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Abstract A field-driven bending method is introduced in this paper according to the coordinate transformation between straight and curved coordinates. This novel method can incorporate with the periodic boundary conditions in analysis along axial, bending, and transverse directions. For the case of small bending, the bending strain can be compatible with the beam theory. Consequently, it can be regarded as a generalized SLLOD algorithm. In this work, the bulk copper beam under bending is analyzed first by the novel bending method. The bending stress estimated here is well consistent to the results predicted by the beam theory. Moreover, a hollow nanowire is also analyzed. The zigzag traces of atomic stress and the corresponding 422 common neighbor type can be observed near the inner surface of the hollow nanowire, which values are increased with an increase of time. It can be seen that the novel bending method with periodic boundary condition along axial direction can provide a more physical significance than the traditional method with fixed boundary condition.

AbstractThis study reports the surface roughness and nanomechanical characteristics of ZnO thin films deposited on the various substrates, obtained by means of atomic force microscopy (AFM), nanoindentation and nanoscratch techniques. ZnO thin films are deposited on (a- and c-axis) sapphires and (0001) 6H-SiC substrates by using the pulsed-laser depositions (PLD) system. Continuous stiffness measurements (CSM) technique is used in the nanoindentation tests to determine the hardness and Young’s modulus of ZnO thin films. The importance of the ratio (H/Efilm) of elastic to plastic deformation during nanoindentation of ZnO thin films on their behaviors in contact-induced damage during fabrication of ZnO-based devices is considered. In addition, the friction coefficient of ZnO thin films is also presented here.

AbstractThis work presents the molecular dynamics approach toward mechanical deformation and phase transformation mechanisms of monocrystalline Si(100) subjected to nanoindentation. We demonstrate phase distributions during loading and unloading stages of both spherical and Berkovich nanoindentations. By searching the presence of the fifth neighboring atom within a non-bonding length, Si-III and Si-XII have been successfully distinguished from Si-I. Crystallinity of this mixed-phase was further identified by radial distribution functions.

Designed experiments were conducted in this paper to study the effect of Au/Ni/Cu or Cu substrate pad metallization on the electromigration reliability of 96.5Sn--3Ag--0.5Cu flip-chip solder joints with Ti/Ni(V)/Cu under bump metallurgy (UBM) under a current stressing condition with an average current density of around 5 kA/[cm.sup.2] at an ambient temperature of 150[degrees]C. Cross-sectional observations on current-stressed solder joints indicate that although Cu metallization results in severe voiding compared with Au/Ni/Cu metallization on the substrate side of the solder joint, the dominant failure has been identified as UBM consumption, and test vehicles with Cu metallization exhibit better electromigration reliability than those with Au/Ni/Cu metallization. The stronger durability against current stressing for test vehicles with Cu metallization may attribute to the lower UBM consumption rate due to the continuous Cu diffusion toward UBM as a result of the concentration gradient. The consumption of UBM is faster for test vehicles with Au/Ni/Cu metallization because Cu diffusion from the substrate pad is retarded by the Ni barrier. [DOI: 10.1115/1.3103945] Keywords: electromigration, reliability, flip-chip, solder joint, substrate pad metallization

The sequential thermal-mechanical coupling analysis, which solves in turn the transient temperature field and subsequent thermomechanical deformations, was performed in this work to investigate thermal characteristics along with fatigue reliability of a thin-profile fine-pitch ball grid array chip-scale package subjected to power cycling. The numerical model was calibrated using steady-state and power cycling experiments. Following the calibrated numerical model different power cycling durations on the thermal characteristics and fatigue reliability of the solder joints were examined. Numerical results indicate that, compared with thermal cycling, power cycling requires many more cycles to achieve a stabilized plasticity index between test cycles. The fatigue reliability would therefore be greatly underestimated if only such an index of the first several cycles is followed in the predictions. [DOI: 10.1115/1.3068294]

This work presents electromigration reliability and patterns of Sn-3Ag-0.5Cu and Sn-3Ag-1.5Cu/Sn-3Ag-O.5Cu composite flip-chip solder joints with Ti/ Ni(V)/ Cu under bump metallurgy (UBM), bonded on Au/Ni/Cu substrate pads. The solder joints were subjected to an average current density of 5 kA/ [cm.sup.2] under an ambient temperature of 150[degrees]C. Under the situation when electron charges flow from the UBM toward the substrate, Sn diffuses from the Cu-Ni-Sn intermetallic compound developed around the UBM toward the UBM and eventually causes the Ni(V) layer to deform. Electromigration reliability of Sn- 3Ag- l.5Cu/ Sn- 3Ag-O.5Cu composite flip-chip solder joints was found to be better than that of Sn-3Ag-0.5Cu solder joints. According to the morphological observations on cross-sectioned solder joints, a failure mechanism is proposed as follows. Since the deformation of the Ni(V) layer as a result of Sn diffusion toward the UBM is considered as the dominant failure, a greater Cu weight content in the solder joints would trap more Sn in the Sn-Cu interfacial reaction and would therefore retard the diffusion of Sn toward the UBM and hence enhance the electromigration reliability. [DOI: 10.1115/1.2957325] Keywords: electromigration, flip-chip, solder joint, reliability, Sn-Ag-Cu, Cu weight content

