Power Electronics Packaging Research Articles

Finite element modeling and analysis of ultrasonic bonding process of thick aluminum wires for power electronic packaging

Ultrasonic wedge bonding of aluminum (Al) wires is a widely applied interconnect technology for power electronic packaging. The joint quality of the wedge bonding is mainly affected by the process parameters and material properties. Inappropriate process parameters will lead to failure modes such as chip surface pit, metal layer peeling off, wire cracking, non-sticking to the pad, etc., which limits the long-term stability of power devices. In order to reach the desired reliability, the design of experiment (DoE) is generally deployed which is costly in terms of time and related materials. Therefore, simulation-assisted analysis is in demand to rapidly narrow down the process windows. In this paper, an ultrasonic bonding model involving thick Al wires (300 μm) was established based on the Finite Element Method (FEM), to optimize process parameters effectively with reduced time and cost. The model was designed in ANSYS utilizing the transient structural mechanics module with various stresses and ultrasonic power, to simulate the relative deformation of the bonded wires and the displacement against the substrate. The result was then verified by ultrasonic wedge bonding samples with 9 sets of process parameters. The stress distributions were simulated and analyzed with the failure modes of tensile strength tests, while the deformation of wires under various process parameters was measured and compared with shear strength tests. Further, the relationship between the failure modes of the joint and the deformation was then analyzed by Response Surface Method (RSM), and the regression equation of the wire deformation and related process parameters was established and fitted with the actual sample's data. Such analysis not only found the optimum range of the deformation of thick Al ultrasonic wire bonds but also quickly provided a range of optimized processes for Al thick wires applying ultrasonic wedge bonding techniques.

Supersaturated Ag-Cu nanoalloy film for high reliability power electronic packaging

Nano-Ag sintering can realize superior thermal, electrical, and mechanical properties for power electronic packaging, while it suffers from unstable microstructure and electrochemical migration. In this work, a supersaturated Ag2.8 wt%Cu nanoalloy film has been developed using pulsed laser deposition, which can be sintered at 250 °C in air for electronic packaging without sacrificing bondability. It was surprisingly found that both thermal stability and electrochemical migration resistance were significantly improved when alloying the Ag with 2.8 wt% Cu. The in situ formed Cu2O/CuO from Ag-Cu nanoalloy existed on the pore surface and along the grain boundaries of Ag/Ag-Cu nanoalloy, which stabilized the microstructure of bondline from 400 h to 1400 h at 300 °C. The synergistic effect of Ag-Cu nanoalloy and Cu oxides realized an obvious protective effect in the process of anodic dissolution, exhibiting a short-circuit time 2.4 times of the pure Ag electrode. The die attach process is compatible with most commercial equipment and Si/SiC dies with improved performance, enabling the supersaturated Ag-Cu nanoalloy to be a promising material for high reliability power electronic packaging.

Ceramic Additive Manufacturing Potential for Power Electronics Packaging

Compared with silicon-based power devices, wide bandgap (WBG) semiconductor devices operate at significantly higher power densities required in applications, such as electric vehicles and more electric airplanes. This necessitates the development of power electronics packages with enhanced thermal characteristics that fulfill the electrical insulation requirements. The present research investigates the feasibility of using ceramic additive manufacturing (AM), also known as three-dimensional (3-D) printing, to address thermal and electrical requirements in packaging gallium nitride (GaN)-based high-electron-mobility transistors (HEMTs). The goal is to exploit design freedom and manufacturing flexibility provided by ceramic AM to fabricate power device packages with a lower junction-to-ambient thermal resistance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\theta \text {JA}}$ </tex-math></inline-formula> ). Ceramic AM also enables incorporation of intricate 3-D features into the package structure in order to control the isolation distance between the package source and drain contact pads. Moreover, AM allows to fabricate different parts of the packaging assembly as a single structure to avoid high thermal resistance interfaces. For example, the ceramic package and the ceramic heatsink can be printed as a single part without any bonding layer. Thermal simulations under different thermal loading and cooling conditions show the improvement of thermal performance of the package fabricated by ceramic AM. If assisted by an efficient cooling strategy, the proposed package has the potential to reduce <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\theta \text {JA}}$ </tex-math></inline-formula> by up to 48%. The results of the preliminary efforts to fabricate the ceramic package by AM are presented, and the challenges that have to be overcome for further development of this manufacturing method are recognized and discussed.

