Solder paste: the magical "adhesive" for electronic soldering

Solder paste: a key material for electronic soldering

In the field of modern electronics manufacturing, solder paste, as a core material, plays an irreplaceable role in the connection process of electronic components. Its emergence has greatly promoted the development of surface assembly technology (SMT) and provided a solid guarantee for the miniaturization and high performance of electronic products.

1. The birth and evolution of solder paste

In the 70s of the 20th century, with the development of electronic products in the direction of miniaturization and lightweight, traditional perforated welding technology was difficult to meet the demand, and surface assembly technology came into being. As a key supporting material for SMT, solder paste has also entered the stage of history. Early solder paste technology was relatively simple and was mainly used for basic soldering needs. However, with the rapid development of the electronics industry, the performance requirements of solder paste are becoming increasingly stringent, driving its continuous innovation. Nowadays, solder paste has become an indispensable and important material in the production of electronic products, and its quality is directly related to the performance and reliability of electronic products.

2. Complex and exquisite composition

Solder paste is a paste-like system made up of a variety of ingredients, each performing its own duties and working synergistically.

1. Solder alloy powder: the "main force" of soldering

Solder alloy micropowders dominate solder pastes, often exceeding 80%. These fine powders are obtained by special treatment of solder alloys, and common preparation methods include chemical reduction, electrolytic deposition, mechanical crushing and atomization milling. Among them, atomization has become the preferred method for preparing powders for solder paste due to its unique advantages. During the atomization process, the prepared solder alloy is heated and melted, and then atomized into tiny powders under the action of CO₂, N₂, Ar₂ and other gases. The powder prepared by this method is spherical or near-spherical, which can not only meet the strict requirements of solder paste for powder shape, particle size, oxygen content and fluidity, but also retain the performance of the original solder alloy to the greatest extent.

The particle size, morphology and uniformity of solder powder have a crucial impact on the printing performance of solder paste. The ideal powder shape should be spherical with uniform particle size to ensure uniformity and resolution of printing and avoid printing defects. In addition, spherical powders have the smallest surface area and have a lower degree of surface oxidation compared to irregular powders. The oxygen content of the powder is one of the key factors affecting the performance of the solder paste, and too high oxygen content will reduce the welding performance of the solder paste. Therefore, during the production process, the particle size range of the powder must be strictly controlled. At present, the particle size of solder powder for the preparation of solder paste in China is generally controlled between 20μm and 45μm, which is defined as type III powder and type IV powder in the surface assembly industry. However, with the continuous advancement of electronic technology, the demand for fine-pitch soldering is increasing, and the graphics on printed boards are becoming more and more detailed, and more and more manufacturers are using ultra-fine powders below 20μm to prepare solder paste. However, the use of ultrafine powders also brings new challenges, and the protection of the powder becomes particularly important due to its increased surface area and the oxygen content on the surface.

