Development of Ag Nanotwinned Film Coated Pre-Formed Silver Sintered Sheets for the Die Bonding of SiC Chips with DBC Ceramic Substrates

Wide-bandgap (WBG) semiconductor materials are expected to be widely utilized in next-generation power devices as modules to replace Si-based devices owing to their higher voltage-blocking capability, higher temperature operation, and switching frequencies, as well as their reduced power loss [1][2]. In WBG devices, die-attached materials that bond WBG dies to substrates are crucial for maintaining high performances in environments reaching 250 °C [3]. Conventional die bonding materials, such as Pb-5Sn solder [4], Au-Sn solder [5], and Bi-based solder [6], are not suitable for use in next-generation packaging because cracks can form at the intermetallic solder interface in high-temperature environments . One of the promising alternative approaches is Ag sinter joining technology, which allows for a sintering temperature below 300 °C at low pressure. As the melting point of Ag is higher than 900 °C, Ag sinter joining can be used in high-temperature environments exceeding 250°C .The quality of die-attach by Ag sinter joining depends on the bond-line cohesion from Ag sintering and interfacial adhesion from atomic inter-diffusion. Therefore, the bonding quality depends on the surface finish such as its chemistry and microstructure, on both device and substrate . Usually, electro-plated Ag, electroless-plated Ni(P)/Ag, electro- and electroless-plated Ni(P)/Au is used as surface finish layer for sinter Ag joining. On the other hand, the electro- and electroless-plated Ni(P) technology was well developed and used widely, which can achieve a sufficient resin adhesion resulting from the chemical interaction between the base metal Ni and the resins. However, this additional metallization layer increase costs due to the additional raw materials and processing steps. Ag paste-based, die-attachments on a bare the direct bonded copper (DBC) substrate or direct bonded aluminum (DBA) seems an ideal option due to reduced cost and a simplified process .Here we report a robust joint by Ag sinter joining technology for different metal interface (Au, Ag, Ni, Cu, Al) in wide band gap power modules. Micron-scale Ag flakes (AgC239, Fukuda Metal Foil, and Powder Co. Ltd, Japan) were used as the Ag fillers. The thermal behaviors and morphology of the Ag paste sintered at different temperatures were firstly studied to understand the sintering behavior of the prepared Ag paste. Sinter Ag joint structure for different metal interface was implemented by a sintering process under temperatures of 250 °C in air without pressure. In addition, each sinter Ag joint structure was investigated to obtain a comprehensive understanding for different metal interface bonding. A possible mechanism was proposed based on the SEM and TEM observation of the cross-sectional part of each sinter Ag joint structure.Fig. 1 shows the sinter Ag joint structure for Ag metallization metal interface, and the die shear strength of sinter Ag joint structure for different metal interface. Sintered Ag shows a microporous network structure and bonding well with the Ag metallization layer at the both SiC chip and substrate side. The die shear strength of each sinter Ag joint structure is larger than 25 MPa which comparable with the value of traditional Sn–Pb solders (19~24MPa. This study helps to understand the Ag sinter joining for different metal metallization interface, enlarger the Ag sinter joining for wide band-gap power modules in high temperature applications. ACKNOWLEDGMENTS This work was supported by the JST Advanced Low Carbon Technology Research and Development Program (ALCA) project “Development of a high frequency GaN power module package technology” (Grant No. JPMJAL1610). The author is thankful to the Network Joint Research Centre for Materials and Devices and Dynamic Alliance for Open Innovation Bridging Human, Environment and Material.

Recently, with the development of power electronics technology, the power modules have been used extensively and the demand for downsizing. At our laboratory, the power assist to support knee moving has been developing. And this product also uses power module to improve energy efficiency. The problem of the power module is thermal fatigue of the junction area between SiC chip and substrate. When downsize the power module, thermal design become more important because it gets more difficult to release heat produced in the module. In this study, assuming practical use of power assist, proposing a model of power module under power cycle test that allows further downsizing and reducing weight was conducted. And thermal design of power module was evaluated by using coupled electrical-thermal-mechanical analysis. The analysis using Finite Element Method (FEM) was operated to evaluate non-uniform temperature distribution in power module was estimated. Based on the results, the model of power module is proposed as effective solution for downsizing without reducing the reliability.

