What Is Silver Paste?
Silver paste is a conductive adhesive consisting of silver particles dispersed in resin.
Solder is used as a conductive bonding method, but with solder the temperature must be raised to nearly 250°C, and the resin parts to be bonded are likely to be damaged by the heat.
On the other hand, silver paste can be sintered at temperatures as low as 100°C, thus minimizing damage to the material. It is often used for the purpose of conducting and fixing electronic components, such as capacitors, to the underlying substrate.
Uses of Silver Paste
Silver paste can be sintered at low temperatures and is widely used for circuit boards of electronic devices, electrodes of displays, and piezoelectric components. In recent years, demand for flexible printed circuit boards (flexible printed wiring boards) has been increasing, and silver paste is used to create wiring on resin film. This type of wiring board can be made at a lower cost than copper foil laminated together.
In dye-sensitized solar cells, which are attracting attention as next-generation solar cells, silver paste is used to create wiring on transparent conductive glass in order to further enhance the conductivity of the conductive glass.
Principle of Silver Paste
Silver paste is a method of obtaining conductivity by bringing the contained silver particles into contact with each other using a hardening reaction caused by heating a resin.
1. Resin
Epoxy resins are mainly used for silver paste, and the relationship between the structure and properties of epoxy resins has been analyzed, and curing agents have also been developed. The curing reaction of epoxy resin proceeds by polymerization reaction between epoxy and curing agent, enabling the creation of a strong three-dimensional bonded structure.
When amines are used as curing agents, polymerization proceeds through the reaction of amines with epoxy groups or amino groups with hydroxyl groups. The initial liquid state changes to a gel-like state with heating, and after a certain time, it transitions to a rubber-like state and finally to a glass-like state.
When the transition to glass is completed, the series of curing reactions is over. The temperature at which the transition to glass occurs is called the glass transition temperature.
2. Silver Particles
As a conductive mechanism, micrometer-sized silver particles contact each other to conduct electricity. To ensure good electrical connection between particles, flat flake-shaped silver particles are generally used instead of spherical particles.
Upon heating, the silver particles are incorporated into the three-dimensional molecular structural changes of the epoxy resin. When heated, the entire material shrinks as it cures, allowing the silver particles to make contact with each other and thus acquire conductivity. In addition to silver particles, gold paste and nickel paste are also available.
Types of Silver Paste
There are many types of resins used as adhesives and conductive particles to be blended, and a great variety of conductive adhesives have been developed and sold in the market. It is necessary to make a selection in consideration of performance, application, cost, usage, and other factors.
Resins used include epoxy, phenol, acrylic, urethane, and silicone resins. For electronic component connection applications, heat reaction curing epoxy systems are the most common.
Epoxy adhesives are characterized by excellent adhesive strength to metals, high heat resistance, and low-volume shrinkage during curing. On the other hand, silver conductive particles are widely used as conductive particles. Silver is common in electronic materials because of its stable conductivity, resistance to oxidation, good storage stability, and high thermal conductivity.
Spherical or flake-shaped particles are used for silver conductive particles, and there are different types depending on the size of the particles and the amount of silver in the mixture, depending on performance.
Other Information on Silver Paste
Thermal Conductivity of Silver Paste
The thermal conductivity of silver alone is very high at 429 W/mK, but the resin used is as low as 1 W/mK, so the overall thermal conductivity of epoxy silver paste is about 30 to 50 W/mK. To increase this thermal conductivity, it is necessary to increase the content of silver particles. However, the amount of resin content decreases accordingly, resulting in a significant decrease in adhesive strength, and manufacturing cost is also a concern.
Furthermore, if the average particle size of silver particles is too small, the issue of heat conduction paths cannot be secured, and if the average particle size of silver particles is too large, sintering becomes difficult. Therefore, in recent years, high thermal conductivity of silver paste with silver nanoparticles has been developed.
This is due to the fact that silver nanoparticles bond the silver particles together and create many paths for heat conduction. Products with thermal conductivity of about 240 W/mK are now available.