Browsing by Author "Mei, Yunhui"

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    Characterizations of Rapid Sintered Nanosilver Joint for Attaching Power Chips
    Feng, Shuang-Tao; Mei, Yunhui; Chen, Gang; Li, Xin; Lu, Guo-Quan (MDPI, 2016-07-12)
    Sintering of nanosilver paste has been extensively studied as a lead-free die-attach solution for bonding semiconductor power chips, such as the power insulated gated bipolar transistor (IGBT). However, for the traditional method of bonding IGBT chips, an external pressure of a few MPa is reported necessary for the sintering time of ~1 h. In order to shorten the processing duration time, we developed a rapid way to sinter nanosilver paste for bonding IGBT chips in less than 5 min using pulsed current. In this way, we firstly dried as-printed paste at about 100 °C to get rid of many volatile solvents because they may result in defects or voids during the out-gassing from the paste. Then, the pre-dried paste was further heated by pulse current ranging from 1.2 kA to 2.4 kA for several seconds. The whole procedure was less than 3 min and did not require any gas protection. We could obtain robust sintered joint with shear strength of 30–35 MPa for bonding 1200-V, 25-A IGBT and superior thermal properties. Static and dynamic electrical performance of the as-bonded IGBT assemblies was also characterized to verify the feasibility of this rapid sintering method. The results indicate that the electrical performance is comparable or even partially better than that of commercial IGBT modules. The microstructure evolution of the rapid sintered joints was also studied by scanning electron microscopy (SEM). This work may benefit the wide usage of nanosilver paste for rapid bonding IGBT chips in the future.
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    A (Permalloy + NiZn Ferrite) Moldable Magnetic Composite for Heterogeneous Integration of Power Electronics
    Ding, Chao; Mei, Yunhui; Ngo, Khai D. T.; Lu, Guo-Quan (MDPI, 2019-06-22)
    Soft magnetic moldable composites (SM2Cs) would be ideally suited for the integration of magnetic components in power electronic converters because they can be formed into magnetic cores by low-temperature and pressure-less processing. However, most SM2Cs have low relative magnetic permeability, typically less than 30, and high core-loss densities at switching frequencies over 1 MHz. To improve their magnetic properties, we combine powders of Permalloy and a NiZn ferrite with an acrylic polymer to formulate a paste of SM2C. The paste can be molded and then cured below 200 °C without pressure to form cores with a relative permeability over 35 and a core-loss density at 1 MHz, 30% lower than those of commercial cores. The ease of its processing and high-performance properties makes the SM2C a good candidate material for the integration of power magnetics.
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    Tailoring a Silver Paste for Additive Manufacturing of Co-Fired Ferrite Magnetic Components
    Liu, Lanbing; Ding, Chao; Mei, Yunhui; Lu, Guo-Quan (MDPI, 2019-03-11)
    Additive manufacturing (AM), or 3D-printing, has the potential for rapid prototyping of innovative designs of magnetic components used in power electronics converters. In this study, we tailored a silver paste as the metal feedstock of an extrusion 3D printer so that the metal would be compatible with a ferrite paste feedstock for 3D-printing of ferrite magnetic components. We focused on adjusting the metal formulation to match its shrinkage to that of the ferrite and to improve adhesion during the co-sintering process of the printed part. We found that a 5 wt % addition of ferrite powder in the metal paste can achieve matched shrinkage and strong adhesion. Evaluation of the co-sintered magnetic components showed no significant defects, such as cracks, warpage, or delamination, between the metal and ferrite. The shear strength between the two sintered materials was greater than 50 MPa, and the electrical resistivity of the sintered metal winding was less than twice that of the bulk silver, which is lower than those of most 3D-printed winding metals reported in the literature.