Preparation process and development trend of gold conductor slurry

This article provides an overview of the development of electronic slurries. It summarizes the development and preparation processes of the various components of traditional gold conductor slurries, as well as the process flow for mixing each component to prepare the gold slurries. Then, it analyzes the problems in the development of the domestic gold slurry industry and makes a certain degree of prediction for the future development trend of gold conductor slurries based on the development of the entire industry.


Introduction
With the rapid development of electronic information technology, military electronic technology has become the key to the national defense force, on the one hand, new materials, new technologies, new processes put forward higher and higher requirements, on the other hand, the development of microelectronic circuits, components or systems mixed integration of high-density assembly technology innovation put forward a higher demand.Electronic paste is a multidisciplinary technology integration of functional materials, is one of the important components of electronic information materials.Common electronic pastes are usually composed of a functional phase, a binder phase or modifier and an organic carrier.Since different electronic pastes have different conductivity requirements, their functional phases are also different, and they are categorized into resistor pastes, dielectric pastes and conductor pastes.The functional phase of conductor pastes is generally composed of precious metals and precious metal mixtures.The functional phase of dielectric pastes is mainly composed of glass, ceramics or a mixture of both.The functional phase of resistive pastes consists mainly of conductive oxides.The bonding phase of the electronic paste is generally composed of glass powder, metal oxides and mixtures of the two, which acts as a binder in the sintering process, so that the electronic paste can be firmly bonded with the substrate, the amount of the bonding phase can affect the sintering temperature is high or low, but also affects the solder properties of the paste after sintering.Organic carriers can regulate the rheological properties of the electronic paste, determines the printing performance of the electronic paste, while the organic carrier content of the high and low impact of the electronic paste sintering shrinkage, warpage and other properties, can directly affect the paste and the substrate of the co-firing match.
As a common precious metal, gold has an extremely low resistivity and excellent chemical inertness second only to silver.At the same time, since gold conductors have no Electromigration tendency in the normal environment and can work in harsh environments, gold conductor pastes are often used in electronic technology industries with high performance requirements.Gold conductor slurry is composed of ultra-fine gold powder with high purity, inorganic binder, and organic carrier.Gold conductor slurry is a necessary material for thick film multilayer systems sintered in harsh environments.By screen printing on the top, bottom, or middle layers, sintering forms thin, dense, and highly conductive conductors between multi-layer wiring.The combination of Low Temperature Co-fired Ceramics (LTCC) technology developed by Hughes Corporation has made it widely used in the field of military electronic equipment.In addition to its applications in the military field, gold conductor pastes also play an important role in electronic technology applications such as hybrid circuits, Large Scale Integration(LSI), semiconductor packaging, and multi-layer wiring circuits [1,2,3] .

