A review is provided, which first considers low-temperature diffusion bonding with silver nanomaterials as filler materials via thermal sintering for microelectronic applications, and then other recent innovations in low-temperature joining are discussed. The theoretical background and transition of applications from micro to nanoparticle (NP) pastes based on joining using silver filler materials and nanojoining mechanisms are elucidated. The mechanical and electrical properties of sintered silver nanomaterial joints at low temperatures are discussed in terms of the key influencing factors, such as porosity and coverage of substrates, parameters for the sintering processes, and the size and shape of nanomaterials. Further, the use of sintered silver nanomaterials for printable electronics and as robust surface-enhanced Raman spectroscopy substrates by exploiting their optical properties is also considered. Other low-temperature nanojoining strategies such as optical welding of silver nanowires (NWs) throu...

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Joining of Silver Nanomaterials at Low Temperatures: Processes, Properties, and Applications

Sintered silver joint is a porous silver that bonds a semiconductor die to the substrate as part of the packaging process. Sintered Ag is one of the few possible bonding methods to fulfill the operating conditions of wide band-gap (WBG) power device technologies. We review the current technology development of sintered Ag as a bonding material from the perspective of patents filed by various stakeholders since late 1980s. This review addresses the formulation of sintered pastes (i.e., nano-Ag, hybrid Ag, and micron Ag fillers), innovations in the process and equipment to form this Ag joint. This review will provide the insights and confidence to engineers, scientists from universities and industry as well as investors who are developing and commercializing the sintered Ag as a bonding material for microelectronic packaging. [DOI: 10.1115/1.4033069]

Identifying the Development State of Sintered Silver (Ag) as a Bonding Material in the Microelectronic Packaging Via a Patent Landscape Study

Low-temperature Ag sintering provides a lead-free die attachment method that is compatible with high-temperature (300 °C) power electronics applications. The reliability of sintered Ag die attach for Si and SiC die has been studied on both thick film substrates for lower current power applications and direct bond copper (DBC) substrates for higher current power applications. Pressureless and low-pressure sintering were evaluated. Sintering with low pressure yielded lower porosity (15–17%) versus pressureless sintering (∼30%). Reliability was evaluated with thermal aging (300 °C) and thermal cycling (–55 °C to + 300 °C) tests. Reliable Ag sintered die attach was achieved with assemblies having Ag-bearing surface finishes on both the die and the substrate. In contrast, the shear strength after 300 °C aging was greatly reduced when Au metallization was used either on the die or on substrate surface. In some cases, low-pressure sintering delayed the failure of the sintered Ag die attach to Au surfaces when aged at 300 °C compared to the pressureless sintering. The reliability with Pd-containing substrate metallizations was intermediate between Ag and Au metallizations. The thermal cycle reliability on DBC substrates was limited by failure at the Cu-to-alumina interface over the wide temperature range, while on the thick film substrates high adhesion was maintained after 1000 thermal cycles.

Reliability of Ag Sintering for Power Semiconductor Die Attach in High-Temperature Applications

A large number of factors such as the increasingly stringent pollutant emission policies, fossil fuel scarcity and their price volatility have increased the interest towards the partial or total electrification of current vehicular technologies. These transition of the vehicle fleet into electric is being carried out progressively. In the last decades, several technological milestones have been achieved, which range from the development of basic components to the current integrated electric drives made of silicon (Si) based power modules. In this context, the automotive industry and political and social agents are forcing the current technology of electric drives to its limits. For example, the U.S Department of Energy’s goals for 2020 include the development of power converter technologies with power densities higher than 14.1 kW/kg and efficiencies greater than 98%. Additionally, target price of power converters has been set below $3.3/kW. Thus, these goals could be only achieved by using advanced semiconductor technologies. Wide-bandgap (WBG) semiconductors, and, most notably, silicon carbide (SiC) based power electronic devices, have been proposed as the most promising alternative to Si devices due to their superior material properties. As the power module is one of the most significant component of the traction power converter, this work focuses on an in-deep review of the state of the art concerning such element, identifying the electrical requirements for the modules and the power conversion topologies that will best suit future drives. Additionally, current WBG technology is reviewed and, after a market analysis, the most suitable power semiconductor devices are highlighted. Finally, this work focuses on practical design aspects of the module, such as the layout of the module and optimum WBG based die parallelization, placement and Direct Bonded Copper (DBC) routing.

