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Blacksburg, VA, United States

Agency: National Science Foundation | Branch: | Program: STTR | Phase: Phase I | Award Amount: 147.39K | Year: 2008

This Small Business Technology Transfer (STTR) Phase I project will investigate the feasibility of employing novel silver pastes for joining power semiconductor devices to achieve 5 times higher temperature cycling capability, 3 times better total module resistance, and device junction temperature over 175 degrees Celsius. A sintering technology for joining semiconductor chips, now being implemented in manufacturing lines of some major European companies, requires a 120-ton press to lower the sintering temperature of silver powders. This significantly complicates the manufacturing process and places critical demands on substrate flatness and thickness of the chips. This project uses materials that can be sintered below 270 degrees Celsius under ambient pressure and have 5 times better thermal and electrical properties than widely used solder alloys, thus will have great commercial potential to improve the electronic assembly process and products. The broader impacts/commercial potential is to the electronics manufacturers in the United States by providing a low-cost manufacturing process for low-temperature sintering technology for joining devices.

Kim W.,Virginia Polytechnic Institute and State University | Luo S.,NBE Technologies, LLC | Lu G.-Q.,Virginia Polytechnic Institute and State University | Ngo K.D.T.,Virginia Polytechnic Institute and State University
Conference Proceedings - IEEE Applied Power Electronics Conference and Exposition - APEC | Year: 2013

A planar power module was developed, and a gate-driver circuit with an over-current protection was planned to integrate into the module. After reviewing several current-sensing methods, the giant-magneto-resistive (GMR) sensor was chosen as a current-sensing method. However, there were several factors that hindered accurate measurement. The high junction temperature of the power dice gave high influence to the operating temperature of the GMR sensor, and the magnetic-flux distribution seen by the GMR sensor was also non-uniform due to skin effect. The temperature response of the GMR sensor was analyzed by experiments, and the GMR sensor showed about 3.45% errors when it sensed 80 Adc and the operating temperature changed by 60°C. To further improve the measurement capability over wide range of operating temperature, an active temperature-compensation method is described. The optimal position of the GMR sensor was found based on FEA simulation as the midpoint of two current paths. At that location, the GMR sensor could consistently sense both current excitations. A test module was fabricated, and preliminary measurement result showed excessive noise that had to be filtered out for accurate measurement. A signal-conditioning circuit was designed using an instrumentation amplifier, and the current measurement between the GMR sensor and a high-bandwidth current probe showed consistent result. The current sensor with signal-conditioning circuit was integrated into the gate-driver circuit, and the concept was verified by experiments. © 2013 IEEE.

Lei T.G.,Virginia Polytechnic Institute and State University | Calata J.N.,Virginia Polytechnic Institute and State University | Lu G.-Q.,Virginia Polytechnic Institute and State University | Chen X.,Tianjin University | Luo S.,NBE Technologies, LLC
IEEE Transactions on Components and Packaging Technologies | Year: 2010

A low-temperature sintering technique enabled by a nanoscale silver paste has been developed for attaching large-area (>100 mm2) semiconductor chips. This development addresses the need of power device or module manufacturers who face the challenge of replacing lead-based or lead-free solders for high-temperature applications. The solder-reflow technique for attaching large chips in power electronics poses serious concern on reliability at higher junction temperatures above 125°C. Unlike the soldering process that relies on melting and solidification of solder alloys, the low-temperature sintering technique forms the joints by solid-state atomic diffusion at processing temperatures below 275°C, with the sintered joints having the melting temperature of silver at 961°C. Recently, we showed that a nanoscale silver paste could be used to bond small chips at temperatures similar to soldering temperatures without any externally applied pressure. In this paper, we extend the use of the nanomaterial to attach large chips by introducing a low pressure up to 5 MPa during the densification stage. Attachment of large chips to substrates with silver, gold, and copper metallization is demonstrated. Analyses of the sintered joints by scanning acoustic imaging and electron microscopy showed that the attachment layer had a uniform microstructure with micrometer-sized porosity with the potential for high reliability under high-temperature applications. © 2006 IEEE.