In the early days of the development of the electronics industry, traditional SnPb solder and solder powder were widely used due to their mature technology and good soldering performance. However, with the increasing global awareness of environmental protection and the deepening of awareness of the dangers of lead pollution, countries have introduced laws and regulations to restrict the use of lead-containing solder. In this context, the research and development of lead-free solder has become a hot topic in the industry. Among them, tin-silver-copper (Sn-Ag-Cu, SAC) lead-free solder is one of the most potential alternative materials due to its properties close to tin-lead solder. For example, Sn3Ag0.5Cu (SAC305) is a classic lead-free solder recommended by the Japan Electronics and Information Technology Industry Association (JEITA), which has excellent physical properties and high-temperature stability, and the bond strength after soldering is comparable to or even higher than that of traditional tin-lead eutectic solder. In the early stages of lead-free development, most manufacturers chose SAC305 as their first choice for lead-free solder. However, due to the rising price of silver, institutions are working on low-silver solder pastes with less than 1% silver in order to reduce costs. For example, Senju has achieved a series of results in the research and development of lead-free solders, filing several patents. In 2004, the company patented SAC solder containing 0% to 8% Ag and 0% to 5% Cu, which met the reflow temperature requirements but was insufficient to eliminate the monument effect. In 2006, another patent was filed for Sn-Ag-Cu-P solder with 0.3% - 0.4% Ag, claiming that the solder has the same solderability, conductivity and mechanical properties as SAC305, while reducing the toxicity problems caused by the reaction between Ag and acid and base. In 2010, an application for a Sn-Ag based solder with an Ag content of 0.2% - 1.0% and trace amounts of Sb, Cu or Ni, Co, Fe, Mn, Cr, Mo or P, Ga, Ge and other elements were added, the addition of these elements can improve the mechanical strength of the alloy, but too high an amount will cause the liquid phase temperature of the alloy to rise. This alloy claims to effectively eliminate the phenomenon of erection during welding. In addition, the low-silver solder paste developed by Koki is Sn0.1Ag0.7Cu0.03Co, with a melting point of 217 - 227°C, and the reduction of silver addition makes the product less affected by market price fluctuations, and the addition of cobalt can prevent the microstructure changes caused by thermal cycling, maintain the dense structure, inhibit the segregation and agglomeration of age-sensitive intermetallic compounds, and reduce the cost by 10% to 20% compared to SAC305. The low-cost silver-free solder developed by Genma Corporation has a composition of Sn0.7Cu0.03Ni0.01Co0.005Ge, a melting temperature of 226 - 228°C, and the addition of nickel and cobalt improves the strength and reliability of the solder at a cost of 54% less than SAC305.

Flux: The "Catalyst" of the Soldering Process

The surface of the metal being welded is very prone to the formation of an oxide layer in the air, which will seriously hinder the welding process and hinder the formation of welds. Therefore, it is necessary to use flux during welding to remove oxides from the surface of the material being welded and promote smooth welding. The flux for solder paste is different from ordinary flux, it not only requires good solder activity, but also needs to be easy to form a paste with the solder powder, can make the solder powder suspended stably, and at the same time have a good protective effect on the solder, and has special properties such as thixotropic characteristics, viscoelasticity and thermal stability. The composition of flux is extremely complex, and it is a large system containing a variety of organic and inorganic substances. Its main components include active agents, film forming agents (protective agents), solvents, catalysts, surfactants, rheological modulators, heat stabilizers, etc.

Active agent: Active agent is one of the key components of flux, and its main function is to remove oxides from the surface of the metal being soldered, reduce the surface tension of the solder, and improve the wettability and expansion of the solder. Common active agents include organic acids, organic halides, etc. During the soldering process, the active agent is able to chemically react with oxides on the metal surface, reducing it to metal elements, thereby creating conditions for the wetting and spreading of the solder. For example, succinic acid and glutaric acid in organic acids can react with metal oxides in acid-base neutralization to produce volatile substances, thereby achieving the purpose of removing oxides.

Film forming agent (protective agent): The film forming agent can form a protective film on the surface of the welded metal during the welding process, preventing the metal surface from oxidizing again, and also helping to improve the mechanical strength and corrosion resistance of the solder joint. When the flux works at the soldering temperature, the film forming agent will gradually decompose and form a uniform film on the metal surface, which can isolate the air and prevent the metal from contacting oxygen, thereby effectively preventing the secondary oxidation of the metal.

Solvent: The main function of solvent is to dissolve activators, film formers and other components, so that the flux has good fluidity and stability, and is easy to evenly apply on the surface of the welded metal. Commonly used solvents include ethanol, isopropyl alcohol, ethyl acetate, etc. The solvent can evenly disperse various solid components in the flux to form a stable solution system, ensuring that the flux can be uniformly covered on the surface of the welded metal during use and play its due role.

Catalysts: Catalysts can accelerate chemical reactions during welding and improve welding efficiency. It can reduce the activation energy of chemical reactions, making various reactions in the welding process more likely to occur, thereby shortening the welding time and improving production efficiency.