Due to cost and process considerations, sometimes a drop-in heat spreader is incorporated in a quad flat package, which is not attached to the die pad. Consequently, due to mismatch of thermal expansions of the components, gapping is inevitable on the unattached interface between drop-in heat spreader and die pad during temperature change. The presence of the gap would potentially decrease the reliability and thermal performance of the package. In this paper, gap distributions on the unattached interface and thermal characteristics of a quad flat package implemented with an unattached drop-in heat spreader are examined through the thermal-mechanical coupling analysis. A numerical procedure is proposed to specify film coefficients as functions of gap openings on separated surfaces. Effects of mesh density and interfacial heat transfer conditions on the numerical solutions are then investigated. [DOI: 10.1115/1.2753988] Keywords: thermal-mechanical coupling analysis, thermal contact, quad flat package (QFP), drop-in heat spreader

We present in this paper parametric studies of board-level reliability of wafer-level chip-scale packages subjected to a specific pulse-controlled drop test condition. Eighteen experiment cells, constructed by varying joint pitch, die thickness, and die size, are proposed and examined numerically. The transient analysis follows the support excitation scheme and incorporates an implicit time integration solver. Numerical results indicate that the drop reliability of the package enhances as the die thickness as well as the die size decreases. Moreover, the package with smaller solder joints and a smaller joint pitch suffers a greater drop reliability concern. [DOI: 10.1115/1.2429717] Keywords: board-level drop test, WLCSP, reliability, JEDEC

The high-temperature operation life test (HTOL) was conducted in this paper to study electromigration phenomena of solder interconnects in a flip-chip package assembly. We examined the fatigue reliability and morphological patterns of three solder compositions: Sn-4Ag-0.5Cu, Sn-3.5Ag-1Cu, and Sn-3Ag-1.5Cu, subjected to two test conditions consisting of different average current densities and ambient temperatures (5 kA/[cm.sup.2] at 150[degrees]C and 20 kA/[cm.sup.2] at 30[degrees]C). It is interesting to realize that as the Cu weight content of the solder composition increases, the fatigue life increases under 5 kA/[cm.sup.2] at 150[degrees]C but decreases under 20 kA/[cm.sup.2] at 30[degrees]C. Observed electromigration morphologies along with computed current density and temperature distributions on solder interconnects from the electrothermal coupling analysis were examined, correlated, and discussed. [DOI: 10.1115/1.2429710] Keywords: electromigration, solder bump, flip-chip, reliability, electrothermal coupling analysis

In this work, thermal characteristics of a board-level chip-scale package, subjected to coupled power and thermal cycling test conditions defined by JEDEC, are investigated through the transient thermal analysis. Tabular boundary conditions are utilized to deal with time-varying thermal boundary conditions brought by thermal cycling. It is obvious from the analysis that the presence of power cycling leads to a significant deviation of the junction temperature from the thermal cycling profile. However, for components away from the die, the deviation is insignificant. Moreover, for low-power applications, temperature histories from coupled power and thermal cycling are approximately linear combinations of temperature histories from pure power cycling and the ones from pure thermal cycling. [DOI: 10.1115/1.2229229] Keywords: power cycling, thermal cycling, JEDEC, transient thermal analysis

In a finite element analysis, when localized behavior of a large model is of particular concern, generally one would refine the mesh until it captures the local solution adequately. Submodeling is an alternative way for solving this kind of problem. It provides a relatively accurate solution at a modest computational cost. For a valid submodeling analysis, the boundaries of the submodel should be sufficiently far away from local features so that St. Venant's principle holds. Moreover, special treatments are required for solving problems that involve path-dependent characteristics. This paper presents a general procedure to perform submodeling analyses for path-dependent thermomechanical problems without a priori assumptions on the structural response. The procedure was benchmarked using a bimaterial strip and demonstrated through analyses on a bump chip carrier package assembly. The procedure is conducive to the numerical assessment of fatigue lives of electronic packages. [DOI: 10.1115/1.1869513]