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Ag sinter-joining technology has become one of the leading candidates to replace conventional high-Pb soldering technology for wide bandgap (WBG) power modules. In many cases of die-attach applications, Ni/Au plating is an inevitable choice for both die-back faces and substrates. In this study, a preheating treatment of the Ni/Au plating was utilized to increase the bond strength of the Ag sinter-joined structure of Cu-to-Cu to the electroless Ni/Au plating. Our research determined that the microstructure of the Ni/Au plating can be modified by performing an optimized preheating treatment. The thickness of the Ni underlayer was fixed at 5 μm, and the Au was plated with two different thicknesses of 0.1 μm and 0.3 μm. Ni/Au plated Cu substrates and Cu chips were preheated at various temperatures from 150°C to 350°C and then bonded with hybrid Ag particles at a low temperature of 250°C with a pressureless sintering process. Without the preheating treatment, average shear strength was 14.2 MPa (as plated Au), however, after a 1 h preheating treatment at 250°C, the interface microstructure change resulted in a 26.3 MPa shear strength, an increase of about 85%. The same improvement effect was also confirmed for the Au plating layer with a thickness of 0.3 μm. Energy dispersive x-ray spectrometry (EDS) and x-ray diffraction (XRD) analysis indicated that the preheating treatment influences the grain structure of the Au plating layer and removes defects, resulting in an increase of bond strength for the Au plated joint structure. [ABSTRACT FROM AUTHOR]

This study reports a novel high-temperature and high-current wire interconnection by printing Ag sinter paste for WBG semiconductor power devices. The Ag sinter paste wire interconnection (APW) was successfully fabricated connecting two Cu electrodes coated by Ag. Both the high-temperature reliability and the high-current reliability of the APW were evaluated by aging test at 250 °C for 1000 h and by electro-migration (EM) test at 2.2 × 104 A/cm2 for 2000 h. In the aging, a resistivity of about 4.1 × 10−6 Ω cm was achieved, which further decreased to 3.4 × 10−6 Ω cm after aging for 1000 h. This reduction can be attributed to further sintering of the sintered Ag network. The initial shear strength was 17 MPa, which, while high enough, was maintained even after an aging test of 1000 h. In the EM test, the resistance of APW was not changed significantly up to 1000 h and then only slightly increased at 2000 h. This only slight change in resistance demonstrates greater stability than that of conventional wire interconnection material such as Al and Cu under high temperature and high current density. The influence of EM on the interface between APW and a Cu substrate was observed by scanning electron microscopy with EDX. The change of resistance of the APW was induced by the formation of Cu oxide at a cathode side, which leads to a crack extension along the interface between APW and a Cu substrate. Thus, the results obtained in the present work show the APW as a promising new alternative to wire bonding technology for WBG power electronic devices. [ABSTRACT FROM AUTHOR]

Some kinds of thermofluid instruments are using Helmholtz resonators for absorbing noises radiated from them. Since the resonance frequency of a Helmholtz resonator depends on the ratio of the resonant volume to the neck size, operating frequency range can be easily tuned and thus the Helmholtz resonator is well-suited for noise control in a wide range of frequency. Besides the noise control application, Helmholtz resonators have been recently applied to acoustic energy harvesting. To date, most of studies on energy harvesting using Helmholtz resonators are employing piezoelectric materials located in the resonant volume and enhancing harvesting efficiency by tuning the resonance frequency and the structure of the added piezoelectric materials. In this study, a piezoelectric (PVDF composite) cantilever was integrated within a Helmholtz resonator. Then, energy harvesting efficiency was maximized by matching and tuning mechanical resonance of the piezoelectric cantilever and acoustic resonance of the Helmholtz resonator. Theoretical analysis including finite element simulation was also performed to confirm our experimental results. To maximize the harvest power, the electrical impedance matching was considered. This study can be applied to acoustic energy harvesting as well as acoustic noise control. [ABSTRACT FROM AUTHOR]