Magnetic components such as inductors and transformers are generally the largest circuit elements in switch-mode power systems for controlling and processing electrical energy. To meet the demands of higher conversion efficiency and power density, there is a growing need to simplify the process of fabricating magnetics for better integration with other power electronics components. The potential benefits of additive manufacturing (AM), or more commonly known as three-dimensional (3D) printing technologies, include shorter lead times, mass customization, reduced parts count, more complex shapes, less material waste, and lower life-cycle energy usage—all of which are needed for manufacturing power magnetics. In this work, an AM technology for fabricating and integrating magnetic components, including the design of manufacturing methodology and the development of the feedstock material, was investigated. A process flow chart of additive manufacturing functional multi-material parts was developed and applied for the fabrication of magnetic components. One of the barriers preventing the application of 3D-printing in power magnetics manufacturing is the lack of compatible and efficient magnetic materials for the printer's feedstock. In this work, several magnetic-filled-benzocyclobutene (BCB) pastes curable below 250 degree C were formulated for a commercial multi-material extrusion-based 3D-printer to form the core part. Two magnetic fillers were used: round-shaped particles of permalloy, and flake-shaped particles of Metglas 2750M. To guide the formulation, 3D finite-element models of the composite, consisting of periodic unit cells of magnetic particles and flakes in the polymer-matrix, was constructed. Ansoft Maxwell was used to simulate magnetic properties of the composite. Based on the simulation results, the pastes consisted of 10 wt% of BCB and 90 wt% of magnetic fillers—the latter containing varying amounts of Metglas from 0 to 12.5 wt%. All the pastes displayed shear thinning behavior and were shown to be compatible with the AM platform. However, the viscoelastic behavior of the pastes did not exhibit solid-like behavior, instead requiring layer-by-layer drying to form a thick structure during printing. The key properties of the cured magnetic pastes were characterized. For example, bulk DC electrical resistivity approached 107 Ω⋅cm, and the relative permeability increased with Metglas addition, reaching a value of 26 at 12.5 wt%. However, the core loss data at 1 MHz and 5 MHz showed that the addition of Metglas flakes also increased core loss density. To demonstrate the feasibility of fabricating magnetic components via 3D-printing, several inductors of differing structural complexities (planar, toroid, and constant-flux inductors) were designed. An AM process for fabricating magnetic components by using as-prepared magnetic paste and a commercial nanosilver paste was developed and optimized. The properties of as-fabricated magnetic components, including inductance and DC winding resistance, were characterized to prove the feasibility of fabricating magnetic components via 3D-printing. The microstructures of the 3D-printed magnetic components were characterized by Scanning-electron-microscope (SEM). Results indicate that both the winding and core magnetic properties could be improved by adjusting the formulation and flow characteristics of the feed paste, by fine-tuning printer parameters (e.g., motor speed, extrusion rate, and nozzle size), and by updating the curing profile in the post-process. The main contributions of this study are listed below: 1. Developed a process flow chart for additive manufacturing of functional multi-material components. This methodology can be used as a general reference in any other research area targeting the utilization of AM technology. 2. Designed, formulated and characterized low-temperature curable magnetic pastes. The pastes are physically compatible with the additive manufacturing platform and have applications in the area of power electronics integration. 3. Provided an enhanced understanding of the core-loss mechanisms of soft magnetic materials and soft magnetic composites at high frequency applications. / Ph. D.

As the first level interconnection in electronic packages, chip attachment plays a key role in the total packaging process. Sintered nanosilver paste may be used as a lead-free alternative to solder for die-attachment at sintering temperature below 300 °C without applying any pressure. Typically, the substrate, such as direct bond copper (DBC) substrates, has surface metallization such as silver or gold to protect the copper surface from oxidation during the sintering process. This study focused on developing techniques for die-attachment on pure copper surface by low-temperature sintering of nanosilver paste. One of the difficulties lies in the need for oxygen to burn off the organics in the paste during sintering. However, the copper surface would oxidize, preventing the formation of a strong bond between sintered silver and copper substrate. Two approaches were investigated to develop a feasible technique for attachment. The first approach was to reduce air pressure as a means of varying the oxygen partial pressure and the second approach was to introduce inert gas to control the sintering atmosphere. For the first method, die-shear tests showed that increasing the oxygen partial pressure (PO₂ from 0.04 atm to 0.14 atm caused the bonding strength to increase but eventually decline at higher partial pressure. Scanning electron microscopy (SEM) imaging and energy dispersive spectroscopy (EDS) analysis showed that there was insufficient oxygen for complete organics burnout at low PO₂ condition, while the copper surface was heavily oxidized at high PO₂ levels, thus preventing strong bonding. A maximum bonding strength of about average 8 MPa was attained at about PO₂ = 0.08 atm. With the second method, the die-shear strength showed a significant increase to about 24 MPa by adjusting the oxygen exposure temperature and time during sintering. The processing conditions necessary for bonding large-area chips (6 mm à 6 mm) directly on pure copper surface by sintering nanosilver paste was also investigated. A double-print process with an applied sintering pressure of less than 5 MPa was developed. Die-shear test of the attached chips showed an average bonding strength of over 40 MPa at applied pressure of 3 MPa and over 77 MPa under 12 MPa sintering pressure. SEM imaging of the failure surface showed a much denser microstructure of sintered silver layer when pressure was applied. X-ray imaging showed a bond layer almost free of voids. Because the samples were sintered in air, the DBC surface showed some oxidation. Wirebondability test of the oxidized surface was performed with 250 μm-diameter aluminum wires wedge-bonded at different locations on the oxidized surface. Pull test results of the bonded wires showed a minimum pull-strength of 400 gram-force, exceeding the minimum of 100-gf required by the IPC-TM-650 test standard. / Master of Science