Development and preparation of gold conductor slurry 2.1 Development andpreparation offunctional phase
The main function of functional phase in thick film conductor slurry is to act as a conductor to conduct electricity.In addition, the functional phase in surface wiring must be able to interconnect with external passive or active devices, and the functional phase in hole filling slurry must have good heat dissipation performance.In order to meet these performance requirements, strict requirements have been put forward for the dispersion, surface morphology, and size of functional phases.In order to achieve good screen printing performance of the gold paste, it is required that the functional phase particle size should preferably be within 5μm, the shape is mostly spherical, quasi spherical, or flaky.Gold powder is generally divided into two types, one is 1-2μmspherical gold powder, a mixture of flake gold powder and spherical gold powder, including hexagonal, triangular, etc., with a thickness generally of 0. 1μm.Particle size between 1-10μm.The particle size of spherical gold powder is relatively small, generally ranging from 0.2 to 0.5μm [4,5,6] .Since Klaus Lutz et al. [7] invented the liquid phase reduction method to prepare spherical gold powders, the preparation of gold powders has been fully developed.Research on the preparation of gold powders abroad has started early and developed relatively mature.However, foreign literature has described its development less, and its preparation process research is mostly published in the form of patents, and its product technology is also more confidential.For example,US4023961, US3771996, US3892557, and US3725035 [8,9,10,11] .Due to the outstanding performance of gold paste, it is widely used in the electronic paste industry, especially in the field of military industry, which promotes the domestic research and development of domestic gold paste, in which the preparation of gold powder has also been deeply studied.The preparation method of gold powder has been studied and applied for many years, and the mainstream preparation process includes mechanical ball milling method, gas-liquid atomization method and chemical reduction method.Chemical reduction method: Chemical reduction method refers to the use of different reducing agents under appropriate conditions from the gold precursor compound solution to reduce the gold, and in the reduction process by adding dispersants to regulate the gold nucleation and growth process, and ultimately get the gold conductor slurry required by the class of spherical or flaky gold powder.The chemical reaction equation is shown below: Homogeneous reduction: HAuCl 4 + reducing agent → Au ↓+ oxidation product + HCl (2)   Heterogeneous reduction:Au + HNO3 + 4HCl → HAuCl4 + NO + H2O (3) In industry, the general process of preparing gold powder by chemical reduction is shown in the figure below: For the control of the microscopic morphology of gold powder, scholars at home and abroad have carried out detailed work and have a more thorough understanding of the growth mechanism and influencing factors of gold nanocrystals.Burda et al [12] reviewed the preparation and study of nanocrystals of different shapes, and the article showed that in the process of preparing gold nanocrystals by the chemical reduction method, the gold undergoes homogeneous nucleation or nonhomogeneous nucleation, which causes the prepared gold powder to form a characteristicThe article shows that in the chemical reduction process of preparing gold nanocrystals, homogeneous or heterogeneous nucleation of gold occurs, which makes the prepared gold powder form a unique shape.Meanwhile, environmental factors such as solution pH and gold liquid concentration affect the reducing ability of the reducing agent, which in turn affects the two competing processes of nucleation and growth of gold powder, resulting in different particle sizes of the prepared gold powder particles.As of 2018, the research on the preparation of domestically produced gold powder in China has been developed, and the various processes are summarized in the following table.Some of the substances are abbreviated as follows Chemical reduction method of preparing gold powder process table [13][14][15][16] Serial number Mechanical ball milling method: the main purpose is to reduce the particle size and get a fine powder.Due to the different types and properties of the milled materials, there are many other effects in the milling process.For example, to change the surface and shape of the particles, improve the bulk density and mobility of the powder, so that the particles grow up agglomerated, as well as to achieve the full mixing of more than two kinds of powder and so on.Therefore, in general the result of ball milling is: brittle materials are easy to get fine powder: ductile materials can be changed from spherical powder to flaky powder; the above two materials may be mixed to get mechanical alloying composite powder.Mechanical ball milling method for the preparation of flaky gold powder is usually used to ball mill spherical gold particles, through the control of solidliquid ratio, time and other parameters to obtain different specifications of flaky gold powder [17] Gas-liquid atomization method: Atomization powder preparation refers to the atomized solution of a certain concentration of chloroauric acid, which is sprayed at a suitable pressure through a carrier gas into a reductant solution of a certain concentration, temperature and pH [18][19][20][21] .It was found that conditions such as temperature and carrier gas affect the morphology of gold powder, with the increase of temperature, the prepared gold powder changed from irregular shape to regular spherical shape, and the particle size of the gold powder was increasing, when the carrier gas was changed from nitrogen to air, the morphology of the gold powder remained unchanged, and the particle size became larger, and when it was changed from the nitrate of gold to the chlorate of gold, the morphology of the prepared gold powder were all of irregular shape.

Teams
For industrial applications, each of the three methods has its own advantages and disadvantages, which are compared in the table below: It can be seen from the comparison that the chemical reduction method is more mature and stable than the other methods, so it is the most common method in the production application.

