Kaushik A. Iyer

Prediction of electrical contact resistance for anisotropic conductive adhesive assemblies

Anisotropic conductive adhesive (ACA) assembly is emerging as one of the most flexible and cost effective packaging interconnect methods in the microelectronics industry. One of the major impediments to the full realization of the fine pitch (<200 /spl mu/m) capabilities of this assembly method is accurate prediction and control of electrical contact resistance. This paper presents a detailed review and direct comparisons of the different models that have been used to predict electrical contact resistance in ACA interconnects. It is found that large discrepancies exist among these models and between contact resistance values experimentally measured and what these models predict. The governing equations and assumptions underlying the models and their implications are examined, and possible rationales for the observed discrepancies and future directions for developing improved models are identified. The study shows that important issues generally not considered in current models are tunneling resistance, multimaterial layers, edge effect, rough surfaces, elastic recovery and residual forces, interaction between nearby particles and variation on the radii of multiple particles.

Analyses of contact pressure and stress amplitude effects on fretting fatigue life

Elastic-plastic finite element analyses of a cylinder-on-plate configuration, studied experimentally, were performed to provide an explanation for the decrease in fretting fatigue life with increasing contact pressure. Three values of normal load, namely 1338 N, 2230 N, and 3567 N, and three stress ratios (0.1, 0.5, and 0.7) were considered. Based on a previously determined dependency between contact pressure and friction coefficient, the effect of coefficient of friction was also evaluated. The deformation remained elastic under all conditions examined. Cyclic, interfacial stresses, and slips were analyzed in detail. The amplification of remotely applied cyclic stress in the contact region is shown to provide a rationale for the effect of contact pressure and stress amplitude on life. Comparisons with previous experiments indicate that the local stress range computed from finite element analysis may be sufficient for predicting fretting fatigue life. Further, the results suggest that the slip amplitude and shear traction may be neglected for this purpose.

Large-scale troughs on Vesta: A signature of planetary tectonics

Abstract Images of Vesta taken by the Dawn spacecraft reveal large-scale linear structural features on the surface of the asteroid. We evaluate the morphology of the Vesta structures to determine what processes caused them to form and what implications this has for the history of Vesta as a planetary body. The dimensions and shape of these features suggest that they are graben similar to those observed on terrestrial planets, not fractures or grooves such as are found on smaller asteroids. As graben, their vertical displacement versus length relationship could be evaluated to describe and interpret the evolution of the component faults. Linear structures are commonly observed on smaller asteroids and their formation has been tied to impact events. While the orientation of the large-scale Vesta structures does imply that their formation is related to the impact events that formed the Rheasilvia and Veneneia basins, their size and morphology is greatly different from impact-formed fractures on the smaller bodies. This is consistent with new analyses that suggest that Vesta is fully differentiated, with a mantle and core. We suggest that impact into a differentiated asteroid such as Vesta could result in graben, while grooves and fractures would form on undifferentiated asteroids.

Solutions for contact in pinned connections

Abstract The pinned connection is a principal joining and load-bearing element in countless structures. A comprehensive analytical solution for the mechanics of a pinned connection would greatly facilitate mechanics and dynamics analyses seeking to address larger issues such as fatigue life and earthquake resistance of entire structures. However, even the most advanced closed-form solutions describing contact conditions in pinned connections involve mathematical approximations for some or all of the following aspects of the problem: plate dimensions, contact area, pin–plate friction and material–property mismatch. The actual problem involving a finite plate, frictional contact and arbitrary material pairs still remains intractable in a rigorous manner through elasticity theory. This study uses the finite element method to obtain solutions when some or all of the above simplifying assumptions are removed, and thereby also evaluates the general applicability of the most advanced closed-form solutions. A surprising finding is that the pin–plate contact pressure and plate tangential stress distributions are practically independent of the material pair as long as both the pin and plate are metallic and the friction coefficient is small. It is also found that the stress concentration factor in the plate is significantly higher when finite dimensions are considered. Among the factors considered, the pin–plate friction coefficient has the greatest effect on the contact pressure and tangential stress distributions.

