The Comparison of Form Drag and Profile Drag of a Wind Turbine Blade Section in Pitching
Extensive wind tunnel tests have been conducted to investigate the unsteady flow field over and behind a 2D model of a 660 kW wind turbine blade section in pitching motion. The surface pressure and wake dynamic pressure variation at a distance of 1.5 chord length from trailing edge were measured by pressure transducers during several oscillating cycles at 3 reduced frequencies and oscillating amplitudes. Moreover, form drag and linear momentum deficit are extracted and compared at various conditions. The results show that the wake velocity field and surface pressure of the model have similar behavior before and after the airfoil beyond the static stall angle of attack. In addition, the effects of reduced frequency and oscillation amplitudes are discussed. Keywords—Pitching motion, form drag, Profile drag, wind turbine.
Unsteady wake measurements of an oscillating flap at transonic speeds
Mesure en soufflerie des profils de sillage stationnaire ou instationnaire d'un profil aerodynamique avec volet oscillant. Mesure des profils de pression et de vitesse en 4 points axiaux. Etude de l'influence de l'angle du volet
How Plasma-Enhanced Surface Modification Improves the Production of Microelectronics and Optoelectronics
Over the past 30 years, plasma, the fourth state of matter, has become a very useful method for surface modification and deposition of various materials. In IC packaging applications, plasma is employed to prepare surfaces for die attach, wirebonding and mold/encapsulation. Moreover, plasma-enhanced contamination removal and surface activation processes improve the reliability and yield, and also enhance the manufacture of advanced technology products. In many optoelectronic devices, plasma-enhanced contamination removal is used to prepare surfaces prior to eutectic die attach and wirebonding. This article examines examples of plasma surface modification in both the microelectronic and optoelectronic industries. S By Dr. James D. Getty, March Plasma Systems, Concord, Calif. This single strip, compact plasma chamber processes BGA-type substrates with argon plasma. Reprinted from the January-February issue of Chip Scale Review. Used by permission; all rights reserved. ©2002 Chip Scale Review. 72 T e c h n i c a l F o r u m chemical energy to remove micron-level contamination. This process employs ablation, where the positive ions bombard the surface. The ablation process can dislodge contamination from the surface, and can roughen the surface on an atomic scale, as revealed by atomic force microscopy.2 The chemical process is widely employed to remove residual materials, typically less than a few microns, such as organic films, and oxidation. The chemical process employs either reduction or oxidation chemistry via the gas-phase radicals. Specific contamination issues in microelectronic and optoelectronic package reliability are poor wirebond pull strength and voiding, due to insufficient solder reflow in eutectic die attach. Wirebond pad contamination can be a by-product from previous processing steps such as die attach epoxy bleed or environmental exposure (i.e., bond pad metal oxidation). A physical, chemical or combined physical-chemical process using argon and oxygen source gases can be employed to prepare the bond pads. An oxygen-based plasma will take advantage of the oxygen radicals to chemically react with the epoxy, producing volatile gas-phase by-products that can be pumped from the vacuum chamber. The success of an oxygen plasma for removal of die bond epoxy bleed has been widely demonstrated.3 In cases where oxidation is of concern, a physical process can be employed to prepare the bond pad surfaces. An argon plasma treatment of PBGA strips has been shown to improve the wirebond pull strength by as much as 24.3 percent.4 Metal oxidation can act as a physical barrier for both wirebonding and solder reflow. A combined physical and chemical process using argon and hydrogen can reduce the metal oxides. For example, reduction of copper oxide to copper is achieved in a hydrogen plasma via the reaction of the hydrogen radicals with the metal oxide. CuO + 2H• → Cu + H2O Even in the absence of a contamination source, ablation will roughen the surface and provide a larger surface area for wirebonding, resulting in improved wirebond uniformity bond to bond.4 Surface Activation Plasma surface activation employs gases, such as oxygen, nitrogen, hydrogen, and ammonia, which, when exposed to the plasma, will dissociate and react with the surface, creating different chemical functional groups on the surface. The different functional groups modify the chemical activity of the surface. The new functional groups have strong chemical bonds with the bulk material and have the capability to further bond with adhesives to promote better adhesion. The functional groups also increase the surface area available for the adhesive and thus will distribute the load over a larger area resulting in improved adhesive strength. Gas selection and surface type determine the functional group that will be substituted on the surface.4 In microelectronic applications, plasma surface activation prior to die attach provides better contact, improved heat transfer and minimal voiding. The purpose of the mold/encapsulant material for semiconductor applications is to