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RESEARCH PAPERS

J. Electron. Packag. 1992;114(1):1-7. doi:10.1115/1.2905436.

To gain a better understanding of the stress state in surface mount joints and their design considerations, an analysis method of bonded joints is developed and consequently three parameters that govern the stress distribution are identified. The solutions are obtained in terms of trigonometric and hyperbolic functions, and then applied to a simplified surface mount device. Contrary to general belief that the surface mount joints are mainly subjected to shear, it is shown that a peel stress also exists in a joint. Especially in a short joint used to connect stiff members as in direct mounting of chips and chip carriers, the magnitude of the peel stress is far greater than the shear stress.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):8-13. doi:10.1115/1.2905447.

Failure of plated-through-holes (PTHs) due to thermomechanical stresses is a well established cause of failure of multilayer printed wiring boards (MLBs). This paper uses the finite element method (FEM) to examine the nature of the stress distribution within the PTH structure when the MLB is subjected to thermal loads. Guidelines are laid out for realistic modelling of material properties and boundary conditions in the FEM model. Parametric studies are conducted to study the qualitative effect of several geometric parameters on the critical stresses in the PTH. Both traditional glass-epoxy (FR-4) MLBs and highly anisotropic Kevlar-polyimide MLBs are examined. Differences in behavior observed between the two materials underline the pitfalls in extending the standard design thumbrules for standard FR-4 MLBs to other MLB materials. The purpose here is to provide guidelines for the reliable design of PTHs. Actual fatigue life predictions are deferred to a later paper.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):14-21. doi:10.1115/1.2905435.

This paper describes an investigation of the forced convection heat transfer and pressure drop characteristics of a regular in-line array of flatpacks for several channel heights and inlet velocities. The work has both practical and theoretical interest since it relates to technical problems now faced by the electronics industry, and it embodies one of the most general heat transfer problems: non-uniform heat release from nonuniform geometries. To predict operating temperatures in situations where the wall temperature distribution is non-uniform, one must use superposition. Both the adiabatic heat transfer coefficient, had , and the superposition kernel functions, g*, are required. The problem can be solved using superposition directly (had and g*) or indirectly (using had and g* to calculate the correct value of hm ). Either way the superposition data is required. This work presents the first full set of superposition data for flatpack arrays. Part 1 presents heat transfer and pressure drop results and part 2 presents a model for heat transfer that is based on the maximum turbulence fluctuations in the channel.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):22-28. doi:10.1115/1.2905437.

This paper describes an investigation of the forced convection heat transfer and pressure drop characteristics of a regular in-line array of flatpacks for several channel heights and inlet velocities. The work has both practical and theoretical interest since it relates to technical problems now faced by the electronics industry, and it embodies one of the most general heat transfer problems: nonuniform heat release from nonuniform geometries. To predict operating temperatures in situations where the wall temperature distribution is nonuniform, one must use superposition. Both the adiabatic heat transfer coefficient, h ad and the superposition kernel functions, g * are required. The problem can be solved using superposition directly (h ad and g *) or indirectly (using h ad and g * to calculate the correct value of h m ). Either way the superposition data is required. This work presents the first full set of superposition data for flatpack arrays. Part 1 presents heat transfer and pressure drop results and part 2 presents a model for heat transfer that is based on the maximum turbulence fluctuations in the channel.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):29-34. doi:10.1115/1.2905438.

Velocimetry, heat transfer, and pressure drop experiments are reported for laminar/transitional air flow in a channel containing rectangular transverse ribs located along one channel wall. The geometry is intended to represent an array of low profile electronic packages. At fixed pumping power per unit channel volume, the heat transfer rate per unit volume is independent of rib-to-rib spacing and increases with decreasing wall-to-wall spacing. The fully developed, rib-average heat transfer coefficient is found to be linearly related to the maximum streamwise rms turbulence measured above the rib-tops. Linear correlations, in terms of a descriptor of the rms streamwise turbulence, are shown to unify heat transfer/pressure drop data for channels containing either two-or three-dimensional protrusions.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):35-40. doi:10.1115/1.2905439.

