High thermal conductivity adhesives with different fillers

High thermal conductivity adhesiveCombining the processability of polymer resins with the high thermal conductivity of fillers, it is widely used in electronic packaging industries such as smartphone chip packaging and high-power LED lighting. Among them, polymer resin provides sufficient adhesive strength and mechanical strength.

High thermal conductivity adhesives have been widely used in the electronics and electrical industries. They can be used as thermal interface materials to dissipate the heat generated by electronic components and extend the service life of electronic devices. Thermal conductive fillers mainly determine their heat dissipation ability. Common fillers include silicon carbide (SiC), aluminum nitride (AlN), aluminum oxide (Al2O3), magnesium oxide (MgO), silicon nitride (SiN), and other metals (Ag, Cu, Al, etc.) and metal oxides.

2.1 Thermal conductive adhesive for non-metallic fillers

According to different usage methods, there are two types of thermal conductive adhesives for non-metallic fillers: insulating adhesive and conductive adhesive. The combination of different fillers plays a decisive role in the performance of thermal conductive adhesive.

Zhang Xiaohui et al. prepared a series of thermal conductive adhesives containing epoxy resin and different fillers (SiC, AlN, Al2O3). The research results indicate that there is a critical point for the filler content. This can be attributed to the effective internal thermal chain. Compared with these fillers, SiC fillers have a higher thermal conductivity when the filler content is 53.9wt%. This is because SiC fillers are inexpensive, have high thermal conductivity, and SiC composite materials also maintain good mechanical properties.

Teng uses surface functionalized inorganic fillers such as BN and MWCNTs alone or in combination to prepare epoxy composite materials. The results indicate that due to the synergistic effect of mixed fillers, the thermal conductivity of mixed filler composite materials is higher than that of single filler composite materials. The thermal conductivity of epoxy composites containing 30% modified BN and 1% functionalized MWCNTs is 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 and prepared particles with different morphologies, including spheres, bamboo, cylindrical tubes, and collapsed tubes, using nano boron nitride as raw material, as shown in Figure 3. The results indicate that the composite material with spherical particles has a lower thermal conductivity, while the composite material with boron nitride collapse has a higher thermal conductivity, and the surface area of spherical particles is larger, resulting in significant heat loss through these surfaces. On the contrary, there is a larger effective contact area between the collapsed BN particles. When heat is transferred linearly along the collapsed BN particles, the heat resistance is very low, so the composite material exhibits good thermal conductivity.

Apart from what was mentioned earlierconductive fillerCommon non-metallic conductive fillers include carbon based fillers such as graphite, carbon black, carbon nanoparticles, and carbon fiber tubes, which are widely used in the printed electronics industry. Among them, graphene and carbon nanotubes, as two ideal high-quality fillers, have received widespread attention. Graphene is a two-dimensional single atomic layer nanomaterial with advantages such as high mechanical strength and strong electrical and thermal conductivity. Its conductivity is 108 S/m, far superior to metal copper and silver. The basic skeleton of the carbon nanotube wall is a hexagonal carbon ring, which has good conductivity and mechanical properties. Its aspect ratio can reach over 1000, making it easier to build conductive pathways. These two high-performance new carbon based conductive fillers have great potential for development and broad application prospects, but their dispersibility is poor, stability needs improvement, and preparation costs are expensive. They have not yet been widely produced and promoted in the market.

2.2 Thermal conductive adhesive for metal fillers

Compared with other metal fillers, copper powder not only has conductivity similar to silver (at 20 ℃, silver has a resistivity of 1.59 × 10-6 Ω· cm and copper has a resistivity of 1.72 × 10-6 Ω· cm), but also as a low-cost and widely sourced base metal, copper has good migration resistance. However, in practical applications, due to its active chemical properties, copper is easily oxidized in air or high temperature environments, generating difficult to conduct copper oxide or cuprous oxide, which increases its resistance. The current focus of research is still on improving the oxidizability of copper, making electronic pastes filled with copper more competitive in the market.

Obtained by silver plating on the surface of copper powderSilver coated Copper PowderAs a conductive phase, it is currently the main method to improve the oxidation problem of copper paste. This method not only improves the disadvantage of copper being easily oxidized, but also reduces the cost of the system compared to pure silver fillers, while having good electrical and thermal conductivity. However, the stability of silver coated copper powder in use is poor, and further research is needed to improve the stability and performance of copper coated silver powder in use.

In metal filling materials, silver has excellent thermal conductivity (pure silver has a thermal conductivity of 400W/(m • K)) and oxidation resistance. In the electronics industry, as the functional phase of conductive paste, silver and its compounds have higher cost-effectiveness. Therefore, their applications and research are also the most extensive, with about 80% of electronic paste products having various silver powders as the main functional phase. When sintering occurs, the interfacial resistance of silver powder will significantly decrease. However, due to the difficulty of sintering at medium temperatures, preparing silver based samples with high thermal conductivity at low temperatures remains a huge challenge. Another drawback of silver conductive adhesive is that under the action of an electric field, silver will undergo electron migration, resulting in a decrease in the conductivity of the conductive adhesive and thus affecting the lifespan of the device.

After extensive research and experimentation, it has been found that the density and resistivity of conductive silver paste thick film paste are significantly affected by the morphology and content of silver powder. Therefore, the conductivity of silver paste can be improved by improving the morphology and particle size of silver powder. Therefore, in order to prepare a conductive paste that is denser and has better conductivity and thermal conductivity after sintering, micron and nanometer sized conductive silver powders can be used for compounding. According to the theory of closest packing of powders, the combination of powders with different particle sizes can reduce the porosity of the separated system, make the sintered conductive film layer denser, and have better conductivity. Moreover, due to the electrical contact between spherical particles and the linear or surface contact between sheet-like particles, the electrical resistance of spherical particles is greater than that of sheet-like particles when the volume and proportion are the same. After coating to a certain thickness, the silver powder flakes overlap in a fish scale shape and have good flowability, making the sintering of the silver paste denser and exhibiting better conductivity. At the same time, the thermal conductivity of the system has also been significantly improved.

Silver paste, as the most widely used thermal conductive adhesive filler, is usually solved by using flake and nano-sized silver powder to address the silver migration problem during use. For the problem of high usage and cost of silver powder in silver paste, it is achieved by doping base metals (Ni, Al, Cu, etc.) or other conductive substances into the silver powder to reduce the amount of silver powder in the system and achieve the goal of cost reduction.