Development and preparation ofbonding phases
The bonding phase occupies an important position in electronic paste, and the addition of glass powder not only effectively reduces the sintering temperature, enhances the adhesion between the conductive paste and the substrate, but also affects the properties of the thick film.The bonding phase generally includes glass powder and metal oxides.Single glass binders may not necessarily meet industrial requirements, and re firing can easily cause a decrease in adhesion.Although individual metal oxides can form a spinel structure with the substrate, significantly improving adhesion, the sintering temperature is high and wire welding is difficult.The mixed glass phase can not only reduce the sintering temperature, but also improve adhesion and facilitate wire welding.Therefore, the performance of bonding relatively thick film electronic pastes is crucial [22] .The early bonding phase was mainly composed of glass powder, and bismuth oxide powder was usually added to improve its properties.The oxides that make up glass powder can be divided into three categories based on their functions:  Hey mainly serve as skeleton structures in glass to support it;  Adjust the physicochemical properties of glass, such as thermal expansion coefficient and glass softening temperature;  Used to enhance the performance of glass while ensuring that the chemical properties of the glass do not change.
During the sintering process, the glass frit melts and flows between the gold particles, affecting the interface between the gold conductor film and the substrate to solidify during cooling and obtain adhesion under wetting and capillary action.The glass layer infiltrates the ceramic matrix and gold particles, mechanically connecting the gold film to the ceramic matrix [23] .However, the uneven distribution of glass in the thick film after sintering affects the wire welding performance of the gold slurry.At the same time, due to the presence of the glass layer, the film layer of this type of slurry is thicker, and there are also high requirements for sintering temperature.
With the development of the industry, the requirements for conductor pastes are gradually increasing.
In order to solve the above problem of bonding phase, various metal oxides are added to glass powder to improve its performance.Its mechanism is that metal oxides react with the substrate to generate complex compounds to increase the binding force between the two.TiO2, CuO, CdO, and other commonly used metal oxides generate spinel like compounds such as CuAl2O4 and CdAl2O4 at the interface between the ribbon and thick film conductors, which have high bonding strength.However, after sintering, there are still some oxides left on the surface of the film layer, and the presence of CuO can cause the surface of the film layer to turn black.At the same time, the addition of metal oxides leads to an increase in the sintering temperature of the gold slurry, resulting in poor co firing matching with the substrate, which affects the performance of the thick film conductor after sintering.By combining the advantages of two oxides and sintering them together, the adhesion of the goldslurry can be increased while effectively controlling its sintering temperature.The mechanism of its action is believed to reduce the interfacial free energy, resulting in an increase in the bonding force between the thick film and the substrate.However, the role of glass powder is different during low-temperature and high-temperature sintering, with low-temperature mainly mechanical bonding and high-temperature mainly generating spinel like compound bonding [24,25] .In 2006, the European Union issued the RoHS ban, which greatly promoted the research and development of electronic products without lead, insulator and other toxic substances.The research on new low melting glass powders at home and abroad mainly focused on phosphate system, Vanadate system, high secret Borate system or the addition of elements that can reduce the melting point, i.e, reduce the sintering temperature of glass application products, such as Bi, Sn, Zn and other oxides [26,27] At present, there are three main methods to prepare glass powder: mechanical ball milling, spray pyrolysis and sol gel method.Among them, glass phase preparation is the most mature, and the most widely used method is mechanical ball milling, which is to weigh the raw materials according to their components, then melt them, then water quench the melted glass powder, and then ball mill the solid obtained to obtain the target glass powder.The general process flow is shown in the following figure: Process flow of glass powder preparation using mechanical ball milling method [28] The ball milling method is convenient and fast, and can prepare glass powder in large quantities.However, the ball milling method cannot obtain ultrafine and uniform glass powder, and it is difficult to control the morphology of the glass powder.During the ball milling process, it may also damage some of the structure of the glass powder and introduce impurities.
Spray pyrolysis is a relatively new method to prepare glass powder at present.Its principle is to obtain highly dispersed powder with relatively uniform particle size and component distribution through micro reaction of liquid droplets.The basic process is shown in the following figure: 3.Flow chart of spray pyrolysis [29] The basic operation and reaction process are these [29] :  Preparing precursor solutions;  Ultrasonic spray machine;  Mass transfer through droplet movement;  Dispersion phenomenon during movement;  Mutual attraction between droplets;  Rapid evaporation when heated during fusion;  When the surface tension of the droplets is dominant, they fuse together for pyrolysis reaction.When the droplets are affected by Coulomb repulsion and the surface tension reaches the critical deformation, they begin to split.When the Coulomb repulsion is  Completely greater than the surface tension, the droplets completely split;  The droplets after division cycle and undergo continuous reactions during division;  The droplets undergo reactions such as evaporation, drying, pyrolysis, nucleation, crystallization, and sintering, ultimately forming spherical powders .
Sol-gel method is also an emerging method for the preparation of glass powder, which is also reacted under liquid solvent conditions, adding the corresponding salts or hydroxides of inorganic oxides under liquid conditions, and realizing homogeneous mixing, and then connecting the metal ions through hydrolysis, condensation, and other chemical reactions, and then evaporation of solvents as well as calcinations can be obtained to obtain the desired powder.In the process of sol-gel preparation of glass powders, the initial substances form a gel by processes such as hydrolysis, condensation and polymerization under suitable conditions.Next, other unhydrolyzed ions in the solution enter the voids of the silica sol to form a network structure in which each ion is uniformly distributed.Finally, the gel is formed by a further process of condensation.The gel is dried and heat-treated to form the desired glass component [31][32][33][34].