Surfactant: Surfactant can reduce the surface tension between the solder and the metal surface being soldered, enhance the wetting ability of the solder, and enable the solder to better spread and adhere to the metal surface. At the same time, surfactants can also improve the dispersion and emulsification properties of fluxes, helping to improve the overall performance of fluxes. For example, nonionic surfactants of fatty acid or aromatic groups can effectively reduce the surface tension generated by contact between solder and lead pin metal, enhance surface wettability, and enhance the penetration of organic acid activators, and in some cases can also act as blowing agents.

Rheological Regulators: Rheological modulators are used to regulate the rheological properties of fluxes, giving them suitable flow and thixotropy during printing and coating processes. During the printing process, the flux needs to have good fluidity so that it can smoothly pass through the printed screen and fill onto the pad. After printing is completed, the flux needs to quickly restore a certain viscosity to prevent collapse and flow, and ensure that the solder paste maintains a good shape and position on the pad. Rheological modulators can adjust this rheological property of the flux as needed to meet the requirements of different process links.

Heat stabilizer: Heat stabilizer can improve the stability of the flux in a high-temperature environment, preventing the flux from decomposing or deteriorating due to high temperature during the soldering process, thereby ensuring that the performance of the flux remains stable throughout the soldering process. In high-temperature processes such as reflow soldering, the flux needs to maintain the stability of its chemical and physical properties within a certain temperature range, and the presence of heat stabilizers can effectively prevent the flux from failing due to high temperatures and ensure the smooth progress of the soldering process.

According to the corrosiveness of flux to solder joints, fluxes can be divided into three categories: non-corrosive fluxes, corrosion-retardant fluxes and corrosive fluxes. From the perspective of chemical composition, fluxes can be divided into organic fluxes and inorganic fluxes. With the continuous development of flux technology, special fluxes have appeared, according to the difference in cleaning methods, this type of flux can be divided into three categories: rosin-based flux, water-based flux, and no-cleaning flux. Among them, rosin-based flux is mainly composed of rosin, which has good soldering performance and low corrosiveness, and is widely used in electronic welding. Water-based flux uses water as a solvent, which has the advantages of environmental protection and easy cleaning, but has high requirements for equipment and process. No-clean flux does not require a special cleaning process after soldering, which can reduce production costs and environmental pollution, but it has stricter requirements for flux formulation and performance, and it is necessary to ensure that the residue after soldering will not adversely affect the performance of electronic products.

Add-ons and additives: "little helpers" for performance optimization

In order to achieve good printability and storage stability of solder paste, in addition to the two main components of solder powder and flux, it is necessary to add a small amount of other components to adjust its performance. For example, adding proppants can prevent delamination of solder paste during storage and use, ensuring uniformity of solder paste. The addition of lubricant can ensure that the solder paste does not have problems such as tailing and sticking during printing, and ensures the printing quality. thixotropic agent and rheological modifier are used to adjust the thixotropic performance of the paste, so that the solder paste has good fluidity during the printing process, and can quickly restore a certain viscosity after printing is completed, prevent the solder paste from collapsing and flowing, and ensure the accurate position of the components before soldering. These add-ons and additives, although used in small quantities, play an integral role in improving the overall performance of the solder paste.

3. Diversified classification system

Classification by alloy composition

Lead eutectic solder paste: The traditional lead eutectic solder paste is mainly composed of tin-lead alloy, which has the advantages of low melting point, good soldering performance, and relatively low price, and has been widely used in the development of the electronics industry for a long time. However, lead's use is increasingly limited due to its potential harm to the environment and human health. For example, Alpha Leaded Solder Paste OL-107E is a halogen-free lead-free solder paste for fine stencil printing surface packaging applications, and while its flux itself is non-toxic, it produces a small amount of reactive and decomposing vapors that need to be completely evacuated from the workspace to ensure the safety of the working environment.

