Optimizing Silver Paste Conductivity with Controlled Convection for Nanowrinkle Growth.

Conductive silver paste plays a crucial role as an interconnecting material between electrodes and circuits in electronic circuits and solar cells. The quality of the silver paste is greatly influenced by the preparation of the conductive-phase silver powder and the sintering process. This study investigated the impact of fluid dynamics on the preparation of silver powder. Combined with X-ray diffractometer characterization and molecular dynamics simulation, the formation mechanism of wrinkled silver powder was explained. The wrinkled silver powder replaced the traditional smooth spherical silver powder, and the point contact between the smooth silver powder turned into a line and surface contact. After mixing and sintering with the micrometer flake silver powder, the electrical conductivity and sintering morphology of the silver paste were improved. Compared with the silver content of conventional silver paste (≥75 wt %), the silver paste of (9.23 ± 0.68) × 10-6 Ω cm can be prepared by curing at 250 °C for 45 min when wrinkled powder/flake powder = 1:1 and silver paste content was only 66.7%. This research work provides a new idea for the morphology control of submicrometer silver powder, which has important applications in the field of low-temperature silver paste for new N-type batteries.

Rheologically Assisted Design of Conductive Adhesives for Stencil Printing on PCB.

Driven by the need to deliver new, lead-free, eco-friendly solder pastes for soldering electronic components to Printed Circuit Boards (PCB), electrically conductive adhesives (ECAs) based on epoxy, carbon nanotubes (CNT), and exfoliated graphite (EG) were designed. The rheology of the adhesives prepared is paramount for the success of the deposition process, which is based on stencil printing. Thus, a rheological analysis of the process was first performed. Then, an experimental protocol was defined to assess the relevant viscoelastic characteristics of the adhesives for stencil printing application. Different composite formulations of epoxy/CNT/EG were produced. Their rheological characteristics were established following the designed protocol and benchmarked with a commercial solder paste. The thermal and electrical properties of the composite formulations were also characterized. As a result, a new, electrically conductive adhesive was delivered with potential to be an eco-friendly alternative to the solder paste currently used in stencil printing of PCB.

A Novel Preparation Method of Electrically Conductive Adhesives by Powder Spraying Process.

In a conventional electrically conductive adhesive (ECA) preparation process, typical ECAs are made by adding an appropriate amount of electrically conductive fillers, such as silver, into a polymer matrix, such as epoxy resin, to form a uniformly dispersed mixture by mixing and stirring operations. However, during the preparation process, secondary pollution and mass loss are caused by the vigorous mixture process. At the same time, the stirring operation introduces many small and stable bubbles, which affect the electrical conductivity of the ECAs. In light of these problems with the conventional preparation of ECAs, we developed a novel ECA preparation method that employs a powder spraying process: silver flakes are sprayed into the epoxy resin with a certain mass fraction to form formulated pastes. The as-prepared ECAs exhibited excellent properties compared with those prepared by the conventional process. This proves that the powder spraying process is feasible and superior to the conventional process.

Easily Synthesized Polyaniline@Cellulose Nanowhiskers Better Tune Network Structures in Ag-Based Adhesives: Examining the Improvements in Conductivity, Stability, and Flexibility.

It is essential to develop a novel and versatile strategy for constructing electrically conductive adhesives (ECAs) that have superior conductivity and high mechanical properties. In this work, easily synthesized polyaniline@cellulose (PANI@CNs) nanowhiskers with a high aspect ratio and excellent solubility in 1,4-dioxane were prepared and added to conventional Ag-containing adhesives. A small amount of PANI@CNs can dramatically tune the structure of the ECAs' conductive network and significantly improve the conductivity of the ECAs. Good solubility of PANI@CNs in solvents brings excellent dispersion in the polymer matrix. Thus, a three-dimensional (3D) conducting network formed with dispersed PANI@CNs and Ag flakes can enhance the conductivity of ECAs. The conductivity of the ECAs (with 1.5 wt% PANI@CNs and 55 wt% Ag flakes) showed three orders of magnitude higher than that of the ECAs filled with 55 wt% Ag flakes and 65 wt% Ag flakes. Meanwhile, the integration of PANI@CNs with Ag flakes in polymer matrices also significantly enhanced the mechanical compliance of the resulted ECAs. The resistivity remained unchanged after rolling the PANI@CNs-containing ECAs' film into a 4 mm bending radius for over 1500 cycles. A bendable printed circuit was fabricated using the above PANI@CNs-containing ECAs, which demonstrated their future potential in the field of flexible electronics.