Peak contact pressure, cyclic stress amplitudes, contact semi-width and slip amplitude: relative effects on fretting fatigue life

Abstract Using Hertzian relationships and a finite element model of a cylinder-on-flat contact configuration, a set of critical fretting fatigue experiments have been conducted to determine the relative effects of six local mechanistic parameters on fretting fatigue life. The six parameters evaluated are: (i) the peak contact pressure, p0; (ii) the maximum local cyclic bulk stress range, ΔσL,max; (iii) the maximum, local cyclic shear stress, τL,max; (iv) the maximum, local cyclic shear stress range, ΔτL,max; (v) the maximum slip amplitude, δmax; and (vi) the contact semi-width, a. The findings are significant. Only the peak contact pressure and the local, maximum bulk stress range have direct effects on fretting fatigue life. The fretting fatigue life is found to have a previously unknown positive, exponential dependence on the peak contact pressure, which enables an elegant recovery of a standard (‘plain’) fatigue life equation. Additionally, interfacial shear tractions appear to have no effect on the life. Although some of the remaining four parameters can be correlated to fretting fatigue life, it is likely through their relationships to either the peak contact pressure or the local, maximum bulk stress range. These inter-relationships have been outlined for a cylinder-on-flat geometry and a general, physically based fretting fatigue life equation has been developed.

Analysis of fretting conditions in pinned connections

A 2-dimensional, finite element model of a pinned connection, involving AA7075-T6 aluminum alloy sheet and aluminum and steel pins, is used to evaluate the local mechanical parameters that control fretting wear damage. These include the contact pressure, the slip amplitude, the tensile stress parallel to the hole interface and a fretting wear (F1) and fretting fatigue (F2) parameter. The connection is subjected to cyclic loading with a peak nominal stress of σ = 125 MPa and R = 0.1. The dimensions and constraints of the model approximate those of multi-riveted panels. Values of interference in the range 0–2% interference, and 2 values of the coefficient of friction, μ=0.2 and μ = 0.5, are examined. The variations of the mechanical parameters with angular position about the hole are defined. Peak contact pressures in the range p = 500–600 MPa are obtained at the pin-bore interface. The slip amplitudes in the range 2 μm < δ < 20 μm, and circumferential tensile stresses as high as σ ≈100 MPa, are produced in regions where the contact pressure is generally above p = 400 MPa. The peak values of the fretting wear parameter, 0.3 kPa m < F1 < 3 kPa m, coupled with small slip amplitudes and specific wear rates cannot account for significant material loss in the stiff model connection examined here. However, the relatively large fretting fatigue parameter values, 6 × 1010 Pa m < F2 < 70 × 1010 Pa m, could promote early fretting fatigue failure in the absence of interference.

Fatigue and Fretting of Self-Piercing Riveted Joints

The fatigue life and fretting characteristics of aluminum alloy 5754-O self-piercing riveted lap joints have been investigated experimentally and analytically. The experimental program involves a set of 27 cyclic tension tests on three different joints consisting of either 1 mm, 2 mm or 3 mm-thick sheet specimens. In most cases (85%), fatigue cracks are found to initiate on the faying surface of the upper sheet, adjacent to the hole, and at an angular location that lies on the sheet loading axis towards the loading end. Three-dimensional finite element analysis of the three joints has also been performed. Computed distributions of local stresses and rivet-sheet slips are interpreted in terms of experimental observations of fatigue life, crack initiation location and fretting damage observations. Significantly, the calculations provide a rationale for the surprising crack initiation location.© 2002 ASME

A computational design-of-experiments study of hemming processes for automotive aluminium alloys