Optimization of Ag-alloy ribbon bonding — An approach to reliable interconnection for high power IC packaging

Ultrasonic ribbon bonding has drawn much attention in power electronic packaging as compared to conventional heavy wire bonding. This study evaluated the bondability of 100 × 1500 μm Ad-4Pd ribbon bonded on direct bonded copper (DBC) substrates metallized with Ni/Pd/Au layers through adjustment of the bonding parameters. The bonding patterns on the wedge bonds along with cross-sectional images of the polished interface were investigated, and the pull force and joint properties were examined by pull and shear tests. Failure codes indicated heel damage resulting from excessive ultrasonic energy, which was explained from the deformation point of view. Furthermore, footprint studies of the sheared surfaces indicated that increasing the bonding power could promote the bonding surface, as evidenced by the transition of the surface morphology from fretting to microwelds and finally fractured pieces of ribbons. This study may be of great importance in explaining the special features of ultrasonic ribbon bonding, as well as in providing a feasible solution for power electronic packaging with a better understanding of the importance of optimizing the bonding parameters.

Interdiffusion and formation of intermetallic compounds in high-temperature power electronics substrate joints fabricated by transient liquid phase bonding

Power electronics packages typically comprise a dielectric substrate bonded to a metal layer and attached to heat sinks using a low-thermal-conductivity solder. These multiple layers increase the effective thermal resistance of the package and are responsible for package failures under cyclic thermal loads. Bonding the aluminum nitride dielectric layer (AlN) directly to a low coefficient of thermal expansion (CTE) aluminum silicon carbide (AlSiC) cold plate using copper‑aluminum (Cu-Al) transient liquid phase (TLP) bonding has been shown to improve the mechanical reliability of the power electronic packages while making the package more compact by bringing the cooling solutions closer to the devices. This study aims to characterize the Cu-Al bonds formed during the TLP bonding of AlSiC with three different power electronics substrates: pure aluminum nitride (AlN), aluminum (DBA), and copper (DBC). The material compositions and microstructures of the bonds were analyzed using scanning electron microscopy (SEM), x-ray spectroscopy (EDS), confocal scanning acoustic microscopy (C-SAM), and x-ray diffraction (XRD). α-Al solid compound was identified as the dominant phase in AlN-AlSiC and Al-AlSiC, with a notable presence of SiC particles. In contrast, three intermetallic phases – θ-CuAl2, η-CuAl, and γ′- Cu9Al4 – were observed in the Cu-AlSiC bond. A computational solid-state diffusion model was developed to predict the intermetallic compounds (IMCs) formed during Cu-Al TLP bonding in each system, which supported the observation that an initial Cu volume fraction of 20 % in the material layers produced a final bond composition of >95 % Al with minimal IMC growth. However, increased Cu concentrations produced higher concentration gradients, leading to increased growth of IMCs, Kirkendall voids, and interstitial cracks.

Reliability of Wide Band Gap Power Electronic Semiconductor and Packaging: A Review

Wide band gap (WBG) power electronic devices, such as silicon carbide metal–oxide–semiconductor field-effect transistors (SiC MOSFETs) and gallium–nitride high-electron-mobility transistors (GaN HEMTs) have been widely used in various fields and occupied a certain share of the market with rapid momentum, owing to their excellent electrical, mechanical, and thermal properties. The reliability of WBG power electronic devices is inseparable from the reliability of power electronic systems and is a significant concern for the industry and for academia. This review attempts to summarize the recent progress in the failure mechanisms of WBG power electronic semiconductor chips, the reliability of WBG power electronic packaging, and the reliability models for predicting the remaining life of WBG devices. Firstly, the typical structures and dominant failure mechanisms of SiC MOSFETs and GaN HEMTs are discussed. This is followed by a description of power electronic packaging failure mechanisms and available packaging materials for WBG power electronic devices. In addition, the reliability models based on physics-of-failure (including time-dependent dielectric breakdown models, stress–strain models, and thermal cycling models), and data-driven models are introduced. This review may provide useful references for the reliability research of WBG power devices.

