Compliant Bump Research Articles

Optimizing energy dissipation in gas foil bearings to eliminate bifurcations of limit cycles in unbalanced rotor systems

High-speed rotor systems mounted on gas foil bearings present bifurcations which change the quality of stability, and may compromise the operability of rotating systems, or increase noise level when amplitude of specific harmonics drastically increases. The paper identifies the dissipating work in the gas film to be the source of self-excited motions driving the rotor whirling close to bearing’s surface. The energy flow among the components of a rotor gas foil bearing system with unbalance is evaluated for various design sets of bump foil properties, rotor stiffness and unbalance magnitude. The paper presents a methodology to retain the dissipating work at positive values during the periodic limit cycle motions caused by unbalance. An optimization technique is embedded in the pseudo-arc length continuation of limit cycles, those evaluated (when exist) utilizing an orthogonal collocation method. The optimization scheme considers the bump foil stiffness and damping as the variables for which bifurcations do not appear in a certain speed range. It is found that secondary Hopf (Neimark–Sacker) bifurcations, which trigger large limit cycle motions, do not exist in the unbalanced rotors when bump foil properties follow the optimization pattern. Period-doubling (flip) bifurcations are possible to occur, without driving the rotor in high response amplitude. Different design sets of rotor stiffness and unbalance magnitude are investigated for the efficiency of the method to eliminate bifurcations. The quality of the optimization pattern allows optimization in real time, and gas foil bearing properties shift values during operation, eliminating bifurcations and allowing operation at higher speed margins. Compliant bump foil is found to enhance the stability of the system.

On the manufacturing of compliant foil bearings

The fabrication of foil bearings is challenging due to the dependence on sheet metal forming to produce the compliant structure. This paper is an attempt to shed light into the foil bearing manufacturing know-how. Design of experiments techniques are used to quantify the effects of the different manufacturing parameters, as well as defining an optimum manufacturing procedure. The effect of manufacturing noise on the static performance of foil bearings is quantified using a Monte Carlo simulation of a bump foil stiffness model. A non-intrusive optical measurement technique which has been developed to measure the formed bump foils is also presented. An uncertainty quantification was performed for the produced foils, showing large uncertainty in the bump dimensions, which significantly affect both nominal bearing clearance and compliance. Finite element simulations are used to model the bump foil forming process that would present potential problems during fabrication, suggesting sharp bends in the bump foil as the main driver for manufacturing deviations. Based on this outcome an improved design for the compliant bump foil with reduced curvature is proposed, manufactured and measured. The novel design allows to reduce springback error by 69% compared to classical bump foils and thus offers an equivalent yet more robust foil bearing design.

Enhanced effects from tiny flexible in-wall blips and shear flow

Fluid motion at high Reynolds number over a flexible in-wall blip (a compliant bump or dip in an otherwise fixed wall) is considered theoretically for a very short blip buried low inside a boundary layer. Only the near-wall shear of the oncoming flow affects the local motion past the tiny blip. Slowly evolving features are examined first to allow for variations in the incident flow. Linear and nonlinear solutions show that at certain parameter values (eigenvalues) intensifications occur in which the interactive effect on flow and blip shape is larger by an order of magnitude than at most parameter values. Similar findings apply to the boundary layer with several tiny blips present or to channel flows with blips of almost any length. These intensifications lead on to fully nonlinear unsteady motion as a second stage, after some delay, thus combining with finite-time breakups to form a distinct path into transition of the flow.

Investigation of Electrical Contact Mechanism for Anisotropic Conductive Adhesive Joints After the Thermocompression

Flexible interconnects are needed that meet assembly requirements for future applications in flexible consumer electronics products. This work investigates the electrical contact mechanism of ultrathin chip-on-flex (UTCOF) package using anisotropic conductive adhesive (ACA), which is highly flexible. In this paper, a 3-D nonlinear finite element (FE) model, which integrates analytical models of ACA joints and the thermal-mechanical behaviors of the UTCOF, is presented. The model is then applied to simulate the electrical contact mechanism for various ACA joints after thermocompression. Moreover, a “death-birth” meshing scheme is utilized to determine the effect of ACA resin temperature on contact resistance of the ACA joints. Multiple particle models are also generated using the ANSYS program. To validate the suitability of the proposed FE analytical model, contact resistance is measured to determine the bonding quality for 80-μm -pitch UTCOF test samples. The interfaces between the silicon chip and substrate for samples bonded under different bonding pressures are observed using cross-sectional scanning electron microscopy. The contact resistances obtained from the 3-D FE models were in good agreement with experimental results and can be used to predict the effects of thermocompression loading, pressure unloading, and cooling to room temperature on electrical contact mechanism in ACA joints after thermocompression. Generally, the gold bump and the compliant bump joined with multiple conductive particles reached stable contact resistance by capturing five to nine conductive particles. Contact resistance of an ACA-bonded UTCOF can be estimated using a proposed empirical model equation, particularly under loading conditions of L = 40%-60%. Overall, this work improves the design of flip chips for flexible electronics and the understanding of the electrical contact mechanism for ACA joints.

