A Study on the Casting Process and Classification of Silver Adhesive Conductive Silver Paste

Silver glueConductive silver paste materialIt has the characteristics of high strength, high hardness, wear resistance, and corrosion resistance, and is widely used in fields such as energy, information, aerospace, electronics, biology, and medicine. The ceramic forming method is an important factor in determining the properties of ceramics, and casting is currently one of the commonly used methods for producing electronic ceramic sheets. Today we will introduce the casting process and its classification.

1、 Casting molding process and its workflow

1. Casting molding

Casting moldingTape casting, also known as Doctor blasting or Knife coating, refers to a molding method in which solvents, dispersants, binders, plasticizers, and other components are added to ceramic powders to obtain a uniformly dispersed and stable slurry, which is then used to produce a thin film of the desired thickness on a casting machine.

Compared with other molding processes, casting has many advantages:

1) The equipment process is simple and can be continuously produced;

2) Can prepare single-phase or multi-phase ceramic thin sheet materials;

3) The product has small defects, uniform performance, high production efficiency, and can be operated continuously;

4) Can be produced in both large and small batches, suitable for industrial production;

5) Very suitable for the preparation of ceramic components for large thin plates, which is the biggest feature of casting and is difficult to achieve through compression or extrusion molding processes.

However, there are also some problems with the casting process, mainly due to the high content of binder, resulting in a large shrinkage rate of the blank during the dispensing process, and the thin plate is prone to defects such as cracking, curling, and uneven thickness.

2. Casting process flow

Casting molding was first applied in the fields of papermaking, painting, and plastics, and is a long-standing molding technology that mainly includesPreparation of slurryThe processes of ball milling, mixing, forming, drying, rubber extrusion, and sintering.

The process flow of extrusion molding is shown in the following figure. Firstly, ceramic powder and dispersant are added to a solvent (water or organic solvent), and particle aggregation is opened by ball milling or ultrasonic oscillation, and the solvent wets the powder. Then, a binder and plasticizer are added, and a stable and uniform slurry is obtained through secondary ball milling; Then, the slurry is molded onto a casting machine to obtain a green body; Then, drying is carried out to evaporate the solvent, and the binder forms a network structure between the ceramic powders to obtain a green film; Next, the green film is machined to obtain the desired specific shape; Finally, the desired finished product is obtained through rubber extrusion and sintering treatment.

Process flow chart of ceramic substrate preparation by flow casting method

2、 Classification of Casting Forming Processes

According to the type of solvent, casting can be divided into two categories: water-based casting systems and organic casting systems. With the continuous improvement and perfection of tape casting technology, a series of special tape casting methods, such as gel tape casting, UV initiated polymerization tape casting and isostatic pressure tape casting, have been studied and developed in recent years.

1. Organic extrusion molding

The application of organic solvent casting technology in ceramic preparation has become relatively mature. Due to the low boiling point of organic solutions, it is beneficial for the drying of the green body. At the same time, organic solutions have good solubility for polymer binders, plasticizers, etc., and do not undergo hydrolysis reactions with ceramic particles, ensuring the stability of the slurry. Therefore, ceramic green bodies formed by organic casting have a uniform internal structure, high strength, good flexibility, and are easy to cut and process. Currently, they are widely used in industrial production and laboratories.

Common solvents for organic tape casting include methyl ethyl ketone, toluene, xylene, ethanol, and trichloroethylene. In practical applications, binary azeotropic solvents such as ethanol/toluene and ethanol/trichloroethylene are commonly used.

advantage:

Due to the compatibility, volatility, low latent heat of evaporation, low surface tension, and ability to prevent ceramic powder hydration of organic solvents, the selection range of additives in organic casting systems is wide, the solvent evaporates quickly, the drying time is short, and it is easy to obtain ceramic thin plates with uniform structure, small defect size of the green body, high strength and good flexibility.

Disadvantages:

Organic solvents have certain toxicity, inevitably causing harm to humans and the ecological environment, and have high production costs. The finished product has a high organic content, low density, and is prone to cracking during the rubber dispensing process.

2. Water based extrusion molding

Water based extrusion molding uses water as a solvent and water-soluble polymers as binders, overcoming the disadvantages of organic extrusion systems such as environmental hazards, high costs, and low product density. It is suitable for large-scale production.

The commonly used adhesives for water-based casting include polyvinyl alcohol (PVA), cellulose (MC), and polyacrylic acid (PAA), while the commonly used plasticizers include dibutyl phthalate (DBP), glycerol (glycerol), polyethylene glycol (PEG), etc.

advantage:

1) Pollution free and environmentally friendly;

2) Low cost and convenient for large-scale production.

Disadvantages:

1) There are few types of water-soluble dispersants and binders, so the range of choices is narrow and the effect is poor;

2) The solvent evaporation rate is low, and the film is not easy to dry, making it particularly unsuitable for casting thicker blanks;

3) When the adhesive content is high and the adhesive is discharged, the shrinkage rate of the blank is large;

4) Due to the presence of hydrogen bonds, the powder tends to agglomerate;

5) Sensitive to changes in process parameters, resulting in lower quality of the green film;

6) The raw material is brittle and prone to cracking during drying.

3. gel tape casting

Gel tape casting is a new process that combines the curing mechanism of gel casting with tape casting method.

This method involves dispersing ceramic powder in an aqueous solution containing organic monomer crosslinking agents to prepare a low viscosity, high solid volume fraction concentrated suspension (with a volume fraction greater than 50%). Then, initiators and catalysts are added to initiate the polymerization of organic monomers under certain temperature conditions, increasing the viscosity of the suspension and leading to in-situ solidification, resulting in a green body with certain strength that can be machined.

The slurry used for water-based gel tape casting is composed of ceramic powder, organic monomer, cross-linking agent, solvent, dispersant, plasticizer and other components.

advantage:

It can greatly reduce the amount of organic matter used in the slurry, increase the solid content of the slurry, thereby improving the density and strength of the green body, greatly reducing environmental pollution, and significantly reducing production costs.

3、 Application

The casting method has become an important method for preparing large-area and ultra-thin ceramic substrates due to its characteristics of simple equipment, continuous operation, high production efficiency, and uniform green body properties. It is widely used in fields such as electronics and energy industries.

It can be used for the production of ceramic tiles for monolithic capacitors, Al2O3 substrates for thick and thin film circuits, piezoelectric ceramic membranes, structural ceramic sheets, capacitors, thermistors, ferrites and piezoelectric ceramic bodies, hybrid integrated circuit substrates, etc. It also has applications in the field of ceramic cover plates for mobile phones