Research on the Application of High Thermal Conductivity Adhesives

Abstract: With the rapid development of electronic technology, electronic products are miniaturized and centralized.

Key words: high thermal conductivity adhesive filler nano silver thermal conductivity

With the rapid development of electronic technology, electronic products have been rapidly developed and popularized, and the assembly density of electronic components has increased and the volume has been continuously reduced, making electronic components and products show a trend of miniaturization, lightening, and compactness.

1 Heat transfer theory of thermally conductive adhesives

Metal materials have thermal vibration of grains, so they have good thermal conductivity

A schematic diagram of conventional die attach is shown in Figure 1

Figure 1 Schematic diagram of die attach

The microscopic surfaces of electronic devices are rough rather than smooth, and their effective contact area is only about 10%

Figure 2 Schematic diagram of heat transfer of electronic components

2 High thermal conductivity adhesives with different fillers

High thermal conductivity adhesives combine the processability of polymer resins and the high thermal conductivity of fillers, and are widely used in electronic packaging industries such as smartphone chip packaging and high-power LED lighting

High thermal conductivity adhesives have been widely used in the electronic and electrical industries, they can be used as thermal interface materials to dissipate the heat generated by electronic components and can prolong the life of electronic equipment

2.
1 Thermally conductive adhesive for non-metallic fillers

Depending on the method of use, there are two types of thermally conductive adhesives for non-metallic fillers: insulating adhesives and conductive adhesives
.
The combination of different fillers plays a decisive role in the performance of the thermal conductive adhesive
.

Zhang Xiaohui and others prepared a series of thermally conductive adhesives containing epoxy resin and different fillers (SiC, AlN, Al2O3)
.
The results show that there is a critical point for filler content
.
This can be attributed to an efficient thermal conduction chain inside
.
Compared with these fillers, the thermal conductivity of SiC fillers is higher when the filler content is 53.
9wt%
.
This is because SiC fillers are inexpensive and have high thermal conductivity, while SiC composites also maintain good mechanical properties
.

Teng used surface-functionalized inorganic fillers such as BN and MWCNTs to prepare epoxy composites either alone or in combination
.
The results show that the thermal conductivity of the hybrid filler composites is higher than that of the single filler composites due to the synergistic effect of the hybrid fillers
.
The thermal conductivity of epoxy composites containing 30% modified BN and 1% functionalized MWCNTs was significantly higher than that of epoxy composites containing 30% pure BN and 1% pure MWCNTs
.

Tang et al.
studied the effect of filler morphology on thermal conductivity, using nano-boron nitride as raw material to prepare particles of different shapes, including spheres, bamboos, cylindrical tubes and collapsed tubes, as shown in Figure 3
.
The results show that composites with spherical particles have lower thermal conductivity, while boron nitride-collapsed composites have higher thermal conductivity, and the spherical particles have a larger surface area, so heat loss through these surfaces is high
.
On the contrary, there is a larger effective contact area between the collapsed BN particles
.
When the heat is transferred along the linear direction of the collapsed BN particles, the heat resistance is very low, so the composite exhibits good thermal conductivity
.

Fig.
3 Different morphologies of BN particles

In addition to the conductive fillers mentioned above, commonly used non-metallic conductive fillers include carbon-based fillers such as graphite, carbon black, carbon nanometers, and carbon fiber tubes.
These new conductive fillers are widely used in the printed electronics industry
.
Among them, graphene and carbon nanotubes have received extensive attention as two ideal high-quality fillers
.
Graphene is a nanomaterial with two-dimensional single atomic layer, which has the advantages of high mechanical strength and strong electrical and thermal conductivity.
Its electrical conductivity is 108 S/m, which is much better than that of metal copper and silver
.
The basic skeleton of the carbon nanotube wall is a carbon six-membered ring, which has good electrical conductivity and mechanical properties, and its aspect ratio can reach more than 1000, making it easier to build a conductive path
.
These two new carbon-based conductive fillers with excellent performance have great potential for development and have broad application prospects, but their dispersibility is poor, their stability needs to be improved, and their preparation costs are expensive, so they have not been mass-produced and promoted in the market.
use
.

2.
2 Thermally conductive adhesive for metal fillers

Compared with other metal fillers, copper powder not only has electrical conductivity similar to silver (at 20°C, the resistivity of silver is 1.
59×10-6Ω·cm, and the resistivity of copper is 1.
72×10-6Ω·cm), And as a cheap and widely available base metal, copper also has good migration resistance
.
However, in practical applications, due to its active chemical properties, copper is easily oxidized in air or high temperature environments, resulting in copper oxide or cuprous oxide that is difficult to conduct electricity, and its resistance increases
.
The current research focus is still to improve the oxidizability of copper, so that the electronic paste with copper as filler has stronger market competitiveness
.

Silver plating on the surface of copper powder to obtain silver-coated copper powder as a conductive phase is the main method to improve the oxidation problem of copper paste at present
.
The method not only improves the disadvantage of copper being easily oxidized, but also reduces the cost of the system compared with pure silver fillers, and has good electrical and thermal conductivity at the same time
.
However, the stability of silver-coated copper powder in use is not good, and further research is still needed to improve the stability of copper-coated silver powder in use and improve its performance
.

Among metal fillers, silver has excellent thermal conductivity [the thermal conductivity of pure silver is 400W/(m•K)] and oxidation resistance
.
In the electronic industry, as the functional phase of conductive paste, silver and its compounds have higher cost performance.
Therefore, its application and research are also the most.
About 80% of the main functional phase of electronic paste products are various types of silver powder
.
When the sintering behavior occurs, the interfacial resistance of the silver powder decreases significantly
.
However, it is still a great challenge to prepare silver-based samples with higher thermal conductivity at low temperatures because sintering is difficult at intermediate temperatures
.
Another defect of the silver conductive adhesive is that, under the action of an electric field, silver will generate electron migration, so that the conductivity of the conductive adhesive decreases, thereby affecting the life of the device
.