Simulation of Compliant Bump Foil Journal Bearing Using Coupled Reynolds Equation and Finite Element Model Method

Based on a “1st generation” compliant foil journal bearing, the performance of foil bearing was simulated according to the model base on Reynolds equation coupled the finite element method. Model considers the bending of top foil, elastic deformation of bump foil, the friction between top foil and bump foil and the interaction between bumps. After compared analysis results to test data, the result of simulation was more accurate than other models, thus this model establish a basis for further study of the compliant foil bearing.

Microjoining of LSI Chips on Poly(ethylene naphthalate) Using Compliant Bump

Microjoining of LSI Chips on Poly(ethylene naphthalate) Using Compliant Bump

Microjoining of LSI Chips on Poly(ethylene naphthalate) Using Compliant Bump

We show that microjoining of LSI chips on a film made of poly(ethylene naphthalate) (PEN), whose glass transition temperature is 155 °C, can be realized by using a cone-shaped compliant bump for flexible electronics. A 20-µm-pitch area array of cone-shaped Au bumps was fabricated on a Si wafer by photolithography and electroplating. The counter electrode on the PEN film was composed of a Au (top)/Ni/Al (bottom) layered structure, where Ni and Au layers were deposited by electroless plating on patterned Al. The bonding was carried out at 150 °C. More than 10,000 connections with 108 mΩ/bump were achieved. Mechanical analysis using finite element method indicates that the joining is due to deformation of the cone-shaped Au bumps.

Integration of Micro-Light-Emitting-Diode Arrays and Silicon Driver for Heterogeneous Optoelectronic Integrated Circuit Device

In this paper, we proposed the possibility of implementing a single chip device for realizing optoelectronic integrated circuits (OEICs). Micro-light-emitting-diode (LED) arrays and a complementary metal–oxide–semiconductor (CMOS) pulse width modulation (PWM) silicon driver were proposed, designed, and fabricated on a single chip. The micro-LED arrays were separated by a dry etching method into 64 pixels of 8×8, each with a size of 30×30 µm2 and operated in 3 V at 100 µA. The PWM Si driver was well operated and modulated using various control signals. Furthermore, to investigate the driver for handling massive parallel information, a simple multifunctional driver was designed, fabricated, and flip-chip-bonded using a gold compliant bump and anisotropic conductive adhesive with micro-LED arrays.

Characteristics of a Novel Compliant Bump for 3-D Stacking With High-Density Inter-Chip Connections

This paper reports on the detailed characteristics of a novel compliant bump for 3-D stacking with high-density inter-chip connections. The novel compliant bump is a cone-shaped bump made of Au. It is fabricated using electroplating of the Au into undercut holes formed in a photoresist. Because the cone bump is easily deformed under a pressing load, it possesses superior properties for inter-chip connection. First, it suppresses a bonding failure by compensating for the bump-height deviation and the nonuniform bonding pressure, and consequently, offers high-density inter-chip connections of which number is at least 30 600 with 20 μm pitch. Second, it reduces the change in the transconductance g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> of the metal-oxide-semiconductor field effect transistor (MOSFET) after chip stacking. In other words, it reduces the strain generation at the Si device level. Third, room-temperature bonding is achieved by mechanical caulking between the cone bumps and the doughnut-shaped electrodes.

Fabrication of Back-Side Illuminated Complementary Metal Oxide Semiconductor Image Sensor Using Compliant Bump

We fabricated a back-side illuminated (BSI) complementary metal oxide semiconductor (CMOS) image sensor in which a very-thin BSI photodiode array chip was stacked on a CMOS read-out circuit chip by compliant bumps. Cone-shaped bumps made of Au were prepared as the compliant bumps. The base diameter was 10–12 µm and the height was 9–10 µm. To fabricate the BSI CMOS image sensor, we developed a novel thin-chip assembly process. The key features of the process are as follows: preparation of a photodiode array wafer and a CMOS read-out circuit wafer, Au cone bump formation, bonding to support glass, thinning of the photodiode array wafer to 21 µm, through silicon via (TSV) formation using Cu electroplating, formation of back-side electrodes, transfer of the photodiode array wafer to a polymer support tape, dicing of the photodiode array wafer, separation of support tape, formation of Ni–Au bumps, dicing of CMOS read-out circuit wafer, and three-dimensional (3D) chip-stacking. The BSI CMOS image sensor thus fabricated has the following specifications: number of active pixels is 16,384 (128 ×128), photodiode size is approximately 18 µm square, photodiode pitch is 24 µm, and fill factor is approximately 55%. No defects were observed in the obtained image frames.

