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TECHNICAL PAPERS

J. Electron. Packag. 1999;121(2):61-68. doi:10.1115/1.2792669.

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.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1999;121(2):69-74. doi:10.1115/1.2792670.

Two forms (paste and film) of isotropically conductive adhesives (CAs) were mechanically loaded in shear mode. The specimens were instrumented with crossed-line gratings so that normal and shear displacements could be measured. The CA paste specimen failed outside the observed region at a stress 28 percent below the manufacturer’s predicted value. In the observed region there were no normal strains, only shear strains restricted to the CA. In the film specimen the conducting particles began breaking away from the matrix epoxy at very low loads. However, the specimen continued carrying the additional loading increments until the load was transferred to the adjacent material.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1999;121(2):75-84. doi:10.1115/1.2792671.

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.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1999;121(2):85-91. doi:10.1115/1.2792672.

Wire bonding, a process of the connection between a semiconductor chip and a lead frame by a thin metal wire, is one of the important processes of electronic packaging. This paper presents failure estimation of a silicon chip and a GaAS chip during a gold wire bonding process. The gold wire bonding process is carried out by pressing a gold ball made at a tip of the gold wire on a semiconductor chip and vibrating it by ultrasonic. High contact pressure is useful for shortening the process cycle, but it sometimes causes failure of the semiconductor chip. Elastic-plastic large deformation contact analyses are performed and the distributions of the stresses in these semiconductor chips are investigated. The possibility of failure of a semiconductor chip under usual wire bonding pressure is pointed out only for a GaAs chip.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1999;121(2):92-98. doi:10.1115/1.2792673.

Stress-strain diagrams showing the influence of strain rate at five different high homologous temperatures for a Sn-Pb solder alloy are used to determine the material parameters in a minimal version of the viscoplasticity theory based on overstress (VBO), accounting for static recovery effects. VBO is a “unified” state variable theory that does not use a yield surface and has three state variables with appropriate growth laws. It is shown that the effects of strain rate can be modeled well by this theory that requires eight material parameters to describe the inelastic behavior. The behavior in tensile and cyclic strain-controlled loadings, ratcheting, and creep is predicted. The results compare very well with the experiments for tensile and cyclic loading. For the other tests no comparisons were made due to the lack of proper experimental data. The model is formulated in tensorial form and can be used for inelastic stress analyses of solder joints.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1999;121(2):99-107. doi:10.1115/1.2792674.

The deformation behavior of two commercial lead-free solder alloys, Indalloy 227 (77.2Sn-20.0In-2.8Ag) and Castin alloy (96.2Sn-2.5Ag-0.8Cu-0.5Sb), over a range of strain rates (10−6 s−1 to 10−2 s−1 ), temperatures (–55°C to 125°C), and isothermal and thermomechanical cycling conditions is experimentally determined. The behavior of these two lead-free alloys is compared to that of 60Sn-40Pb solder. In order to minimize the number of experiments, yet capture the significant features of the deformation behavior and obtain the simple Norton power law constants, strain-rate jump tests are successfully used. This minimal test information is also shown to be sufficient to determine a complete set of material parameters for modern unified thermoviscoplastic models that can capture the dominant strain rate and temperature effects including the thermomechanical cycling behavior. As an exercise, the parameters for the Bodner-Partom model are determined for both lead-free alloys and predictions of thermomechanical cycling are shown.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1999;121(2):108-115. doi:10.1115/1.2792664.

In the present paper, a methodology is described for the integrated thermal analysis of a laminar natural convection air cooled nonventilated 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 finite volume computational fluid dynamics/heat transfer (CFD/CHT) approach on a coarse grid. Thermal information from 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 bilinear interpolation and further employed in board and component level thermal analyses as various boundary condition combinations. Thus, thermal analyses at all levels were connected. The component investigated is a leadless ceramic chip carrier (LCCC). The integrated analysis approach was validated by comparing the results for a LCCC package with those obtained from detailed system level thermal analysis for the same package. Two preferred boundary condition combinations are suggested for component level thermal analysis.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1999;121(2):116-121. doi:10.1115/1.2792665.

A component aligned through the use of liquid solder bumps behaves like a mechanical oscillator system. The force constants for liquid solder bumps are derived analytically for simple model systems as well the damping factor and the resonance spectrum. It appears that for practical bump sizes the force constant in the vertical direction is an order of magnitude larger than the horizontal one. The damping factor is small and the system can easily be brought into resonance. From the derivation of the force constants combined with the dissociation energy a Morse potential can be constructed depicting the asymmetry in the potential energy curve.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J. Electron. Packag. 1999;121(2):122-126. doi:10.1115/1.2792666.

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 39.5 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 68.4 Hz to 146.9 Hz, or 115 percent higher.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1999;121(2):126-127. doi:10.1115/1.2792667.
Abstract
Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1999;121(2):127-134. doi:10.1115/1.2792668.

Wire sweep has been recognized as one of the major defects in encapsulation of electronic components by transfer molding. The phenomenon is very complicated as it is sensitive to a large number of parameters. In this experimental work, where a 160L QFP package used as the test vehicle, the detailed time-dependent wire displacement is measured for the following two different flow initial conditions: (i) the wire is immersed in the liquid and is displaced due to the acceleration of the flow from rest to the steady-state velocity, and (ii) the wire is surrounded by the ambient air and is displaced first due to the passage of the liquid front and then due to the hydrodynamic load. Significant differences have been observed between the two cases, with important implications for analytical and numerical studies of wire sweep.

Commentary by Dr. Valentin Fuster

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