Mei Y.,Tianjin University | Mei Y.,Virginia Polytechnic Institute and State University | Lu G.-Q.,Virginia Polytechnic Institute and State University | Chen X.,Tianjin University | And 2 more authors.
IEEE Transactions on Device and Materials Reliability | Year: 2011

The low-temperature joining of semiconductor chips by sintering of silver paste is emerging as an alternative lead-free solution for power electronics devices and modules working in a high-temperature environment. A promising die-attachment material that would enable the rapid implementation of the sintering process is nanoscale silver paste, which can be sintered at temperatures below 300 °C without an external pressure. In this paper, we report our findings on the silver migration in sintered nanosilver electrode-pair patterns on an alumina substrate. The electrode pairs were biased at an electric field ranging from 10 to 100 V/mm and at a temperature between 250 C° and 400 °C in dry air. The leakage currents across the electrodes were measured as the silver patterns were tested in an oven. Silver dendrites formed across the electrode gap were observed under an optical microscope and analyzed using scanning electron microscopy and energy dispersive spectroscopy (EDS). The silver migration was found in the samples tested at 400 °C, 350 °C 300 °C, and 250 C°. The measurements on the leakage current versus time were characterized by an initial incubation period, called lifetime, followed by a sharp rise as the silver dendrites were shorting the electrodes. A simple phenomenological model was derived to account for the observed dependence of lifetime on the electric field and temperature. The EDS mappings revealed the significant presence of oxygen on the positive electrode but the complete absence on the negative electrode. A mechanism involving the oxidation of silver and the dissociation of silver oxide at the anode was suggested. We suggest that the migration of a sintered nanosilver die attachment can be prevented in high-temperature applications through packaging or encapsulation to reduce the partial pressure of oxygen. © 2011 IEEE.

Agency: NSF | Branch: Standard Grant | Program: | Phase: | Award Amount: 150.00K | Year: 2013

This Small Business Innovative Research (SBIR) Phase I project is aimed at demonstrating the feasibility of an electronic packaging technology for manufacturing power electronics modules that are critical for electrical energy processing in a wide range of systems, such as hybrid or electric vehicles, renewable energy generators, and the power grid. Recent advances in power semiconductor devices and substrate technology require packaging schemes which optimize the performance of each component for further increases in reliability, density, and high-temperature performance. The best route for meeting this need is to explore three dimensional package architectures which have previously been a barrier for manufacturing using solder techniques. This project will build on the commercialization success of a nanomaterial technology for device interconnection, to develop and implement an innovative three dimensional package architecture which can be force cooled equally well from both sides. The nanomaterial, which already boasts significant increases in thermal and electrical conductivity, is known to provide high reliability and high temperature joints for device interconnection. In addition, processing requirements can be tailored to significantly simplify fabrication of architectures which are difficult to create using existing solder and epoxy connection schemes. Utilizing the processing benefits of the nanomaterial die attachment, the specific technical objectives are: (1) development of a manufacturable process with the nanomaterial for fabricating the planar power modules; (2) testing of the modules under applied continuous current; and (3) evaluation of the module reliability under temperature/power cycling tests and (4) characterization of failure mechanisms. The double-side cooled planar power module technology enabled by the nanomaterial would lead to a highly competitive product in the market place.

The broader impact/commercial potential of this project would strengthen United States? manufacturing base in the field of power electronics. Power electronics modules are the central processing units for electrical energy conversion and are crucial to the nation?s economy and security. Energy applications, specifically those that provide independence from petroleum, require more efficient conversion of electrical power, and demand for reliability and sustainability of the nation?s power infrastructure requires an increasingly greater number of electrical conversions. Currently, the market of power electronics modules is dominated by products made in Europe and Asia. Successful commercialization of the technology developed in this project would usher in a competitive US manufacturer of power modules to the growing power electronics market. The success would further strengthen commercialization effort of the nanomaterial product developed under a NSF STTR program and directly translate to economic growth for Southwest Virginia. Success of this program would also serve as a good educational and business model for transferring fundamental knowledge developed under NSF?s support into the commercial world. It would present students an ideal case study to experience technological and economical impacts of their research activities.

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