Numerical simulations of the flow pattern and forced convective heat transfer in geometries such as those encountered in cooling systems for electronic devices are presented. For Reynolds numbers above the critical one, Rc , these flows exhibit a traveling wave structure with laminar self-sustained oscillations at the least stable Tollmien-Schlichting mode frequency. Supercritical oscillatory flow induces large-scale convective patterns which lead to significant mixing and correspondingly heat transfer augmentation. Three techniques of heat transfer enhancement by flow destabilization are compared on an equal pumping basis: active flow modulation, passive flow modulation and supercritical flow destabilization. It is found that the best enhancement system regarding minimum power dissipation corresponds to passive flow modulation in the range of low Nusselt numbers. However, supercritical flow destabilization becomes competitive as the requirement for a higher Nusselt number begins to dominate the design choices.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):41-47. doi:10.1115/1.2905440.

An experimental study has been conducted to reveal the relevant heat transfer mechanisms which exist within an infrared reflow oven. Simulated card assemblies are used and their transient thermal responses, induced by combined radiative and convective heating, are measured. A simple numerical model is developed with which relevant heat transfer mechanisms are identified and quantified. The study shows that radiative and mixed convective heat transfer processes induce a variety of system thermal responses. Model predictions, which incorporate measured forced convection heat transfer coefficients and accurate descriptions of surface-to-surface radiative exchange, are in excellent agreement with experimental data for cases where the thermally induced buoyancy forces within the oven air are relatively small. The results of the experimental and analytical study provide guidelines for the development of more sophisticated models of the infrared reflow process.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):48-54. doi:10.1115/1.2905441.

A numerical model is developed that describes the infrared reflow soldering of surface mounted components onto prewired circuit cards. The model predicts convective conditions within the reflow oven and uses these conditions, in conjunction with a radiative heat transfer analysis and a two-dimensional transient conduction analysis, to predict the thermal response of a card assembly as its components are soldered. The model is described, and its performance is illustrated through presentation of base case simulations. Parametric simulations are performed to determine the sensitivity of the card assembly thermal response to variations in oven conditions and uncertainties in the convective, as well as radiative heat transfer portions of the model.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):55-62. doi:10.1115/1.2905442.

Experiments have been performed using water and FC-77 to investigate heat transfer from an in-line 1 x 10 array of discrete heat sources, flush mounted to protruding substrates located on the bottom wall of a horizontal flow channel. The data encompass flow regimes ranging from mixed convection to laminar and turbulent forced convection. Buoyancy-induced secondary flows enhanced heat transfer at downstream heater locations and provided heat transfer coefficients comparable to upstream values. Upstream heating extended enhancement on the downstream heaters to larger Reynolds numbers. Higher Prandtl number fluids also extended heat transfer enhancement to larger Reynolds numbers, while a reduction in channel height suppressed buoyancy driven flows, thereby reducing enhancement. The protrusions enhanced the transition to turbulent forced convection, causing the critical Reynolds number to decrease with increasing row number. The transition region was characterized by large heater-to-heater variations in the average Nusselt number.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):63-70. doi:10.1115/1.2905443.

Forced convection boiling experiments have been performed for an in-line 1 x 10 array of discrete heat sources, flush mounted to protruding substrates located on the bottom wall of a horizontal flow channel. FC-72, a dielectric fluorocarbon liquid, was used as the heat transfer fluid, and the experiments covered a range of flow velocities, degrees of fluid subcooling, and channel heights. The maximum heater-to-heater surface temperature variation was less than 2.5°C and was insensitive to channel height under conditions of fully developed nucleate boiling. Although the fluid velocity influenced the heat flux for partially developed nucleate boiling, its influence was muted for fully developed nucleate boiling. The heat flux associated with a departure from nucleate boiling increased with increasing velocity, subcooling, and channel height; however, the effect of channel height was only significant when all of the upstream heaters were energized.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):71-80. doi:10.1115/1.2905444.