Advancements in the Structural Stiffness and Damping of a Large Compliant Foil Journal Bearing: An Experimental Study

This paper presents the results of an experimental investigation into the dynamic structural stiffness and damping characteristics of a 21.6‐cm(8.5in.)-diameter compliant surface foil journal bearing. The goal of this development was to achieve high levels of damping without the use of oil, as is used in squeeze film dampers, while maintaining a nearly constant dynamic stiffness over a range of frequencies and amplitudes of motion. In the experimental work described herein, a full compliant foil bearing was designed, fabricated, and tested. The test facility included a non-rotating journal located inside the bearing. The journal was connected to an electrodynamic shaker so that dynamic forces simulating expected operating conditions could be applied to the structurally compliant bump foil elements. Excitation test frequencies to a maximum of 400Hz at amplitudes of motion between 25.4 and 102μm were applied to the damper assembly. During testing, both compressive preload and unidirectional static loads of up to 1335 and 445N, respectively, were applied to the damper assembly. The experimental data from these tests were analyzed using both a single degree of freedom model and an energy method. These methods of data analysis are reviewed here and results are compared. Excellent agreement in results obtained from the two methods was achieved. Equivalent viscous damping coefficients as high as 1050N.s∕cm(600lbf.s∕in) were obtained at low frequencies. Dynamic stiffness was shown to be fairly constant with frequency.

On the Frictional Damping Characterization of Compliant Bump Foils

High-speed rotor systems use either fluid film or rolling element bearing supports, depending upon their design and operating constraints. Regardless of bearing type used, these systems require specific bearing and support stiffness and damping characteristics to achieve the desired stable and low vibration operation. Building upon the technology of thin metallic corrugated bump foils presently used in a particular class of film riding hydrodynamic bearings, a novel corrugated bump foil damped mount is introduced which provides stiffness and damping for application with rolling element bearings. These damping elements are capable of operating at elevated temperatures where implementation of conventional squeeze film dampers is ruled out. The frictional damping results from micro-slip motions between the bump foils and the mating surfaces. A semi-empirical model, based on a one degree of freedom model was developed in which damping is replaced by an equivalent frictional force in order to gain insight into the dynamic friction coefficient of the individual damping element interfaces. Experimental results, obtained in the form of hysteresis loops were compared to the developed model with good agreement. The variation in damping and dynamic coefficient of friction was found to be dependent primarily upon three factors: vibration frequency, amplitude of motion and applied static load. These parameters were tested within the range of 50–1400 Hz, 2.54–12.7 micron and 45–135 N, respectively. The tests were conducted at room and 538°C ambient temperatures under both dry and vapor phase lubricated conditions. Using the resulting empirical data, several bearing dampers were designed, built and tested in a small, high-speed gas turbine engine simulator. The tested novel foil dampers were capable of operating reliably under extremely high levels of shaft imbalance (i.e., 320 times greater than the air bearing supported with specification of 0.0002 oz-in) even while operating at temperatures to 560°C. These results show the great potential for wide application of these dampers on gas turbine engines and high-speed rotating machinery.

Compliant bumps for adhesive flip-chip assembly

Flip-chip-on-glass (FCOG) is susceptible to electrical opens for a variety of reasons including, but not limited to, movement in the Z-axis caused by flip-chip, adhesive CTE and water absorption of the adhesive. Flip-chip assembly to co-fired ceramic and laminate substrates suffers from these problems as well as others, such as bow or twist in the substrate and bond pad height irregularities. Success with adhesive flip-chip connections to these substrates has, to date, been limited. Commercially available adhesives have either failed to produce reliable bonds, or have suffered from long cure time or a lack of reworkability. A solution to these problems has been demonstrated by forming compliant bumps on the chip or substrate bond pads using a photo-imagable polymer coated with a thin layer of gold. Bumps 17 /spl mu/m tall with diameters between 17 /spl mu/m and 95 /spl mu/m have been fabricated and bonded. The resulting compliant bump structure provides 30% of the bump height (5 /spl mu/m) within the elastic compression range. This compliance eliminates many of the demands placed on the assembly adhesives by other electrical contacting methods (such as solid metal bumps or particles). Compliant bumps allow the use of commercially available, fast curing, easily reworkable adhesives for reliable flip-chip assembly. MCC's compliant bumps have been mechanically cycled from minimum compression (needed for electrical contact) to maximum compression (based on diminishing compression distance versus applied force) 1000 times, with minimal degradation of the polymer core or metal overcoat. Assemblies have been subjected to temperature cycling and steam pot aging with substantial improvement in reliability when compared to assemblies using solid metal bumps. Using compliant bump technology, low temperature rework has been demonstrated with compliant bumped chips on glass, laminate and MCM-C substrates. Chips or substrates with compliant bumps are re-usable, a significant advantage over conventional gold bump processes where the bump structure is permanently deformed by the bonding process.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>