Abstract Hemming is a three-step sheet-folding process utilized in the production of automotive closures. It has a critical imact on the performance and perceived quality of assembled vehicles. Using a two-dimensional finite element model, this paper presents a design-of-experiments (DOE) study of the relationships between important hemming process parameters and hem quality for aluminium alloy AA 6111-T4PD flat surface-straight edge hemming. The quality measures include roll-in/roll-out of the hem edge as well as the maximum true strain on the exposed bent surface. The finite element (FE) model combines explicit and implicit procedures in simulating the three forming subprocesses (flanging, pre-hemming, and final hemming) along with the corresponding springback (unloading). The results show that the pre-hemming die angle and the flanging die radius have the greatest influence on hem edge roll-in/roll-out, while pre-strain and the flanging die radius impact the maximum surface strain significantly. The computational DOE results also provide the basis for process parameter selection to avoid hem surface cracking and particular insights for achieving acceptable formability.

A review of the Solar Probe Plus dust protection approach

The Solar Probe Plus (SPP) spacecraft will go closer to the Sun than any manmade object has gone before, which has required the development of new thermal and micrometeoroid protection technologies. During the 24 solar orbits of the mission, the spacecraft will encounter a thermal environment that is 50 times more severe than any previous spacecraft. It will also travel through a dust environment previously unexplored, and be subject to particle hypervelocity impacts (HVI) at velocities much larger than anything previously encountered. New analytical methodologies and designs have been developed to meet this environments extreme micrometeoroid protection challenge while also fulfilling the missions low mass requirement. These new analytical capabilities and protection system concepts could produce similar benefits if applied to Earth orbiting and deep space missions. The SPP dust study was developed to overcome the velocity limitations in the existing micrometeoroid and orbital debris (MMOD) analysis capability. In this study, we developed the hydrocode modeling techniques needed to characterize the material behaviors for a high-shock particle impact event. An additional novel development was an algorithm to calculate the particle flux on specific spacecraft surfaces. Our approach predicts particle impacts for a given spacecraft geometry, including the aforementioned effects. In addition, our approach introduces a size-velocity particle correlation, which lowers the shielding needed for a given protection level. This paper covers the new analytical capabilities developed for the SPP dust environment and how they dramatically lower the mass of the protective systems. The paper also discusses the application of these new analytical capabilities to spacecraft protection in the LEO debris field.

Modeling Articulated Human Body Dynamics Under a Representative Blast Loading

Blast waves resulting from both industrial explosions and terrorist attacks cause devastating effects to exposed humans and structures. Blast related injuries are frequently reported in the international news and are of great interest to agencies involved in military and civilian protection. Mathematical models of explosion blast interaction with structures and humans can provide valuable input in the design of protective structures and practices, in injury diagnostics and forensics. Accurate simulation of blast wave interaction with a human body and the human body biodynamic response to the blast loading is very challenging and to the best of our knowledge has not been reported yet. A high-fidelity computational fluid dynamic (CFD) model is required to capture the reflections, diffractions, areas of stagnation, and other effects when the shock and blast waves respond to an object placed in the field. In this effort we simulated a representative free field blast event with a standing human exposed to the threat using the Second Order Hydrodynamic Automatic Mesh Refinement Code (SHAMRC). During the CFD analysis the pressure time history around the human body is calculated, along with the fragment loads. Subsequently these blast loads are applied to a fully articulated human body using the multi-physics code CoBi. In CoBi we developed a novel computational model for the articulated human body dynamics by utilizing the anatomical geometry of human body. The articulated human body dynamics are computed by an implicit multi-body solver which ensures the unconditional stability and guarantees the quadratic rate of convergence. The developed solver enforces the kinematic constraints well while imposing no limitation on the time step size. The main advantage of the model is the anatomical surface representation of a human body which can accurately account for both the surface loading and the surface interaction. The inertial properties are calculated using a finite element method. We also developed an efficient interface to apply the blast wave loading on the human body surface. The numerical results show that the developed model is capable of reasonably predicting the human body dynamics and can be used to study the primary injury mechanism. We also demonstrate that the human body response is affected by many factors such as human inertia properties, contact damping and the coefficient of friction between the human body and the environment. By comparing the computational results with the real scenario, we can calibrate these input parameters to improve the accuracy of articulated human body model.© 2011 ASME