SELECTING A TEMPERATURE TIME HISTORY FOR PREDICTING FATIGUE LIFE OF MICROELECTRONICS SOLDER JOINTS

Temperature cycling tests are standard industry practice for determining the thermomechanical fatigue life of solder joints. Industry-standard temperature profiles usually start from room temperature, then go to a high temperature, then to a cold temperature, and then back to room temperature. In addition, most of the time, the temperature profile contains dwell times at the highest and lowest temperatures. The dwell time at a high temperature corresponds to the on-state storage of the device, and the cold-temperature dwell time simulates the off-state storage of the device. In this study, the actual temperature history of a Ball Grid Array (BGA) package in a laptop computer was measured in situ. Experimental reliability studies were conducted using the in situ measured temperature history as well as industry-standard temperature history. This article presents the influence of temperature history on solder joint fatigue life. In order to measure deformations in the solder joint under cycling loading, a new Moire interferometry grating replication technique was developed to be able to measure strain field during fatigue testing.     

A lubrication theory for fluids with microstructure

Experimental observations indicate that viscosity of fluids adjacent to solid surfaces is altered as compared to bulk viscosity measured away from the surfaces. It is well-established that in thin films of less than several molecules thick, the classical Reynolds theory of lubrication breaks down and there is at present no microscopic model for the flow behavior of a collection of molecules near a solid wall. In this paper, a nonlocal lubrication theory of fluids with microstructure is presented to provide a basic theory for calculatin the rate at which a thin film of liquid drains from between two molecularly smooth, solid surfaces as they are forced together. The viscosity moduli, proposed by the theory are in excellent agreement with experiments for all film thicknesses down to zero. Exact solutions are obtained for drainage curves and for the spread of thin films on a rotating surface. Theoretical calculations are in perfect agreement with several experimental observations.     

Investigation of Inclined Turbulators for Heat Transfer Enhancement in a Solar Air Heater

ABSTRACT

In the present study, the effect of forward inclined turbulators on the heat transfer enhancement in a duct is investigated, for forced convection. Turbulator configurations with three different pitch ratios and three different inclination angles are investigated for seven Reynolds numbers within the range 500–50,000. Investigations are performed experimentally as well as computationally, within a computational fluid dynamics framework. A distinguishing feature of the latter has been the employment of a turbulence model, the transitional shear stress transport model that is applicable throughout the presently considered range of Reynolds numbers containing laminar, transitional, and turbulent regions. At the beginning of the study, measurements and predictions are validated against analytical and empirical expressions known for a plain duct. The results obtained for turbulators configurations indicate that Nusselt number increases with the inclination angle but decreases with the pitch ratio. The influence of the inclination angle on the Nusselt number and thermal enhancement factor is found to be stronger than that of the pitch ratio. For all Reynolds numbers and for all configurations, the thermohydraulic performance is observed to increase, leading to thermal enhancement factors within the range 2–5. In all cases, a quite good agreement of the predictions and experiments is observed, which increases the confidence in the accuracy of both approaches.     

Deposition of AgGaS<Subscript>2</Subscript> thin films by double source thermal evaporation technique

In this study, polycrystalline AgGaS₂ thin films were deposited by the sequential evaporation of AgGaS₂ and Ag sources with thermal evaporation technique. Thermal treatment in nitrogen atmosphere for 5 min up to 700 °C was applied to the deposited thin films and that resulted in the mono phase AgGaS₂ thin films without the participation of any other minor phase. Structural and compositional analyses showed the structure of the films completely changes with annealing process. The measurements of transmittance and reflectance allowed us to calculate the band gap of films lying in 2.65 and 2.79 eV depending on annealing temperature. The changes in the structure with annealing process also modify the electrical properties of the films. The resistivity of the samples varied in between 2 × 10³ and 9 × 10⁶ (Ω-cm). The room temperature mobility depending on the increasing annealing temperature was in the range of 6.7–37 (cm² V⁻¹ s⁻¹) with the changes in carrier concentrations lying in 5.7 × 10¹³–2.5 × 10¹⁰ cm⁻³. Mobility-temperature dependence was also analyzed to determine the scattering mechanisms in the studied temperature range with annealing. The variations in the electrical parameters of the films were discussed in terms of their structural changes.     

The investigation of structural, electrical, and optical properties of thermal evaporated AgGaS2 thin films

AgGaS₂ (AGS) thin films were deposited onto glass substrates by sequential thermal evaporation of AgGaS₂ single crystalline powder and excess silver (Ag) interlayer. Systematic optimization to obtain single phase AgGaS₂ thin films was carried out by changing the thickness of the excess silver layer. The structure and composition of as-grown and annealed films were studied by means of X-ray diffraction and energy dispersive X-ray analysis, respectively. The optical properties of AGS thin films determined by transmittance and reflection measurements showed that they had quite high absorption coefficient with the values around 10⁴ (cm⁻¹). The calculated band gap values were found to be between 2.30 and 2.75 eV depending on annealing temperature. The refractive index (n) and extinction coefficient (k) of the films were determined by the envelope method. Finally, photo-electrical measurements under different illumination intensities were carried out, and different sensitizing and recombination centers were defined.     

Incompressible micromorphic fluid model for turbulence

A theory of incompressible micromorphic fluids is introduced as a rational model for turbulence studies. Balance laws and constitutive equations are given. The theory is then applied to obtain the solution of the turbulent channel flow problem. Turbulent velocity, gyrations, Reynolds stresses, root-mean squares of longitudinal and transverse turbulent velocities, and turbulent shear stress are given.     

Theory of anisotropic micropolar fluids

A continuum theory of anisotropic fluids is introduced. Balance laws are based on the micropolar continuum mechanics. Properly invariant constitutive equations are established and restricted by the second law of thermodynamics. The field equations are solved for the shear flow of rod-like suspensions in viscous fluids.     

Theory of nonlocal thermoelasticity

By means of the nonlocal thermodynamics and the axiom of objectivity a set of constitutive equations is developed for the nonlocal thermoelastic solids. Constitutive functionals are linearized, and, together with the balance laws of nonlocal continuum mechanics, the field equations are obtained for the displacement and the temperature fields of the linear theory. The surface physics relevant to thermoelasticity is shown to be included in the theory.     

Line crack subject to shear

Field equations of nonlocal elasticity are solved to determine the state of stress in the neighborhood of a line crack in an elastic plate subject to a uniform shear at the surface of the crack tip. A fracture criterion based on the maximum shear stress gives the critical value of the applied shear for which the crack becomes unstable. Cohesive stress necessary to break the atomic bonds is calculated for brittle materials.

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Résumé Les équations de champ d'élasticité non locale sont résolues afin de déterminer l'état de tension au voisinage d'une fissure linéaire dans une tôle élastique soumise à cisaillement uniforme à la surface de l'extrémité d'une fissure. Un critère de rupture basé sur la tension de cisaillement maximum fournit la valeur critique du cisaillement appliqué, pour lequel la fissure devient instable. La contrainte de cohésion nécessaire pour rompre les liaisons atomiques est calculée dans le cas des matériaux fragiles.

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The present work was supported by the Office of Naval Research.