Rapid and low temperature sintering bonding using Cu nanoparticle film for power electronic packaging

It is of great interest but very difficult to achieve both rapid (few minutes) and low temperature (<200 °C) sintering bonding using Cu nanoparticles, which is mainly due to the diffusion dominated process and complex organics in the nanoparticle paste. In present work, a copper nanoparticle film (CNF) fabricated by pulsed laser deposition (PLD) has been developed. Under the reduction of ethylene glycol (EG) and protection of inert atmosphere, it can be sintering bonded at even 140 °C, having adequate shear strength higher than that of Pb-5Sn. A remarkable high strength of 65 MPa for Cu-DBC joints and 50 MPa for Si-DBC joints were achieved when sintered at 250 °C for only 1 min. The rapid and low temperature sintering of CNF is mainly attributed to the efficient oxide film removal by EG at low-temperature, since it is an organic free dry film without the complex organics which decompose or evaporate at high temperature. The relationships among joint strength, porosity of microstructure and interfacial connection ratio have also been discussed.

Achieving high-efficiency electrically insulating ceramic layer formed on SiCp/Al composite by bipolar pulsed PEO for novel integrated strategy

Thermal conductive insulating ceramic materials are urgently needed in the packaging applications for modern electronic devices owing to their multi-functionality. The electrically insulating Al2O3 ceramics with large thickness and high compactness is designed on SiCp/Al baseplate via plasma electrolytic oxidation (PEO) technique, which provides a new idea for the package structure of the IGBT power module. However, porous characteristics of PEO inorganic ceramics hinder effective electrical insulation and heat conduction in practical applications. Herein, the bipolar pulsed (BP) and unipolar pulsed (UP) PEO are tailored to obtain the ceramics respectively. In contrast, the BP mode enables the ceramic layer significantly thickened and densified due to the soft sparking discharge. Combined with outstanding electrical resistivity over 1012 Ω·cm, high breakdown voltage of 716 ± 38 V, high dielectric strength of 15.9 ± 0.8 V/μm, the BP ceramic layer exhibits superior performance in electrical insulation. Moreover, the BP shows excellent thermal cycling stability (>200 cycles), quite good thermal stability, and dielectric stability in the temperature range of 25 °C to 300 °C to realize high-temperature electrical insulation. This work paves a novel way to design and optimize power electronic module package materials and structures.

Effects of bonding pressures on microstructure and mechanical properties of silver–tin alloy powders synthesized by ball milling for high-power electronics packaging

Silver (Ag) sintering is widely adopted as a die attach method for power electronics packaging. However, oxidation, massive grain growth, and increasing porous structure occur in sintered Ag joints at high temperatures, resulting in decreasing mechanical properties of the die attachment in power devices. To solve the problems of sintered Ag joints, a novel die attach material consisting of silver–tin (Ag–Sn) alloy particles is developed in this study. Ag–Sn alloy powders were newly fabricated by high-energy ball milling technique. The microstructural evolutions of the Ag–Sn alloy joints bonded at different pressures were systematically analyzed. With increasing high-temperature storage time at 300 °C, the reactions between sintered Ag–Sn alloy joints and Cu substrates increased, which resulted in the formation of Cu3Sn or Ag-rich diffusion layer. The relationships between microstructures and the formation of each phase were discussed. In addition, the robust sintered Ag–Sn alloy joints exhibited low porosity without pore coarsening and limit oxidation at high temperatures, indicating a dense and stable microstructure of sintered Ag–Sn alloy joints. As a result, high bonding strength did not deteriorate even after high-temperature storage for 2000 h. It is concluded that the novel Ag–Sn alloy paste produced by high-energy ball milling has long-term high-temperature reliability to be used as a die attach material for the power electronics packaging, thus enabling high-power devices to have higher operating temperatures and thermal stability.

Overview of Current Thermal Management of Automotive Power Electronics for Traction Purposes and Future Directions

The design of the thermal management solution has a significant impact on the reliability and power density of power electronics (PEs). As the electric vehicle (EV) industry moves toward increasing the efficiency and output power, the cooling system must effectively remove the excess heat dissipated in PEs. The main heat-generating components are the semiconductor switches, but other components, such as bus bars and power capacitors, also dissipate heat and require cooling. Currently, indirect, direct, and double-sided cooling methods are the most common in EVs and account for 14%–33% of the total volume of traction inverters. However, PE packaging sizes are expected to decrease, while the heat dissipation continues to increase; hence, advanced cooling technologies are being investigated. This article aims to review the thermal management strategies for major PE components in EVs as well as their failure modes since high temperatures can be detrimental to the performance of PEs. Cooling designs that are currently implemented in EVs and future cooling trends for the next generation of PEs are reviewed as well.