A Novel Seedless TSV Process Based on Room Temperature Curing Silver Nanowires ECAs for MEMS Packaging.

The through-silicon-vias (TSVs) process is a vital technology in microelectromechanical systems (MEMS) packaging. The current via filling technique based on copper electroplating has many shortcomings, such as involving multi-step processes, requiring sophisticated equipment, low through-put and probably damaging the MEMS devices susceptible to mechanical polishing. Herein, a room temperature treatable, high-efficient and low-cost seedless TSV process was developed with a one-step filling process by using novel electrically conductive adhesives (ECAs) filled with silver nanowires. The as-prepared ECAs could be fully cured at room temperature and exhibited excellent conductivity due to combining the benefits of both polymethyl methacrylate (PMMA) and silver nanowires. Complete filling of TSVs with the as-prepared 30 wt% silver nanowires ECAs was realized, and the resistivity of a fully filled TSV was as low as 10-3 Ω·cm. Furthermore, the application of such novel TSV filling process could also be extended to a wide range of different substrates, showing great potential in MEMS packaging, flexible microsystems and many other applications.

In Situ Generation of Photosensitive Silver Halide for Improving the Conductivity of Electrically Conductive Adhesives.

Electrically conductive adhesives (ECAs) can be regarded as one of the most promising materials to replace tin/lead solder. However, relatively low conductivity seriously restricts their applications. In the present study, we develop an effective method to decrease the bulk electrical resistivity of ECAs. KI or KBr is added to replace the lubricant and silver oxide layers on silver flakes and to form photosensitive silver halide. After exposure to sunlight, silver halide can photodecompose into silver nanoparticles that will sinter and form metallic bonding between/among flakes during the curing process of ECAs, which would remarkably reduce the resistivity. The modified micro silver flakes play a crucial role in decreasing the electrical resistivity of the corresponding ECAs, exhibiting the lowest resistivity of 7.6 × 10-5 Ω·cm for 70 wt % loaded ECAs. The obtained ECAs can have wide applications in the electronics industry, where high conductance is required.

Hydrothermal Fabrication of Silver Nanowires-Silver Nanoparticles-Graphene Nanosheets Composites in Enhancing Electrical Conductive Performance of Electrically Conductive Adhesives.

Silver nanowires-silver nanoparticles-graphene nanosheets (AgNWs-AgNPs-GN) hybrid nanomaterials were fabricated through a hydrothermal method by using glucose as a green reducing agent. The charge carriers of AgNWs-AgNPs-GN passed through defect regions in the GNs rapidly with the aid of the AgNW and AgNP building blocks, leading to high electrical conductivity of electrically conductive adhesives (ECA) filled with AgNWs-AgNPs-GN. The morphologies of synthesized AgNWs-AgNPs-GN hybrid nanomaterials were characterized by field emission scanning electron microscope (FESEM), and high resolution transmission electron microscopy (HRTEM). X-ray diffraction (XRD) and laser confocal micro-Raman spectroscopy were used to investigate the structure of AgNWs-AgNPs-GN. The resistance of cured ECAs was investigated by the four-probe method. The results indicated AgNWs-AgNPs-GN hybrid nanomaterials exhibited excellent electrical properties for decreasing the resistivity of electrically conductive adhesives (ECA). The resistivity of ECA was 3.01 × 10-4 Ω·cm when the content of the AgNWs-AgNPs-GN hybrid nanomaterial was 0.8 wt %.