The demand for conductive paste in the photovoltaic industry and electronic packaging field is undergoing a transformation from high silver content to low silver or even silver free. Although silver powder has excellent conductivity and chemical stability, it is expensive, has limited resources, and is prone to electromigration. In contrast, copper's conductivity is second only to silver and its cost is about 1/100 of silver. Therefore, using low-cost copper powder instead of silver powder has become an important way to reduce costs. However, the surface of copper powder is prone to oxidation to form a thin layer of electrically insulating oxide, resulting in a serious decrease in conductivity and reliability. This makes preventing copper powder oxidation a core technical challenge in achieving the replacement of silver paste with copper paste.

Oxidation mechanism and performance degradation of copper powder

Copper is different from metals such as aluminum and nickel in that it is difficult to form a dense and stable intrinsic passivation layer on its surface. Therefore, the exposed copper surface will be continuously oxidized and corroded by oxygen and water vapor in the air. The smaller the particle size and larger the specific surface area of copper powder, the easier it is to rapidly oxidize to produce products such as cuprous oxide (Cu ₂ O) and copper oxide (CuO). This oxide insulation layer significantly reduces the conductivity of copper powder and hinders particle sintering connection, resulting in degradation of the performance of the conductive paste. Especially during the sintering process of the front electrode of photovoltaic cells (often requiring high temperatures exceeding 500 ℃), if copper powder is not protected, it will be severely oxidized and unable to form a good metal conductive network. In addition, in high temperature and high humidity environments, the growth of oxide layer can also cause the conductivity to deteriorate over time, affecting the lifespan of the device. Therefore, inhibiting surface oxidation of copper powder is crucial for maintaining its conductivity, sintering activity, and long-term stability.

The main methods for surface antioxidant treatment of copper powder

Researchers and engineers have developed various surface antioxidant treatment techniques to address the issue of copper powder being prone to oxidation. Constructing a physical or chemical protective layer on the surface of copper powder can block oxygen contact or passivate active sites, thereby slowing down or even preventing oxidation from occurring. The main methods include organic coating protection, inorganic coating, self passivation alloying modification, and surface reduction passivation treatment. The following text introduces the principles and typical developments of each method separately.

Organic coating protection
Fatty acid and polymer coating: Long chain fatty acids such as oleic acid and stearic acid can bond with copper surfaces through carboxyl groups, forming a hydrophobic organic layer to isolate oxygen and moisture. Soaking copper powder in an oleic acid acetone solution for surface modification treatment can form a protective film of oleate on the surface of the powder. Experiments have shown that the purity and antioxidant properties of fine copper powder treated with this method are improved. Similarly, resins or polymers can also be used to coat the surface of copper powder. For example, the in-situ polymerization of polyaniline on the surface of copper powder to form a conductive polymer coating can effectively improve the antioxidant storage stability of copper powder in air. In addition, adding a certain amount of polymer binder (such as ethyl cellulose, acrylic resin, etc.) to pre coat copper powder during the preparation of conductive adhesive has also been proven to reduce the oxidation of copper powder and extend the shelf life of the slurry.

Silane coupling agent modification: One end of the organic silane coupling agent molecule contains a hydrolyzable silane group, which can react with copper surface oxides/hydroxyl groups to form silicon oxygen bonds, and the other end has an organic functional group to provide hydrophobic protection. The formation of an organic silicon film on the surface of copper powder through silane treatment can have a dual effect of anti-oxidation and improving dispersibility. Introducing silane coupling agent KH-902 into epoxy conductive adhesive to treat copper powder, the results showed that adding 3% significantly improved the oxidation resistance of copper powder during high-temperature curing process, and made the copper powder more evenly dispersed in the colloid. Research has shown that silane coupling agents can effectively inhibit the oxidation of copper powder below about 200 ℃, and their use in low-temperature curing slurry systems can help improve the compatibility and stability of the powder with organic carriers.

Inorganic coating
Precious metal coating: Coating a dense layer of inert metal on the surface of copper powder is an effective means of completely preventing oxidation. Silver coated copper powder has achieved mature technology and application. By chemical displacement plating or chemical reduction plating, silver shell layers of tens to hundreds of nanometers can be deposited on the surface of copper particles, forming a "copper core silver shell" structure. The silver shell layer isolates copper from the environment, greatly enhancing its antioxidant stability, while utilizing the high conductivity of silver to ensure excellent conductivity of the composite powder. Silver coated copper powder is expected to replace most pure silver powder and has been applied in fields such as conductive adhesives, electromagnetic shielding coatings, conductive inks, etc. It has also been used in photovoltaic slurries to achieve a silver reduction solution by reducing the silver content to 20% -50%. Another type is nickel plated copper powder, which can also act as a barrier layer to prevent copper oxidation. However, nickel has lower conductivity, and the thickness of the coating needs to be controlled to balance oxidation resistance and conductivity. In addition, copper thin wires coated with palladium/gold have been maturely applied in electronic packaging. For example, bonded copper wires are often coated with a layer of palladium and extremely thin gold to prevent copper from oxidizing and becoming brittle before high-temperature secondary bonding. Overall, the precious metal coating method has the best antioxidant effect, but still introduces a certain amount of precious metals, resulting in higher costs and precise control of coating thickness.

The application prospects of copper powder in conductive pastes and electronic packaging are broad, but oxidation has been the main obstacle between laboratory achievements and actual products. Recent studies have shown that various strategies such as organic coating, inorganic coating, self passivation alloying, and surface reduction passivation can significantly improve the antioxidant properties of copper powder, enabling it to maintain excellent conductivity within a wide process window. Different methods have their own advantages and disadvantages, and need to be selected or combined for specific applications.

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