Kazuhiko Sugiura

Mechanical Deformation of Sintered Porous Ag Die Attach at High Temperature and Its Size Effect for Wide-Bandgap Power Device Design

The mechanical properties of sintered Ag paste with microporous structure have been investigated by tensile and shear tests, focusing on the temperature-dependent plastic deformation at various temperatures from 25°C to 300°C, corresponding to the target operating temperature range of emerging wide-bandgap semiconductor devices. Specimens were prepared by sintering hybrid Ag paste consisting of microflake and submicron spherical Ag particles, simulating a typical bonding process for power semiconductor die attach. Mechanical tests revealed that the unique microstructure caused a brittle-to-ductile transition at temperature of around 160°C, remarkably lower than that of bulk Ag. The obtained Young’s modulus and shear modulus values indicate obvious softening with increasing temperature, together with a remarkable decrease in Poisson’s ratio. These plastic behaviors at elevated temperature can be explained based on Coble creep in the microporous network structure. Fracture surfaces after tensile and shear tests indicated unique features on scanning electron microscopy, reflecting the variation in the ductile behavior with the test temperature. Furthermore, these temperature-dependent mechanical parameters were employed in three-dimensional finite-element analysis of the thermomechanical stress distribution in wide-bandgap semiconductor module structures including Ag paste die attach of different sizes. Detailed thermal stress analysis enabled precise evaluation of the packaging design for wide-bandgap semiconductor modules for use in high-temperature applications.

6-in-1 Silicon carbide power module for high performance of power electronics systems

The excellent characteristics (low power loss, high speed/high temperature operation) of SiC semiconductors can contribute to realizing smaller power converter with a higher power output. Using our own packaging technology of double-sided cooling and SiC devices, we have developed the new, small 6-in-1 power module with high output power density. If the inductance of the main circuit is large, it will cause a large surge voltage when switched. Therefore, we have incorporated the optimum low inductance structure into this module, and have made it possible to drive the SiC device at high speed. Using this module, we have built a prototype of inverter unit with 75 kW output power, and have achieved an efficiency of 99% and a power density of 100 kW/L.

Self-healing of cracks in Ag joining layer for die-attachment in power devices

Sintered silver (Ag) joining has attracted significant interest in power devices modules for its ability to form stable joints with a porous interconnection layer. A function for the self-healing of cracks in sintered porous Ag interlayers at high temperatures is discovered and reported here. A crack which was prepared on a Ag joining layer was closed after heating at 200 °C in air. The tensile strength of pre-cracked Ag joining layer specimens recovers to the value of non-cracked specimens after heating treatment. Transmission electron microscopy (TEM) was used to probe the self-healing mechanism. TEM images and electron diffraction patterns show that a large quantity of Ag nanoparticles formed at the gap with the size less than 10 nm, which bridges the crack in the self-healing process. This discovery provides additional motivation for the application of Ag as an interconnection material for power devices at high temperature.

Low-Stress Design for SiC Power Modules with Sintered Porous Ag Interconnection

The mechanical properties of sintered porous Ag-paste are investigated by tension test and shear test in the temperature range from 25 °C to 300°C. Stress-strain curves of sintered porous Ag-paste are measured at different temperatures. The Poissons ratio, which is calculated by Youngs modulus and shear modulus, decreased from 0.31 at 25 °C to 0.11 at 300 °C. In addition, 3D finite element model (FEM) is constructed for six types of module structures under thermal cycling analysis, which focuses particularly on the stress of sintered porous Ag-paste at different temperatures to find an optimized low-stress structure for the long-term reliability. The obtained results in this study suggested that the sintered porous Ag-paste may survive longer and continue to functions at high temperature variations because of the large plastic deformation.

High-Temperature Die Attachment Using Sn-Plated Zn Solder for Power Electronics

Pure Zn is a Pb-free die-attach material considered suitable for application in high-temperature electronics such as widebandgap semiconductor devices, as it has both the requisite electrical/thermal conductivity and an excellent thermal shock resistance. The high melting point of Zn (419.5 °C), however, means that die-attachment using pure Zn would require higher bonding temperatures (>430 °C), whereas a lower temperature is more desirable for mass production. This paper, therefore, presents a bonding method based on using Sn-plated Zn solder (Sn/Zn/Sn structure) to achieve a lower bonding temperature than pure Zn die attachment. The resulting shear strength of this bonding exceeds 25 MPa at 350 °C for 30 min, which is attributed to a uniform and complete Ni-Zn IMC (γ-Ni5Zn21) reaction layer. Furthermore, this bonding strength is retained beyond 25 MPa without serious degradation, even after thermal shock testing within a temperature range of -50 °C-300 °C; and thus, the Sn-plated Zn solder proposed has great potential as a die-attach material for high-temperature applications such as power devices.

