Systematic Analysis of the Impact of Bump Height Manufacturing Errors on the Unbalance Response of a Foil Journal Bearing

The present work exhibits the numerical investigation of the bump height-manufacturing errors on the unbalance response of an aerodynamic foil journal bearing. This is the first study on the impact of manufacturing errors based on an important number of samples. A statistical analysis predicts the mean values of the characteristics and the standard errors of the mean. The paper presents the most important aspects of the numerical model that was used and the way it was implemented for the unbalance analysis of a four degrees-of-freedom rotor. It was considered that the bump height-manufacturing errors had a normal distribution (i.e., each bump had a different random height) around the design (mean) height value. The standard deviation of the bump heights (the same for all bumps) is a measure of the magnitude of the manufacturing errors. The results give a qualitative but above all quantitative overview of the impact of machining errors on some characteristics of aerodynamic foil bearings.

Thin Film Characterization on Cu/SnAg Solder Interface for 3D Packaging Technologies

Copper is a commonly used interconnect metal in microelectronic interconnects due to its exceptional electrical and thermal properties. Particularly in applications of the 2.5 and 3D integration, Cu is utilized in through-silicon-vias (TSVs) and flip chip interconnects between microelectronic chips for providing miniaturization, lower power and higher performance than current 2D packaging approaches. SnAg capped Cu pillars are a common high-density interconnect technology for flip chip bonding. For these interconnects, specific properties of the Cu surface, such as roughness and cleanliness, are an important factor in the process to ensure quality solder bumps. During electroplating, tight processing parameters must be met so that defects are avoided, and high bump uniformity is achieved. An understanding of the interactions at the solder and Cu pillar interface is needed, based on the electroplating parameters, to determine the best method for populating solder on the wafer surface. In this study, surface treatment techniques such as oxygen plasma cleaning were performed on the Cu surfaces and the SnAg plating chemistry for depositing the solder were evaluated through hull cell testing to qualitatively determine the range of current densities to investigate. It was observed that current density while plating played a large role in solder bump deposition morphology. At the higher current densities greater than 60 mA/cm2, bump height non-uniformity and dendritic growth are observed and at lower current densities, less than or equal to 60 mA/cm2, uniform, continuous bump height occurred.

Bumping Co-planarity Collocation for Different UBM Size by Geometry Integration

Bumping co-planarity is a Cu pillar bump characteristic, that can impact to the joint quality of subsequent flip chip bonding process. The plated bump height variation correlates with lesser co-planarity values. Co-planarity can be minimized by bumping process, however the bumping process window is not adequate for some design features. For example, dummy bump or structure drawback features. This paper provides a methodology to improve co-planarity by collocating oval and circular bump which integrates the solder volume of different bump shapes. The final solder formation is different due to the geometry variation from the oval shape and circular shape. The final solder height can be calculated by mathematical integral from as-plated solder volume. Hence, better co-planarity can be achieved by the proposed method to collocate different bump shapes. The Cu pillar bump collocation design rules can be optimized to minimize co-planarity during initial design realization to minimize quality risks during fabrication..

Numerical Investigation of Dorsal S-Shaped Inlet Flow Characteristic and Effects of Related Parameters

The air intake design system of the dorsal intake combined with the S-shaped inlet has been widely used in various combat aircraft due to its good stealth characteristics. In this paper, the numerical simulation of the flow characteristic and influence law of various parameters for a typical air intake design with dorsal S-shaped inlet was carried out using the in-house large-scale parallel computational fluid dynamics (CFD) solver. Firstly, the numerical method was introduced and the solver was preliminarily validated by the well-known RAE M2192 inlet model. Then, the numerical calculation of the target air intake design was conducted and the distribution of shockwave at the entrance area, the second flow in the inlet and the flow at the exit section plane were analyzed in proper order. In addition, the influence of the bump height and lip sweep angles on the inlet performance was also studied. The simulation results show that the unique S-shaped design in the inlet will result in flow separation and secondary flow, ultimately causing total pressure loss, and different external geometry parameters have a great influence on the inlet performance. Within a certain range, proper reduction of the bump height or lip sweep angle can improve the inlet performance.