Development and Preparation of Organic Carriers
Organic carrier is dissolved in an organic solvent polymer solution, it is the functional phase and bonding phase particles of the carrier, can control the rheological properties of the slurry, adjust the viscosity of the slurry, so that the solid form of the conductive phase, the bonding phase and other solid particles of the mixture dispersed into a fluid characteristics of the slurry in order to facilitate the brush to the substrate, the formation of the required graphics.Usually the gold paste contains 5% to 30% of the organic carrier, in the paste sintering process of organic carrier will gradually evaporate, combustion, leaving the functional phase and bonding phase to get the target conductor graphics.The organic carrier content not only affects the screen printing performance of the gold paste, but also affects the shrinkage of the paste during sintering, which in turn affects the co-firing match between the conductor and the substrate [35] .The organic carrier used in the current paste is basically composed of organic solvents, modifiers, thixotropic agents, etc.The modifiers include thickeners, active agents, coupling agents, leveling agents and other additives [36] .Take conductive silver paste as an example, its organic carrier composition is shown in the following table: Table 3.Composition of organic carriers for a conductive silver paste [38] Chemical

Preparation of gold conductor slurry
After preparing the gold powder and bonding phase, mix them with the organic carrier solution prepared in proportion, place them in a planetary mixer for pre mixing, and fully mix them.After that, use a three roll rolling machine to fully roll the slurry to ensure that the viscosity and fineness of the slurry meet the requirements, and then the finished slurry is produced.The preparation process diagram is shown in Figure 4: It is worth noting that due to the volatility of organic carriers, the slurry should be stored at a low temperature and sealed after preparation to avoid any volatile components in the slurry that may affect its performance.

Problems faced by domestic gold conductor slurry
The first problem faced by the gold paste is the problem of localization.The preparation of gold paste and its application is inseparable, therefore, the finished paste in addition to the requirements of its excellent electrical properties and welding performance, and raw ceramic tape co-firing match is also a factor that must be considered.LTCC gold paste mainly includes inner layer gold paste, outer layer gold paste, bonding alloy paste, filling holes gold paste, through-hole gold paste, gold, platinum and palladium paste, etc.More than ten kinds of raw ceramic tape and the whole conductor paste matching consistency, resulting in the development of the series of gold paste products is difficult.At present, the mature LTCC conductive gold paste and raw ceramic tape are mainly foreign products, and China is in the initial stage.Among them, Guiyan Platinum is the largest domestic gold paste producer, its cooperation with the 14 Institute of China Electric Power to achieve the Ferro based on the production of domestic gold paste, but the gold powder is still imported from abroad, completely independent of the localization of the gold paste is still not yet available.And due to the current military industrial enterprises used in the raw ceramic tape is basically Ferro, DuPont and other foreign products (CLP 13 part of the use of self-produced raw ceramic tape), the domestic research and development of the gold paste team are based on imported raw ceramic tape to carry out the research and development, resulting in a narrower range of applications, which is unfavorable for the large-scale application of the gold paste and its application.The localization of the problem is brought about by the price of gold paste, imported from abroad, Ferro, DuPont products, for example, the price of raw materials (gold prices averaging about 450 yuan / gram) price, the average price of 800-900 yuan / gram, gold powder can also be sold separately as a product, the average price of 700 yuan / gram.While the localized gold paste selling price is only about 650 yuan / gram.This leads to the market price of gold paste was high.If you need to solve this problem, but also many researchers in the same industry to work together.Secondly, the problem faced by the gold pulp is the problem of research and development efforts, the current research and development of the gold pulp is generally carried out by enterprises, colleges and universities do not invest much in this, which leads to the industry's R & D personnel, technology, difficult to attack, due to the expensive price of raw materials, the enterprise's related inputs should be considered quite a lot, so in the industry as a whole has caused a vicious circle.The solution to this problem is to look forward to the implementation of relevant national policies.