Microstructural evolution, fracture behavior and bonding mechanisms study of copper sintering on bare DBC substrate for SiC power electronics packaging

Robust bonding of Cu quasi-nanoparticles sintering for Ag coated chip and bare copper substrate was achieved. The effect of temperature, pressure and time on the sintering bonding strength and microstructural evolution was deeply studied. 36.5 MPa shear strength was achieved when applied 5 MPa pressure. By increasing to 30 MPa, it shows the best die shear strength of 116 MPa, accomplished with a sintering temperature of 250 °C for 3 min. Temperature also influenced the shear strength a lot. Between 210 °C and 230 °C can already provide strength over 30 MPa. When increased to 270 °C, the strength was extremely enhanced to over 120 MPa. Inspection on the fracture behaviors and cross-section of sheared off samples was conducted by SEM, EDS, and XRD. It is found that low bonding performance is due to both of the incomplete burnt out of organics and incomplete Cu QNPs sintering. In addition, high bonding is account for the positive effect of pressure and temperature on promoting the necking growth, sintering networking formation, pores isolation and brittle-ductile fracture transformation. The recommended sintering profile is 250 °C, 3 min, 20 MPa. Finally, the feasibility for SiC MOSFET power electronics DA was verified by testing its static characteristics at both room temperature and 150 °C.

Evaluating Cu Printed Interconnects “Sinterconnects” versus Wire Bonds for Switching Converters

This paper demonstrates the feasibility of the printed copper (Cu) paste interconnects for applications in power semiconductor modules and switching converters. Copper sinter paste interconnects denoted as “Sinterconnects” have been recently introduced as an alternative to wire-bonding technology for power electronic device packaging. However, the electrical domain properties of these novel interconnects have not yet been investigated in detail. To address this research opportunity, this paper evaluates the performance of two different types of Sinterconnects applied to multi-chip, insulated gate bipolar transistor (IGBT) power modules. First, parasitic or stray inductances of these Sinterconnected systems are calculated analytically and by using three-dimensional finite element (FE) analysis. In addition to that, resistivity (ρ) of those has been analysed and compared with conventional wire bond technology. Finally, the performances of the Sinterconnects in power device assemblies are experimentally investigated. Two Sinterconnect structures (i.e., printed Cu paste and Cu clip attach) as well as a state-of-the-art wire-bonded IGBT module, have been integrated into a switching DC-DC converter and benchmarked. Experimental measurements show how converters with Sinterconnects enable efficient power conversion.

Simulation of the Effect of Frequency and Aspect Ratio on Ribbon Bond Wire Performance

The frequency dependent impedance and power loss are essential in the design of bond wires in power electronics packages. In this paper, the simulation of the effect of frequency and aspect ratio on the performance of the ribbon bond wire is presented. A copper ribbon bond wire is considered and 2D FEM electromagnetic simulation is carried out. The current density, resistance, and power losses obtained for power frequency using simulation are compared with the calculated values. The performance parameters are obtained for the frequency range from 200 Hz to 20 MHz. Also simulation is carried out to obtain the performance of the ribbon bond wires for the aspect ratios 6, 24, 54, 96, and 150.

Pressureless Sintered-Silver as Die Attachment for bonding Si and SiC Chips on Silver, Gold, Copper, and Nickel Metallization for Power Electronics Packaging: The Practice and Science

