Those familiar with solder paste know that solder paste is a gray paste. It is a new type of welding material with SMT. It is a paste mixture formed by mixing solder powder, flux, other surfactants, thixotropic agents, etc. Mainly used in SMT industry PCB surface resistance, capacitance, IC and other electronic components welding. Generally speaking, solder paste is a material used to connect part electrode and PCB pad. The material is an alloy mainly composed of tin, which can be used to connect part electrode and PCB after curing.

But our solder paste is just a general name. Because solder paste has different characteristics, just like a child, a child can be classified according to gender, age, height, weight and other characteristics. Of course, solder paste is the same. Solder paste can be divided into high-temperature solder paste, medium temperature solder paste and low-temperature solder paste; It can also be divided into lead-free solder paste and lead solder paste, etc. Today, let the solder paste manufacturer of Hongqiao Group introduce the lead-free solder paste in the solder paste in detail.
Definition of lead-free solder paste
The lead-free solder paste does not absolutely prohibit the existence of lead in the solder paste, but requires that the lead content must be reduced to less than 1000ppm (<0.1%), which means that the electronic manufacturing must meet the lead-free assembly process requirements. "Electronic lead-free" is also commonly used to generally mean that the content of six toxic and harmful materials, including lead, must be controlled within 1000 ppm.
Composition of lead-free solder paste
The lead-free solder paste is mainly composed of tin/silver/copper, with silver and copper replacing the original lead.
In the tin/silver/copper system, the metallurgical reaction between tin and minor elements (silver and copper) is the main factor determining the application temperature, curing mechanism and mechanical efficiency. According to the binary phase diagram, there are three possible binary eutectic reactions between the three elements. A reaction between silver and tin forms the eutectic structure of tin matrix phase at 221 ° C and ε Combined phase between metals (Ag3Sn). Copper reacts with tin to form eutectic structure of tin matrix phase at 227 ° C η The combining phase between metals (Cu6Sn5). Silver can also react with copper to form rich silver at 779 ° C α Xianghe is rich in copper α Phase of eutectic alloy. However, in the current study, no phase transition was found at 779 ° C in the measurement of the curing temperature of the tin/silver/copper triple compound. This means that it is likely that silver and copper will react directly in the triple compound. In terms of temperature kinetics, it is more suitable for the reaction of silver or copper with tin to form intermetallic compounds of Ag3Sn or Cu6Sn5. In this case, the tin/silver/copper triple reaction can be expected to include tin matrix phase ε The combined phase (Ag3Sn) and η The combining phase between metals.
Development of lead-free solder paste
Now all industries pay attention to environmental protection, and lead-free solder paste is no exception. By developing new lead-free solder paste products to replace the original harmful lead solder, we can break the monopoly of foreign lead-free solder paste brands in the domestic market, solve the key information technology in the lead-free green manufacturing process, and greatly promote the development of China's electronic information industry manufacturing technology.
The main research contents of lead-free solder paste are closely combined with the key data problems in the current green manufacturing technology, and the lead-free solder paste alloy system is studied for the processability and reliability of lead-free solder paste; In view of the problems of lead-free solder paste, such as poor wettability, excessive residue and poor insulation efficiency, scientific and technological research on the optimization of lead-free solder paste flux formulation system was carried out, so as to develop new lead-free solder paste products suitable for the manufacture of large quantities of products with high reliability requirements.
The research of new lead-free solder paste is the key technology for the industry to solve: solve the problem of selecting new welding materials in the electronic manufacturing industry, and truly realize green manufacturing; Provide lead-free soldering materials with independent intellectual property rights, and break the current situation that foreign lead-free solder paste products monopolize the market in China.
Melting point of lead-free solder paste
The melting point of lead-free solder paste should be low, as close as possible to the eutectic temperature of 63/37 tin lead alloy of 183 ℃. If the eutectic temperature of new products is only a few degrees higher than 183 ℃, it should not be a big problem, but there is no such lead-free solder that can be really promoted and meets the welding requirements; In addition, before developing lead-free solders with lower eutectic temperature, the melting interval temperature difference of lead-free solders should be reduced as much as possible, that is, the temperature range between the solidus and liquidus should be reduced as much as possible. The minimum solidus temperature is 150 ℃, and the liquidus temperature depends on the specific application. Tin bars for wave soldering: below 265 ℃; Tin wire: below 375 ℃; Solder paste for SMT: below 250 ℃, the reflow temperature is generally required to be lower than 225~230 ℃.