After a lot of research and experiments, it is concluded that the compactness and resistivity of conductive silver paste thick film paste are significantly affected by the morphology and content of silver powder
.
Therefore, the electrical conductivity of the silver paste can be improved by improving the morphology and particle size of the silver powder
.
Therefore, in order to prepare a denser conductive paste with better electrical and thermal conductivity after sintering, micro-scale and nano-scale conductive silver powder can be selected for compounding
.
According to the theory of the closest packing of powders, the combination of powders with different particle sizes can reduce the porosity of the split system, so that the sintered conductive film layer is denser and has better conductivity
.
Moreover, since the spherical particles are in the form of electrical contact, and the contact between the flake particles is line contact or surface contact, this makes the resistance of spherical particles greater than that of flakes under the same volume and proportion.
particles
.
After coating to a certain thickness, the flaky silver powders overlap in a fish-scale shape and have good fluidity, which makes the sintering of the silver paste more densified and shows better electrical conductivity
.
At the same time, the thermal conductivity of the system has also been significantly improved
.

Silver paste is the most widely used thermally conductive and conductive adhesive filler.
In order to solve the problem of silver migration in use, flake and nano-scale silver powder are usually used to solve it
.
For the problems of large usage and high cost of silver powder in silver glue, the silver powder is doped with base metals (Ni, Al, Cu, etc.
) or other conductive substances to reduce the amount of silver powder in the system to achieve the purpose of reducing costs.

.

2.
3 Low temperature sintering nano-silver high thermal conductivity conductive adhesive

Currently, as the electronics industry continues to integrate more functions in smaller packages, the electrical, thermal and mechanical properties of existing die attach materials such as solder and conductive epoxy will not meet the higher demands for performance and reliability.
request
.
Due to the high electrical and thermal properties of silver, micron-scale silver pastes are widely used in microelectronic packaging
.
However, the high sintering temperature (>600°C) makes it unsuitable for semiconductor device interconnection
.
Other techniques for reducing the processing temperature of silver interconnects, mainly through external pressure applications, are either not fully developed or technically difficult to achieve and cost prohibitive
.
In order to solve this problem, more and more scholars have begun to study low-temperature sintered nano-silver as a new type of chip attach material
.
Therefore, nanosilver has the potential to be a lead-free, high-performance interconnect material, especially for semiconductor device-metal-substrate interconnects
.

Chung et al.
made nano-scale silver paste by die-cutting 30nm silver powder in an organic binder system with ultrasonic stirring
.
Some important aspects of nanosilver paste are introduced, especially its electrical, thermal and thermomechanical properties after low temperature sintering as a substitute for solder/epoxy in chip attachment of semiconductor devices
.
The resistivity measured from a screen-printed resistor on an insulating substrate was 2.
6 × 105 Ω·cm, which was sintered at a temperature of 280 °C for about 10 min
.
The thermal conductivity obtained by the laser flash method is 240W/(m·K), and the density of sintered silver is about 80%, both of which are lower than those of bulk; the coefficient of thermal expansion (CTE) of sintered silver measured by a dilatometer is 19×10-6/℃, almost the same as bulk silver
.
The microstructure of the sintered joint does not contain the macroporosity observed in reflow soldering
.
These results demonstrate that nanoscale silver pastes sintered at low temperatures are excellent substitutes for solder or epoxy for die attach
.

3 Ways to improve thermal conductivity

High thermal conductivity adhesives not only have excellent thermal conductivity, but also have good mechanical properties and bonding properties
.
With the rapid development of miniaturization and integration of electronic components, new challenges have been raised, especially how to improve the thermal conductivity of adhesives
.

The wettability of the filler surface will affect the dispersion of the particles in the matrix resin, the bonding between the matrix and the filler, and the interfacial thermal barrier between the filler particles and the matrix resin, so the filler surface should be modified
.
For example, the thermal conductivity of MgO particles can be changed from 1.
16W/(m·K) to 2.
136W/(m·K) by pretreating the surface of MgO particles with coupling agent
.

The compounding method of filler and matrix resin, different filler configuration when mixing different filler particles, and different particle size distribution have influence on the thermal conductivity and viscosity of the composite material
.
In addition, the temperature and pressure during the molding process can also affect the performance of thermally conductive adhesives
.

Reducing the particle size of the filler can improve the thermal conductivity
.
For example, the thermal conductivity of nano AIN filler is about 320W/(m•K), while that of ordinary AIN filler is about 36W/(m•K)
.
In addition, the highly oriented fillers and the formation of oriented fibrous structures can significantly improve thermal conductivity
.
Adding seeds to SiC particles and orienting the seeds, the thermal conductivity of the orientation can reach 120W/(m•K), which is about 3 times that of ordinary SiC
.

4 Conclusion

With the development of miniaturization of electrical and electronic components, heat dissipation has become a top priority
.
The rapid development of the communication industry undoubtedly provides broad prospects for the application of thermally conductive materials in aerospace, electrical insulation, electronic packaging and other fields
.
Traditional thermal conductive materials not only have poor processability and lack of raw materials, but also most of the macromolecular polymers of their matrix materials have poor thermal conductivity, which makes the use of traditional materials extremely limited
.
In order to meet the development needs of society and science and technology, the research and development of new high thermal conductivity materials will bring more opportunities and challenges for different fields
.