Future development trends
New gold paste--organic gold paste.There is no bonding phase in the organic gold slurry, which is composed of organic compounds of gold, organic metal salts, and organic carriers [37] .After sintering, the organic compounds of gold decompose into gold, and the surface of gold is covered by decomposition products of multiple layers of organic metal salts.The bonding strength between this decomposition product and the substrate is better than that of traditional thick film gold conductor slurry, and the obtained gold conductor wire welding strength is also significantly better than that of traditional slurry.After practical application, organic gold slurry has been successfully used in the manufacturing of multi-layer wiring top conductors.It can be foreseen that in the future, organic gold slurry will replace some applications of thick film gold conductor slurry.At present, in addition to the research and production of single metal slurries in China, such as common gold, silver, and copper conductor slurries.The domestic research and development team has also developed binary alloy slurries such as silver palladium alloy slurries and platinum palladium alloy slurries, as well as research and application of silver coated copper electronic slurries.Compared to single metal slurries, alloy slurries utilize relatively low priced metals, which can effectively reduce the cost of conductor slurries.O. V. Belousov [38] et al. studied the relationship between the particlesize of gold and palladium and the oxidation Reduction potential, and observed the morphology, size and composition of bimetallic particles, which greatly promoted the slurry research of gold series multi-component alloys.The current research difficulty of the gold series multi element alloy slurry lies in the issue of co firing matching of different metals on the substrate and the impact of different metal layering on the conductivity during the sintering process.If these problems can be solved, the gold series multi element alloy slurry is also an important development direction for future gold conductor slurry.Traditional gold slurry is expensive and the demand for it is not high for individual enterprises.The preparation of a small amount of gold slurry often has unstable quality, which is also a problem that hinders the application of gold slurry.The main reason for the poor stability of gold slurry is that the size distribution of the prepared functional phase gold powder is too wide, which is due to the constantly changing content of oxidants and reducing agents during the wet reduction process.To prepare functional phase gold powder with basically uniform particle size.The research institute has conducted some research on this and proposed a method for continuously preparing micrometer sized gold powder in patent CN114833334A [39] .This method innovatively separates the nucleation and growth stages of gold powder, resulting in a narrow particle size distribution of the obtained gold powder and high stability of the prepared gold slurry.This also provides a new approach for the wet preparation of gold powder and other metal ultrafine powders.Based on the same principle, crystal seeds can be manually added to the solution in the early stage of reduction, and the crystal seed method can be used to reduce and grow gold.Similarly, gold powder with appropriate particle size can be obtained.However, this method may be affected by the crystal seed morphology, and the prepared gold powder morphology may have irregular problems.In addition to improving the preparation process of gold powder, secondary screening can also be carried out on the prepared gold powder, and suitable particle size gold powder can be prepared through sieving or flotation methods.This method can control the particle size of gold powder within a controllable range, but its process is complex, the raw material loss is high, and this method requires high equipment requirements.At present, it is still being explored, but it is one of the development directions of traditional gold slurry in the future.Traditional gold slurry has also developed in the bonding phase,with the bonding phase mainly developing towards low melting point and pollution-free direction.Inorganic substances with similar properties to lead oxide, such as bismuth oxide and tin oxide, are used to replace lead oxide with good wettability with gold.Currently, phosphate glass systems such as SnO-SnO2-P2O5 [40,41] and SnO-P2O5 [42,43], as well as bismuth oxide glass systems such as Bi2O3-B2O3 and Bi2O3-SiO2-B2O3 [44] , are well developed.The future development is to improve the performance of the bonding phase by adding oxides such as zinc oxide, which can reduce the melting point, to these glass systems, or by adding oxides such as Silver oxide, which have good bonding with the substrate.The selection and use of organic carrier is towards the direction of environmental protection, high performance and low cost.In summary, organic gold conductor slurry and multiple alloy slurry will be the mainstream new type of gold conductor slurry in the future.For traditional gold conductor slurry, the future will develop towards large-scale, high stability, high performance, low consumption, and pollution-free direction.

Figure 1 .
Figure 1.Flow chart of gold powder preparation by chemical reduction method

Table 2 .
Comparison table of gold powder preparation methods