Comparing the mechanical and thermal-electrical properties of sintered copper (Cu) and sintered silver (Ag) joints

Silver (Ag), gold (Au), and copper (Cu) have been utilized as metals for fabricating metal-based inks/pastes for printed/flexible electronics. Among them, Cu is the most promising candidate for metal-based inks/pastes. Cu has high intrinsic electrical/thermal conductivity, which is more cost-effective and abundant, as compared to Ag. Moreover, the migration tendency of Cu is less than that of Ag. Thus, recently, Cu-based inks/pastes have gained increasing attention as conductive inks/pastes for printed/flexible electronics. However, the disadvantages of Cu-based inks/pastes are their instability against oxidation under an ambient condition and tendency to form insulating layers of Cu oxide, such as cuprous oxide (Cu2O) and cupric oxide (CuO). The formation of the Cu oxidation causes a low conductivity in sintered Cu films and interferes with the sintering of Cu particles. In this review, we summarize the surface and interface designs for Cu-based conductive inks/pastes, in which the strategies for the oxidation resistance of Cu and low-temperature sintering are applied to produce highly conductive Cu patterns/electrodes on flexible substrates. First, we classify the Cu-based inks/pastes and briefly describe the surface oxidation behaviors of Cu. Next, we describe various surface control approaches for Cu-based inks/pastes to achieve both the oxidation resistance and low-temperature sintering to produce highly conductive Cu patterns/electrodes on flexible substrates. These surface control approaches include surface designs by polymers, small ligands, core-shell structures, and surface activation. Recently developed Cu-based mixed inks/pastes are also described, and the synergy effect in the mixed inks/pastes offers improved performances compared with the single use of each component. Finally, we offer our perspectives on Cu-based inks/pastes for future efforts.

Surface and Interface Designs in Copper-Based Conductive Inks for Printed/Flexible Electronics.

Conductive inks have attracted tremendous attention owing to their adaptability and the convenient large-scale fabrication. As a new type of conductive ink, copper-based ink is considered to be one of the best candidate materials for the conductive layer in flexible printed electronics owing to its high conductivity and low price, and suitability for large-scale manufacturing processes. Recently, tremendous progress has been made in the preparation of cooper-based inks for electronic applications, but the antioxidation ability of copper-based nanomaterials within inks or films, that is, long-term reliability upon exposure to water and oxygen, still needs more exploration. In this review, we present a comprehensive overview of copper inks for printed electronics from ink preparation, printing methods and sintering, to antioxidation strategies and electronic applications. The review begins with an overview of the development of copper inks, followed by a demonstration of various preparation methods for copper inks. Then, the diverse printing techniques and post-annealing strategies used to fabricate conductive copper patterns are discussed. In addition, antioxidation strategies utilized to stabilize the mechanical and electrical properties of copper nanomaterials are summarized. Then the diverse applications of copper inks for electronic devices, such as transparent conductive electrodes, sensors, optoelectronic devices, and thin-film transistors, are discussed. Finally, the future development of copper-based inks and the challenges of their application in printed electronics are discussed.

Copper inks for printed electronics: a review.

Robust bonding and thermal-stable Ag–Au joint on ENEPIG substrate by micron-scale sinter Ag joining in low temperature pressure-less

Pressureless and low-temperature sinter-joining on bare Si, SiC and GaN by a Ag flake paste

Ag sinter joint is gaining greater attention for use with wide-bandgap die attach modules in high-temperature applications. In this study, we have investigated both the necking growth of Ag grain and the mechanical properties of sintered Ag with different Ag particle shapes in a low-temperature, pressure-less sintering process in air and N2. The necking growth ratio of Ag grains plays an important role in the mechanical properties of sintered Ag. Large necking growth corresponds to stronger tensile fracture strength of sintered Ag paste and stronger die shear strength. Self-generated Ag nanoparticles (AgNPs) with of a size less than 10 nm were observed under an air atmosphere, an atmosphere that produces larger necking growth than does that of N2. In addition, self-generated AgNPs were observed in sintered Ag flake particles specimens. Ag flake particles also lead to larger necking growth than that of sintered Ag spherical particle specimens. Self-generated AgNPs accumulated together due to their high surface energy, thus improving the necking growth of Ag grain and leading to a stronger fracture strength. In addition, it was revealed that both the oxygen element and the grain residual strain of initial Ag particles play important roles in the AgNPs self-generation during the sintering process. This study enhances our understanding of the sintering mechanism of Ag particles and the relationship between grain necking growth and its mechanical properties.

Necking growth and mechanical properties of sintered Ag particles with different shapes under air and N2 atmosphere

https://onlinelibrary.wiley.com/doi/pdf/10.1002/admi.201901550

Well-Controlled Decomposition of Copper Complex Inks Enabled by Metal Nanowire Networks for Highly Compact, Conductive, and Flexible Copper Films

Effect of electroless Cu depositions for micro-via structure and thermal cycling reliability

Sintering of silver is a popular method for forming interconnections in power electronics. Owing to their large size and spherical shape, micron- and submicron-sized Ag particles synthesized by a polyol method (denoted as polyol Ag particles) are not expected to undergo low-temperature, pressureless sintering. However, previous studies have shown sound bonding with shear strength of more than 40 MPa at 200 °C with micron and submicron polyol Ag particles. In this work, to understand the bonding mechanism of polyol Ag particles, the sintering behaviors of two Ag pastes, one with polyol Ag particles and another based on hybrid Ag particles consisting of micron-sized Ag flakes and submicron-sized Ag particles, were investigated without any applied pressure at 175 °C via transmission electron microscopy. During the sintering process, Ag nanoparticles formed in situ can significantly accelerate the sintering of the Ag paste, resulting in low electrical resistivity of the sintered Ag paste (9.8 × 10−6 Ω·cm) after only 5 min of sintering at 175 °C. The Ag nanoparticles were likely generated from the reduction of residual Ag ions or the Ag complex in the paste. The results were also verified by washing the Ag particles or adding Ag ions into the paste.

https://link.springer.com/content/pdf/10.1007/s10854-019-02161-8.pdf

Fast and low-temperature sintering of Ag paste due to nanoparticles formed in situ

Effect of polyol synthesis on sintering of microsized Ag particles

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