Low-pressure sintering bonding with Cu and CuO flake paste for power devices

Low-temperature sintering bonding has been proposed as an alternative technique for the soldering to overcome such high operating temperature in wide-gap semiconductor power devices. Ag nanoparticle sintering is one of the candidates in die-attach bonding, but there are certain obstacles for mass production mainly due to the high cost of silver. In addition, metal nano-particle paste including Ag nanoparticle paste bonding needs to apply certain high pressure of MPa order. For mass productions, it is necessary to decrease the applying pressure during the bonding process. In the present study, the authors make flake-shaped Cu based particles by using mechanical milling for improving the contact area between the particles to decrease the required pressure. The die-bonding with Cu flake pastes was carried out at 300 °C with a formic acid. Resulting die-shear strength exceeds 15 MPa for bonded at 300 °C for 60 minutes low pressure (0.4 MPa). Moreover, Cu flake pastes with polyethylene glycol (PEG) solvent showed solid interface layer like bulk Cu. Thus, the Cu flake PEG paste is one of the most promising bonding materials with the remarkably high strength of the sintered bonding.

Prominent interface structure and bonding material of power module for high temperature operation

Sintered Ag is well known for die-attach materials, suitable for Ag metalized interfaces with a self-healing function of generated cracks. A remaining risk of sintered Ag bonding may be possible degradation of interfacial strength at high temperatures. Molding process is thus important for supporting the die-attach in the encapsulated power module once certain adhesion strength is assured between a lead-flame and mold resin. We propose a prominent interface structure using novel bonding materials for electronic power modules targeting high-temperature operation.

First failure point of a SiC power module with sintered Ag die-attach on reliability tests

We have investigated finite element method (FEM) modeling of thermal stress analysis in a SiC power module using sintering Ag die-attach, and explored the reliability focusing on the initial cracking point caused by the thermal stress. The analysis results by FEM models support our experimental observations in the reliability tests of the fabricated modules, confirming that using direct-bonded-copper (DBC) as a module substrate effectively reduces the thermal stress. Such an FEM analysis is thus useful for designing power modules to assure their sufficient reliability in harsh environments expected for automotive applications.

Effect of size and shape of Ag particles for mechanical properties of sintered Ag joints evaluated by micro-compression test

This research focuses on revealing the effect of size and shape of Ag particles for mechanical properties of no-pressure sintered Ag by using micro-compression test. Specimens were fabricated by focused ion beam with the dimension of 5 μm × 5 μm × 15 μm. The stress-strain curve suggested that the sintered Ag subjected a stable elastic deformation and a large plastic deformation. The mechanical properties of sintered Ag such as elastic modulus, and yielding stress, were evaluated, which strongly depend on the size and shape of used Ag particles. In addition, with the evaluation of shear strength for the various sintered Ag particles, it was found that sintered Ag with Ag nanoparticles has a low elastic modulus and low bonding quality due to many voids formation during the sintering process. In addition, sintered Ag micro flake shows the shear strength over 30 MPa in no-pressure sintering condition. The value was far larger than the shear strength of sintered Ag using Ag particles with spherical shape. These results will be helpful to understand from a technological perspective for reliable applications of sintered Ag and a fundamental interest in understanding its mechanical properties with different size and shape of Ag particles.

Development of Packaging Technology for High Temperature Resistant SiC Module of Automobile Application

Aiming for application to the inverter system of HEV and EV, we have developed a novel packaging technique for SiC power devices based on Nickel Micro Plating Bonding (NMPB) technique. We implemented heat resistant mounting of SiC schottky barrier diode (SBD) on the TO247 type package and confirmed the rectifying behavior even after the high temperature storage for 500hr at 250°C without any significant degradations. We also fabricated one-leg inverter modules mounting SBDs and MOSFETs using newly designed lead frames for NMPB process. The module showed normal rectifying and switching behavior even at high temperature such as about 250°C.