provide adequate mechanical strength, adhesion to various package components, good corrosion and chemical resistance, matched CTE to the materials it interfaces with, high thermal conductivity and high moisture resistance in the temperature range used. Plasma Surface Modification Advanced Technology Source Gas Processes Application Argon (Ar) Contamination Removal–Ablation Wirebond Die Attach Cross Linking Substrate Polymer–Metal Adhesion Oxygen (O2) Contamination Removal–Chemical Wirebond Oxidation Process (Organic Removal) Die Attach Surface Activation Mold and Encapsulant Adhesion Etch Photoresist Removal Nitrogen (N2) Surface Activation Mold and Encapsulant Adhesion Hydrogen (H2) Contamination Removal–Chemical Wirebond Reduction Process (Metal Oxide Eutectic Die Attach Removal) Carbon Tetrafluoride Etch Polymer Etch–Fiber Stripping (CF4) and Oxygen (O2) Photoresist Removal or Sulfur Hexafluoride Thin Film Etch–Oxides, Nitrides (SF6) and Oxygen (O2) Process Applications for Plasma Surface Enhancements Figure 1. This automated, in-line plasma tool is designed for surface modification processes. The upstream and downstream transfer mechanisms and the compact high-density plasma chamber are contained within a single enclosure. Reprinted from the January-February issue of Chip Scale Review. Used by permission; all rights reserved. ©2002 Chip Scale Review. 73 The ability to form good adhesion with package components and to remain bonded is of paramount importance, since delamination along the interfaces is a major reliability issue for plastic-encapsulated microcircuits. Plasma treatment has been demonstrated to improve the bond strength at the plastic encapsulant, gold-plated copper leadframe interface via an enhanced chemical compatibility with the molding compound.2 Initial studies have also indicated that plasma treatment of nickel surfaces with water-based plasma improves the adhesion of the mold compound to the nickel surface. Etch Plasma etch is characterized by the chemical reactivity of the discharge. The etching process utilizes source gases that dissociate within the plasma, creating a mixture of highly reactive species. The advantage of this chemical plasma is its chemical selectivity. The process chemistry can be optimized so that one material can be selectively etched in the presence of other materials. For example, the dissociation of carbon tetrafluoride (CF4) and oxygen in the appropriate concentrations produces highly reactive oxy, oxyfluoro, and fluoro radicals that rapidly break carbon-carbon bonds within numerous materials. The reaction at the solid interface produces volatile by-products, which are pumped from the vacuum system. Plasma etch has many applications specific to semiconductor and optoelectronic processing, including photoresist removal, thin film etch, and polymer etch. In optoelectronic manufacturing, plasma etch has been employed to produce stripped fibers through the controlled removal of the urethane acrylate buffer coating. Conventional optic fibers are composed of a cylindrical core covered by a cladding material and a buffer material that encases the cladding. The core is the light-carrying element, and the cladding promotes the total internal reflection in the fiber. Stripping the buffer is required for various applications, including hermetic sealing, pigtailing of laser diodes, fiber arrays, fiber Bragg gratings, and amplifier seeding. Fiber Bragg gratings, for example, are widely used in the fabrication of devices for dense wavelength division multiplexing (DWDM). Figure 2 displays a fiber with the buffer material removed and the glass cladding and core exposed. A critical requirement in the removal of the fiber buffer is to completely remove the urethane acrylate polymer while maintaining the intrinsic T e c h n i c a l F o r u m Figure 2. Illustration displays a fiber with the buffer material removed, and the glass cladding and core exposed. Gas phase plasma is an electrically neutral mixture of electrons, ions, radicals, photons, recombination products and neutrals created by the application of energy, such as radio frequency (RF), to a source gas contained within a vacuum chamber. (The figure summarizes the active species that are present in an oxygen plasma.) Free electrons initiate the process; exposure of the free electrons to the external energy source allows the electrons to gain sufficient kinetic energy, so that a collision with another atom or molecule will result in the formation of ions and radicals. The reactive radical species are capable of chemical work where the ionized atom and molecular species are capable of physical work through sputtering. Photon emission within a plasma is a result of the excited neutrals, ions and free radicals formed in the plasma losing their excess energy. The wavelength of the emission is sufficient to break chemical bonds, and can be useful when treating polymeric materials. Though non-equilibrium plasma has elect ron energies in excess of 10,000°C, critical for sustaining the plasma through atomic and molecular ionization and dissociation processes, it maintains a gas stream temperature that is less than 100°C. The low process temperature is important to products that are temperature sensitive.—JG The Fourth State of Matter This illustration summarizes the active species that are present in an oxygen plasma. PLASMA Free Radical (highly reactive)