Lead-free non-eutectic solder paste: With the increasingly stringent environmental protection requirements, lead-free non-eutectic solder paste has become a hot spot for research and application.

Sn - Ag (-Cu) system: The Sn - Ag series has a eutectic composition of Sn3.5Ag and a eutectic point of 221°C. Among them, the most mature is the Sn-Ag-Cu(SAC) series, such as the SAC305 mentioned above, which has become a typical representative of lead-free solder due to its excellent performance. However, due to the high price of silver, in order to reduce costs, enterprises and research institutions continue to develop low-silver solder paste. For example, Senju has applied for a series of patents by adjusting the alloy composition and adding trace elements, and is committed to developing lead-free solder with excellent performance and reasonable cost. The low-silver solder paste component developed by Koki is Sn0.1Ag0.7Cu0.03Co, which reduces the cost by reducing the amount of silver added, and the addition of cobalt effectively prevents the microstructure changes caused by thermal cycling, maintains the dense structure, and inhibits the segregation and agglomeration of age-sensitive intermetallic compounds. Genma has developed a low-cost silver-free solder that improves the strength and reliability of the solder by adding elements such as nickel and cobalt, significantly reducing costs.

Sn-Cu series: The eutectic composition of Sn-Cu is Sn0.7Cu, and this series of solders also has certain application prospects, its cost is relatively low, and it has certain applications in some fields that are cost-sensitive and do not have particularly demanding performance requirements.

Classification according to flux characteristics

Rosin-based solder paste: with rosin as the main soldering aid component, it has good soldering performance and low corrosiveness, relatively little residue after soldering and is easy to clean, and is widely used in electronic soldering, especially suitable for electronic product welding with high reliability requirements. For example, in the manufacturing of some high-end electronic products, rosin-based solder paste can ensure the quality and reliability of solder joints without causing excessive corrosion and pollution to circuit boards and components.

Water-based solder paste: using water as a solvent, it has the advantages of environmental protection and easy cleaning. During the soldering process, water-based flux can effectively remove oxides from the metal surface, promoting the welding process. Moreover, since its solvent is water, it can be easily removed by washing after welding, reducing environmental pollution. However, water-based solder paste has high requirements for equipment and processes, requiring specialized equipment to control parameters such as humidity and temperature during the soldering process to ensure welding quality.

Clean-free solder paste: No need for a special cleaning process after soldering, reducing production costs and environmental pollution. During the soldering process, the components in the flux can quickly cure after the soldering is completed, forming a protective film, which will not adversely affect the performance of electronic products and prevent the solder joints from being eroded by the external environment. This solder paste has obvious advantages in large-scale production and some fields with strict cost control, which can greatly improve production efficiency and reduce production costs.

4. Application and future prospects of solder paste

In microelectronics manufacturing, solder paste is widely used in chip-to-chip and chip-to-PCB board interconnects. This interconnect process plays a crucial role in enhancing the performance and stability of the device. For example, in the manufacturing process of electronic products such as smartphones and computers, a large number of chips and electronic components need to be soldered through solder paste to achieve electrical connections and signal transmission. With the continuous development of electronic products in the direction of miniaturization and high performance, the performance requirements for solder paste are also getting higher and higher. In the future, the research direction of solder paste will mainly focus on the following aspects: first, further optimize the alloy composition and develop lead-free solder with better performance and lower cost to meet the dual needs of environmental protection and cost control; The second is to carry out innovative research on flux, such as the microencapsulation of flux, which wraps the active ingredients of the flux in a microcapsule and releases it as needed during the soldering process, which can effectively improve the performance and stability of the flux; The third is to add nanoparticles to the flux to give full play to the advantages of microalloys and avoid causing malignant interactions, further improving the soldering performance and reliability of solder paste. In short, solder paste, as a key material in the field of electronic soldering, will continue to develop in continuous technological innovation and provide strong support for the progress of the electronics industry.