Low-Temperature Die Bonding of SiC Chips with DBC Ceramic Substrates Using High-Density Ag (111) Nanotwinned Films

Recently, electrical vehicle (EV) charger demand has been grown rapidly due to electrical vehicle development, so we designed a full SiC power module for EV charger system. The power consumption and thermal performance of EV charger module were evaluated by simulation software. Besides, we did the thermal resistance measurement of full SiC module and calibrated the Rjc (junction-case) .The simulation results show that we can reduce the size of heatsink because of higher system efficiency caused by full SiC power module. The advantage of using SiC chip in EV charger power module lose less power than silicon power module because of SiC chip’s electrical characteristic.

New power module prototypes are being designed to take advantage of the high operating temperature and high power density benefits of using SiC chips. Thus, monitoring the temperature distribution in the power module is critical. This work investigates the temperature distribution in a power module using Infrared Thermography in steadystate and transient test conditions. The methodology, results and observations from the tests are discussed in this work.

The ongoing development of power modules for electronics in the field of electromobility, charging infrastructure and photovoltaic based on SiC chips come up with a strong demand on tailored thermal management materials with high thermal conductivity, tailored coefficient of thermal expansion, soldering workability and a way for industrial production. In this paper the developed routine for the industrial production of metal diamond composites inspired by the wafer technology for semiconductors will be presented using the example of copper diamond composite. For the material integration into power modules by soldering the composite needs a cover with pure copper on both sides. The cross section, thermal conductivity and coefficient of thermal expansion will be displayed for the single composite, the covered composite and after integration into active metal braze (AMB) substrate. Aspects for the further integration into a power module design will be discussed and shown for one example.

A new DBC-based hybrid packaging and integration method is proposed in this paper. A multilayer power module is formed by a direct-bond-copper (DBC) and a window cutting printed circuit board (PCB). The SiC chips and PCB are placed and soldering on the DBC. Al bonding wires are used for connecting the chips and the PCB. A full SiC half-bridge power module is designed and fabricated in compact size. The parasitic inductance of power loop is only 3.38 nH. The passive device and gate drive circuit can be easily integrated on the PCB. Based on the idea, a highly integrated 2.5kW 300 kHz TCM synchronous rectification (SR) is designed. The power density achieves 379 W/in3 and maximum efficiency reaches 98.4%.

SiC power semiconductors can safely operate at a junction temperature of 500degC. Such a high operating temperature range can substantially relax or completely eliminate the need for bulky and costly cooling components commonly used in silicon-based power electronic systems. However, a major limitation to fully realizing the potential of SiC and other wide band-gap semiconductor materials is the lack of qualified high-temperature packaging systems, particularly those with high-current and high-voltage capabilities required for power conversion applications. This paper proposes a new hybrid power module architecture that allows wide bandgap semiconductor power devices to operate at a junction temperature of 300degC. The concept is based on the use of double metal or DCB leadframes, direct leadframe-to-chip bonding, and high temperature encapsulation materials. The leadframes, serving as both the external leads and the internal interconnect to the semiconductor chips, need to provide excellent high temperature stability, adequate electrical and thermal conductivity, and a coefficient of thermal expansion (CTE) closely matching that of SiC. The SiC chips are sandwiched between and bonded to the top and bottom leadframes using a brazing or adhesion process. Extensive electrical, thermal, and mechanical modeling has been performed on this new concept. Several prototypes are fabricated, and a finite element model is evaluated. Packaging architecture and materials considerations are discussed.