An investigation of pulsed, laser drilling in a partially transparent medium was conducted. The study included both theoretical and experimental analyses. The theoretical analysis included development of a computer simulation to model the laser drilling process—a three-dimensional finite difference solution with temperature-dependent thermal properties, finite sample geometry, and experimentally determined laser beam characteristics. Both qualitative and quantitative correlation of the theoretical and experimental results was good with successful prediction of hole shapes and minimum error in the theoretically predicted cross-sectional areas of the laser-drilled holes ranging approximately ± three percent over the range of energies per laser pulse considered in this study. Results of calculations and experiments demonstrated the importance of the shape and irradiance distribution of the incident laser beam on the quality of laser-drilled holes in Al2 O3 samples.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):81-87. doi:10.1115/1.2905445.

The flow structure and local heat transfer characteristics of two-dimensional slot jets impinging on heated protrusions has been investigated. The spent air was constrained to exit at one end of the channel, forming a crossflow. The effects of three parameters on the heat transfer were examined for an array of five protruding heat sources. They include the jet slot width, distance between the jet exit and the protrusion, and the average jet Reynolds number. Laser-Doppler velocimeter measurements were made to detail the mean and turbulent flow structure in the channel. Experimental results reveal that the flow and heat transfer are dominated by turbulent transport even for Reynolds numbers as low as 300. Two transport mechanisms were identified affecting the heat transfer. The first was jet impingement, the second being crossflow of the spent air. A complex interaction between the two mechanisms was observed. At low nozzle-protrusion spacing with large slot jets the heat transfer was dominated by the crossflow, whereas for high nozzle-protrusion spacing and small jets, transport was dominated by jet impingement. It is postulated that the highest average Nusselt number occurs when the jets and the crossflow influence act with near-equal intensity.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):88-92. doi:10.1115/1.2905446.

Low cycle fatigue of solder joints is one of the major kinds of failures in second level interconnections of an electronic package. The fatigue failure is caused by thermal strains which are created from a mismatch of coefficients of thermal expansion (CTE) that occurs between two levels of packaging. As the package approaches smaller dimensions, measurements of thermal strains in the solder interconnections become very difficult. In this paper, moire interferometry technique was applied to evaluate the thermal strains in the second level interconnections for both conventional pin-in-hole (PIH) packages and surface mount components. The coefficient of thermal expansion of each component was measured. Thermal strain distributions in the solder interconnections were determined, and reliability issues were discussed. The strains in solder joints of the PIH components were much higher than those of the stacked surface mount components. Even though the surface mount components had a lower inherent strength, their overall mechanical reliability was much higher since they had practically no localized strain concentrations.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J. Electron. Packag. 1992;114(1):93-96. doi:10.1115/1.2905448.

The results of automated measurements of flank wear of drills used in circuit board drilling are presented. The measurement technique uses a machine vision system to increase measurement speed and reduce measurement variability. A flank area wear parameter is compared to the commonly used flank width wear measurement, and is shown to be more consistent, in agreement with similar work by T. I. Liu using multifaceted crankshaft drills.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 1992;114(1):96-99. doi:10.1115/1.2905449.

The spacecraft electronic assemblies are subjected to severe environmental conditions during testing, launching and during orbit mission. The success of the mission depends upon the proper functioning of these critical electronic modules. The structural analyses using Finite Element Methods (FEM) assure the integrity of these components. A typical box, Array Drive Electronics (ADE), for TIROS satellites [1] is discussed in this article. The NASTRAN software was used to perform the stress and modal analyses of the box assembly with five circuit boards, covers and mounting feet. The stress analysis was performed for a static solution. As a conservative loading, 3 sigma load factor was used in the calculation of the acceleration values from the random vibration test conditions. Margins of safety were calculated. Design and material changes were recommended. The mode shapes fall in to three groups as explained in the text.

Commentary by Dr. Valentin Fuster

BOOK REVIEWS

J. Electron. Packag. 1992;114(1):100. doi:10.1115/1.2905434.
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Abstract
Commentary by Dr. Valentin Fuster

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