Power module electronics in HEV/EV applications: New trends in wide-bandgap semiconductor technologies and design aspects

SiC devices are promising for outperforming Si counterparts in high-frequency applications due to its superior material properties. Conventional wirebonded packaging scheme has been one of the most preferred package structures for power modules. However, the technique limits the performance of a SiC power module due to parasitic inductance and heat dissipation issues that are inherent with aluminum wires. In this article, low parasitic inductance and high-efficient cooling interconnection techniques for Si power modules, which are the foundation of packaging methods of SiC ones, are reviewed first. Then, attempts on developing packaging techniques for SiC power modules are thoroughly overviewed. Finally, scientific challenges in the packaging of SiC power module are summarized.

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Review of Packaging Schemes for Power Module

https://hal.archives-ouvertes.fr/hal-00874458/document

Migration issues in sintered-silver die attaches operating at high temperature

With the emergence of new power semiconductor devices, the switching speeds in power converters are increasing. The stray inductances of switching cells must, therefore, be minimized to limit overvoltages on transistors. One relatively new approach, called power chip-on-chip (PCoC), considers the integration of power dies, one on top of the other, directly in the busbar. This allows for the reduction of the stray inductance. This paper first presents the implementation of a PCoC module using classical packaging techniques. Then, a description of the different technological steps for the realization is outlined. Finally, experimental characterization results confirm the lower stray inductances offered by the PCoC package compared with the planar one.

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Implementation and Switching Behavior of a PCB-DBC IGBT Module Based on the Power Chip-on-Chip 3-D Concept

Silver sintering die attach is a promising technology for high temperature power electronics packaging. In this paper, we evaluate its performances in terms of thermal resistance and high temperature stability. The thermal resistance is measured on test vehicles assembled under different conditions, using silver pastes from NBETech and Heraeus, with different process parameters. The stability test is performed by storing samples at an elevated temperature (300 deg C) in air, with a strong voltage bias (1100 V) for several hours.

https://hal.archives-ouvertes.fr/hal-00707733/document

Evaluation of silver-sintering die attach

With the emergence of new power semiconductor devices, the switching speeds in power converters are increasing. The stray inductances of switching cells must therefore be minimized to limit the over-voltages on transistors. One relatively new approach, called Power Chip-on-Chip (PCo), considers the integration of power dies, one on top of the other directly in the busbar. This allows for the reduction of the stray inductance. This paper first presents the implementation of a PCoC module using classical packaging techniques. Then a description of the different technological steps for the realization is outlined. Finally, experimental characterizations show the interest of the proposed PCoC module compared with planar ones.

Realization and characterization of an IGBT module based on the power chip-on-chip 3D concept

Silver sintering is a promising alternative to high melting point (HMP) solders which contain lead. Indeed, it offers better thermal and electrical properties, and can operate at higher temperature. Currently, several implementations of this technique are available, based on various silver particles sizes and sintering additives. This paper presents a review of the different implementations, and gives practical details about one of them, based on silver nanoparticles. One specific aspect is highlighted: the metal finish of the DBC substrate. It has a major impact on the quality of the sintered joint: with some finishes, the adhesion is excellent (more than 50 MPa), while it is poor with some others (lower than 10 MPa).

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Raphaël Riva

Papers

Migration issues in sintered-silver die attaches operating at high temperature

Implementation and Switching Behavior of a PCB-DBC IGBT Module Based on the Power Chip-on-Chip 3-D Concept

Evaluation of silver-sintering die attach

Realization and characterization of an IGBT module based on the power chip-on-chip 3D concept

Die attach using silver sintering. Practical implementation and analysis