Low-temperature silver (Ag) sintering is emerging as a cutting-edge die-attach technology for power electronics packaging. However, the industry still has concerns about completely accepting this technology. One of the reasons is a lack of knowledge about the mechanism and process of sintered silver bonding on various metallization. Before, we reported a preliminary assessment of the die-shear strength and microstructures of Si dummy device attached on Ag, Au, and Cu metallization. This research will go deeper into the practice and science of attaching Si and SiC devices with sintered-Ag on substrates with up to seven most widely used metallizations, including electroplated Ag, Ni/Au, and Ni, electroless-plated Ni(P)/Ag, Ni(P)/Au, and Ni(P), and native Cu on DBC substrate. All sintered-Ag die-attach were pressureless in the air at temperatures ranging from 220 °C to 300 °C. The mechanical, electrical, and thermal parameters, as well as the microstructural analyses, were assessed. The purpose of this research is to gain a better understanding by: 1) discussing effects of devices and substrate metallizations on the mechanical, electrical, and thermal characteristics; 2) reviewing fundamental mechanisms of interfacial adhesion for sintered-Ag bonding, and 3) providing practical considerations for developing Si and SiC sintered-Ag die-attaching profiles on the widely used substrate metallization.

A low cost multi-shapes designed sintering composite paste: A strengthening method of sintered interconnect for die attach in high temperature applications

In this work, we proposed a multi-shape designed Ag-Cu bimetallic pastes for die attach sintering in power electronic packaging. We found a good adaptation of Ag flakes and Cu sphere particles in the sintered microstructure of the multi-shapes pastes. Under the “rolling effects” of the Cu particles, Ag flakes become more compact with larger plastic deformation. Further analysis indicates that sintering properties is highly dependent upon the shapes of the particles. The fracture interface is also highly dependent upon particle shape combination in sintering paste. For Ag flake- Cu sphere combination, more residual sintered masses were found at the fracture interface, which indicate a good strength matching of the sintered bulk and the interface. The sintered die attach connection with optimized particle shape combination perform a maximum shear strength up to 47.42 MPa, which is obtained at a much lower cost than normally used Ag paste.

Joint Analysis and Reliability Test of Epoxy-Based Nano Silver Paste Under Different Pressure-Less Sintering Processes

Abstract Recent years, the sintered silver paste was introduced and further developed for power electronics packaging due to low processing temperature and high working temperature. The pressure-less sintering technology reduces the stress damage caused by the pressure to the chip, improves reliability, and is widely applied in manufacturing. Currently, most existed studies are focused on alcohol-based sintered silver pastes while resins have been demonstrated to improve the bonding properties of solder joints. Hence, the performance and sintering mechanisms with epoxy-based silver paste need to be further explored. In this work, a methodology for multifactor investigation is settled on the epoxy-based silver paste to reveal the relationship between the strength and the different influence factors. We first analyzed the characteristics of commercialized epoxy-based silver paste samples, including silver content, silver particle size, organic composition, sample viscosity, and thermal conductivity. Samples were then prepared for shear tests and microstructure analysis under different pressure-less sintering temperatures, holding time, substrate surface, and chip size. Full factor analysis results were further discussed in detail for correlation. The influence factors were ranked from strong to weak as follows: sintering temperature, substrate surface, chip size, and holding time. Finally, a thermal cycling test was carried out for reliability analysis. Epoxy residues are one of the possible reasons, which result in shear strength decreasing exponentially.

High-Temperature Deformation Behavior of Sintered Nanocu Paste Used in Power Electronics Packaging: A Constitutive Modeling and Stochastically Equivalent Finite Element Characterization

High-Temperature Deformation Behavior of Sintered Nanocu Paste Used in Power Electronics Packaging: A Constitutive Modeling and Stochastically Equivalent Finite Element Characterization

Rapid and Low Temperature Sintering Bonding Using Cu Nanoparticle Film for Power Electronic Packaging

Rapid and Low Temperature Sintering Bonding Using Cu Nanoparticle Film for Power Electronic Packaging

Microstructural Evolution and Bonding Mechanisms of Nano Copper Sintering for Power Electronics Packaging Industrial Application

Microstructural Evolution and Bonding Mechanisms of Nano Copper Sintering for Power Electronics Packaging Industrial Application

High thermal conductivity diamond-doped silver paste for power electronics packaging

A thermal management material with high thermal conductivity was prepared by sintering silver paste doped with Ag/TiC double-coated diamond at 5 MPa and 260 °C and the effects of diamond particle size, coating and concentration were investigated. The thermal conductivity of the composite sample doped with 25 vol% 40–50 μm double-coated diamond is 243 W/m·K, which is ∼74% higher than that of the undoped sample, i.e., 139 W/m·K. In addition, the resistivity and shear strength of the composite paste were characterized.