In this study, a lead frame based 6-in-1 power module with 6 SiC MOSFET devices aimed for automotive applications is proposed. Different from the conventional power modules, the direct bonded copper (DBC) substrates and the wire-bond interconnections are eliminated from the proposed power module. As such, the proposed power module is with features of lightweight and thin thickness by removing the heavy and thick DBC substrates, as well the high loop wire bond interconnections. Furthermore, the proposed power module is with excellent thermal performance by implementing double side liquid cooling solution. The computational fluid dynamics (CFD) model has been created for the thermal simulation and the thermal performance of the proposed power module has been evaluated. The effects of thermal interface material (TIM) layer, the thickness of the Cu LF and the flow rate of the coolant in each heat sink on the junction temperature of the SiC chips have been simulated and analyzed. The thermal design guidelines for the proposed 6-in-1 power module are extracted and the optimized thermal design is implemented to the power module.

Development of high power SiC devices for rail traction power systems

Effect of copper over-pad metallization on reliability of aluminum wire bonds

Silicon carbide power devices are intended and to enter new application regimes in power electronics, in fact, they are enabling components mainly if higher switching frequencies in power electronics are considered. This trend can be clearly observed since power density can be increased and efforts towards passive components and other mechanical contributions to the system can be reduced. However, this trend imposes new challenges towards the surrounding of the chips in form of the package itself and the whole system around. Stray components like inductances and impedance elements become crucial elements in the whole circuit what results in the fact that a simple exchange of silicon chips by silicon carbide in a given package can be ruled out. In addition different considerations regarding the thermal design especially in power modules arise when SiC chips are considered, triggered by the fact that the cost balance between assembly and chip is shifted compared to silicon based solutions. Thus, different optimization criteria can be used, leading to new design approaches for power modules. The following paper will give a first inside how those boundary conditions can be implemented in innovative solutions using SiC components.

Growth of Cu nanotwinned films on surface activated SiC chips

Today, SiC devices are adopted more and more in various applications. Though SiC material has better thermal conductivity than Si material, effectively dissipating the heat through the power module can also significantly improve the thermal performance of SiC power modules. In this paper, a power module design with ceramic heatsinks has been devised and produced to improve the thermal performance and structure of power modules. The proposed power modules remove many manufacturing and assembly steps during the production process since there are few layers between chips and the heatsink. With different manufacturing processes, including machining and 3D printing, two types of power modules with Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ceramic heatsink have been made. The measured junction-ambient thermal resistance values under forced cooling conditions for these two types of power modules are 3.38°C/W and 3.61 °C/W respectively. The heatsink can be further optimized with advanced structures using 3D printing. In the switching test, the new SiC power modules worked well under 650 V and 2 kW. Moreover, simulation results illustrate that the thermal resistance and the maximum strain of the new package with AlN ceramic heatsinks are lower than those of the conventional structure, demonstrating great potential of this new design.

In this study, a novel Cu lead frame (LF) based double side cooling SiC power module is proposed and developed. The proposed SiC power module eliminates the conventional direct bonded copper (DBC) substrates by implementing a dedicated copper lead frame. Meanwhile, the proposed power module is capable for double side liquid cooling scheme by employing the flat copper clips at the top side of SiC devices. Furthermore, the high temperature endurable materials, i.e. epoxy molding compound (EMC), die attachment (DA) and lead free solder, are evaluated and identified for the proposed power module. In addition, the processes for interconnects (i.e. die attach and solder joints) formation and package encapsulation is optimized for the power module assembly. Lastly, the adhesive dielectric thermal interface material (TIM) with high thermal conductivity is recommended to bond the power module with the heat sink. The proposed power module has been fabricated with identified materials and gone through the specified reliability assessments, e.g. unbiased highly accelerated stress test (uHAST), temperature cycling (TC) test (−40∼150°C) for 1,000 cycles, high temperature storage (HTS) test at 200°C for 1,000hrs and power cycling test (PCT) ( $\Delta \mathrm{T}=150^{\circ}\mathrm{C}$ ) for 50,000 cycles. Failure analysis has been conducted for the failed samples.