Numerical analysis on shape memory alloy–based adaptive shock control bump

A three-dimensional adaptive shock control bump made of shape memory alloy is proposed for transonic wings. The methodology to adaptively change the configuration of the airfoil using the shape memory alloy bump to reduce the shock strength and wave drag is numerically demonstrated using an airfoil RAE2822. The shape memory alloy bump is trained to have a flat initial shape with certain initial strain and can swell up when thermally activated. Boyd–Lagoudas phenomenological model is implemented in finite element method and used to compute the two-dimensional profile and the height of the shape memory alloy bump during thermal activation. The results show that the shape memory alloy bump can generate a considerable deflection due to the reverse phase transformation when thermally activated. The dependence of aerodynamic characteristics of the wing on the height of the shape memory alloy bump and the angle of attack is investigated using computational fluid dynamics method. The results show that there is an optimal bump height for a given angle of attack and the bump with a given height is effective only in certain range of angle of attack. Optimization of bump height and the corresponding driving temperature are carried out under variable angles of attack with the lift-to-drag ratio as the objective function.

Influence of Donut-Shaped Bump on the Hydrodynamic Lubrication of Textured Parallel Sliders

The influence of donut-shaped bump texture on the hydrodynamic lubrication performance for parallel surfaces is presented in this paper. A mathematical equation has been applied to express the shape of three-dimensional donut-shaped bump texture. Numerical simulation of the pressure distribution of lubricant between a textured slider and a smooth, moving slider has been performed to analyze the geometrical parameters' influence on the hydrodynamic performance for textured surfaces. The numerical results show that the convex of the donut-shaped bump provides a microstep slider, which can form a convergent wedge and build up hydrodynamic pressure. Optimum values of horizontal spacing and bump height are obtained to maximize the hydrodynamic pressure. It is also noted that the average pressure increases monotonically with the increase of bump radius, but decreases with the increase of vertical spacing and dimple depth, respectively.

Numerical Analysis of the Impact of Manufacturing Errors on the Structural Stiffness of Foil Bearings

The dynamic characteristics of foil bearings operating at high rotation speeds depend very much on the mechanical characteristics of the foil structure. For this reason, the stiffness and damping of the structure of foil bearings are problems that are the focus of many analyses. The mechanical characteristics of the foil structure (top and bump foil) are analyzed either by using a simple approach obtained for an isolated bump modeled as a beam or with more elaborate ones taking into account the three-dimensional nature of the bumps and their mutual interactions. These two kinds of models give different foil structure stiffness, with lower values for the simplified model. However, the published experimental results of the foil bearing structure tend to validate the simplified model. The present paper explains the differences between the simplified and the elaborate models by taking into account the manufacturing errors of the foil structure. A three-dimensional model based on the nonlinear theory of elasticity is developed. The model offers a unique insight into the way the bearing structure deforms when the rotor is incrementally pushed into the foil structure. Three realistic manufacturing errors, bump height, bump length, and radius of the bump foil, are analyzed. Bump height and length vary following a normal distribution with a given standard deviation while the radius of the bump foil is given a waviness form with an imposed peak-to-peak amplitude. Three to five cases were calculated for each kind of error. Results show that only the manufacturing errors of the bump height affect the stiffness of the foil structure by diminishing its values. Height errors of 20 μm standard deviation (4% of the average bump height and 60% of the radial clearance) may induce a 40–50% reduction of the stiffness of the foil structure, i.e., in the range of the predictions of the simplified model.

The Mechanism of the Low-K Stress Reduction in Chip Assembly by Shorter Solder Bump Height

To solve the low-k delamination in chip assembly for high-end servers, our hypothesis is proposed that the low-k stress is determined by the bending moment and the stress relaxation of a joint. In our hypothesis, the low-k stress decreases as the joint height (SnAg bump height) becomes shorter, such as below 80μm, in 150μm-pitch joints. Our hypothesis is supported by simulation, in which the low-k stress is investigated as a function of the joint height, the joint material and also the joint width (the joint pitch). Finally, experiments are performed to evaluate the low-k delamination as a function of the joint height and our hypothesis is also supported by experiments.