model accounts for underfill over regions of solder in an approximate manner by using overlay elements, and is calibrated using a three-dimensional finite element model. The model constant for the enhanced EP model is derived by fitting model predictions (combination of two-dimensional and three-dimensional model results) to experimental results for a given temperature history. The accuracy of the enhanced EP model is then verified for a different loading profile. The modeling not only reveals the influence of underfill material on solder joint durability, but also provides the acceleration factor to assess durability under life cycle environment, from accelerated test results. Experimental results are used to validate the trends predicted by the analytical model. The final goal is to define the optimum design and process parameters of the underfill material in FCOB assemblies in order to extend the fatigue endurance of the solder joints under cyclic thermal loading environments.

Nonlinear Dynamic Analysis of Surface Mount Interconnects: Part II--Applications

Abstract: Using the unified constitutive model and the finite element procedure presented in Part I (the preceding paper), a material nonlinear time domain dynamic analysis of a solder joint was studied for low cycle and high cycle fatigue. Thermal effects were not included in order to understand the dynamic behavior of a Pb40/Sn60 solder joint without noise-effects from thermal behavior. The latter decision was a result of observations reported in Steinberg (1988), that having in-phase or out-of-phase thermal loading in conjunction with vibrations makes a significant difference in the fatigue life. The study of fatigue under concurrent loading will be the subject of another paper.

Thermoelastic Modeling of a PWB With Simulated Circuit Traces Subjected to Infrared Reflow Soldering With Experimental Validation

Abstract: A bare, four copper layer printed wiring board with simple trace patterns was built for modeling and experimental validation purposes. In-plane elastic properties of the core materials in the board were measured as a function of temperature. Thermoelastic lamination theory was utilized to predict the warpage of the board when subjected to an infrared reflow process, with emphasis on studying the influence of thermal gradients through the board, its support conditions and CTE differential on the warpage process. Board layers with traces were approximated with quasi-homogeneous effective properties obtained using micromechanics theory. An experimental system that employs the shadow moird technique in a simulated infrared reflow environment was used to evaluate the warpage for comparison to modeled results.

An Investigation of PWB Layout by Genetic Algorithms to Maximize Fatigue Life

Abstract: 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 thermo-mechanical fatigue of surface mounted leadless chip carriers on an FR4 epoxy board is used. We have found that by utilizing appropriate formulabased engineering approximations, the efficiency of Genetic Algorithms in finding near-optimal and optimal 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.

Integrated thermal analysis of natural convection air cooled electronic enclosure

Abstract: Traditional thermal analyses of electronic equipment are based on conduction type solvers at the printed wiring board (PWB) level. These require specifications of convection coefficients on the board and component surfaces, which are in most realistic applications unknown. In recent years, with the advancement in computational fluid dynamics (CFD)/computational heat transfer (CHT) tools, system level simulations have been undertaken to evaluate thermal performance of electronic equipment. Due to the large computational time and storage requirements involved in such simulations, the grid sizes in these are usually not fine enough to obtain adequate details at the board and component levels. In order to accurately predict thermal performance of the complete electronic system, all levels of modeling must be performed in an integrated and efficient manner. In the present paper, a methodology is described for the integrated thermal analysis of a laminar natural convection air cooled electronic system. This approach is illustrated by modeling an enclosure with electronic components of different sizes mounted on a printed wiring board. First, a global model for the entire enclosure was developed using a CFD/CHT approach on a coarse grid. The solution to the global model was obtained using a finite volume method. Thermal information frommore » the global model, in the form of board and component surface temperatures, local heat transfer coefficients and reference temperatures, and heat fluxes, was extracted. These quantities were interpolated on a finer grid using Lagrangian polynomials and further employed in board and component level thermal analyses as various boundary conditions.« less

Minimization of Heat Sink Mass Using CFD and Mathematical Optimization

Abstract: This paper describes the use of CFD and mathematical optimization to minimise heat sink mass given a maximum allowable heat sink temperature, a constant cooling fan power and heat source Heat sink designers have to consider a number of conflicting parameters Heat transfer is influenced by, amongst others, heat sink properties (such as surface area), airflow bypass and the location of heat sources, whilst size and/or mass of the heat sink needs to be minimized This multiparameter problem lends itself naturally to optimization techniques In this study a commercial CFD code, STAR-CD, is linked to the DYNAMIC-Q method of Snyman Five design variables are considered for three heat source cases Optimal designs are obtained within six design iterations The paper illustrates how mathematical optimization can be used to design compact heat sinks for different types of electronic enclosures

Hygrothermal Fracture Analysis of Plastic IC Package in Reflow Soldering Process

Abstract: The purpose of this paper is to evaluate the delamination and fracture integrity of the integrated circuit (IC) plastic package under hygrothermal loading by the approaches of stress analysis and fracture mechanics. The plastic small outline J-lead (SOJ) package with a dimpled diepad under the reflow soldering process of infrared (IR) heating type is considered. On the package without a crack, the stress variation according to the change of the design variables such as the material and shape of the package is calculated and the possibility of delamination is considered. For the model fully delaminated between the chip and the dimpled diepad, J-integrals are calculated for the various design variables and the fracture integrity is discussed. From the results, optimal values of design variables to prevent the delamination and fracture of the package are obtained. In this study, the finite difference method (FDM) program to obtain the vapor pressure from the content of moisture absorbed into the package is developed.

Effect of Edge and Internal Point Support of a Printed Circuit Board Under Vibration

Abstract: This paper presents an investigation into the vibration of a PCB that is supported on its three edges by two wedge retainers and a plug-in connector Using a vibration test fixture to couple the PCB structure to an electromagnetic shaker, experiments were conducted to determine its dynamic response The wedge retainer and connector are modeled as simply supported condition with appropriate rotational spring stiffnesses along their respective edges It is found that these supports behave somewhere between the simply supported and clamped boundary conditions and provide a percent fixity of 395 percent more than the classical simply supported case It is further found that a single internal point constraint that would yield the maximum fundamental frequency is located at the intersection of the nodal lines of mode 2 and mode 3 This has the effect of significantly increasing the PCB’s fundamental frequency from 684 Hz to 1469 Hz, or 115 percent higher

Investigation of a New Lead Free Solder Alloy Using Thin Strip Specimens

Abstract: The thermomechanical behaviors of a new lead free solder and eutectic 63Sn37Pb are investigated in this paper. A series of tests including tensile, creep, and fatigue, are carried out on a computer controlled 6-axis mini-fatigue tester. The thin strip specimen is used in this research, which is specially designed and verified to be suitable for the testing of solder alloys and comparable to the data from the literature. Based on the experimental study, the new lead free solder alloy shows very attractive characteristics and may have potential applications in electronics packaging products.