Tin paste is the main process material in surface mount technology (SMT). In various SMT processes such as PCBA assembly and manufacturing, solder paste is used to connect the leads or endpoints of surface assembled components to the solder pads on the printed circuit board, directly affecting the welding quality and reliability of PCBA. Defects generated by solder paste account for 60% -70% of defects in SMT, so it is particularly important to have a deep understanding of solder paste and to use it in a standardized and reasonable manner.
1、 Composition of solder paste
Solder paste=solder powder (METAL)+flux (FLUX): Tin powder is usually manufactured by nitrogen atomization or rotary disc method, and then screened by wire mesh. Flux is composed of binders (resins), solvents, activators, thixotropic agents, and other additives. It plays a crucial role in the entire process of solder paste from silk printing to welding.
In general, the weight ratio of 10% solder paste to 90% tin powder (90% tin alloy powder); The volume ratio of 50% solder paste to 50% tin powder.
2、 Important characteristics of solder paste
The solder paste mainly has the following four characteristics:
1. Liquidity
2. Delaminability
3. Continuous printing
4. Stability
Tin paste is a fluid with fluidity. The fluidity of materials can be divided into ideal, plastic, pseudo plastic, expansive, and thixotropic, with solder paste belonging to the thixotropic fluid. The percentage content of solder paste alloy, powder particle size, temperature, flux amount, and lubricity of thixotropic agent are the main factors affecting the viscosity of solder paste. In practical applications, the optimal viscosity is generally determined based on the type of solder paste printing technology and the thickness printed on the PCB.
3、 How to choose the appropriate solder paste?
Facing various processing techniques, how to choose a suitable solder paste? The following are for your reference:
1. Lead free solder paste and lead-based solder paste
The choice of lead-free or lead-based solder paste depends on customer requirements and market demand. With the increasing awareness of environmental protection, lead-free products are generally accepted in the market, but it does not mean that there is no market for lead-based solder paste. Many electronic merchants still use lead-based solder paste in order to save costs or in applications such as military, aerospace, and power control that require high reliability. If your product is aimed at markets such as Europe, the United States, Japan, and South Korea, it is still wise to choose lead-free solder paste.
2. Washable solder paste and non washable solder paste
Whether to choose water washed solder paste depends on the processing technology after reflow soldering during the PCBA processing and the cleanliness of the PCB board. The residual rosin on the PCB panel may cause short circuits due to the heating, melting, deformation, and moisture absorption of large components during product use, reducing product reliability and service life. In this scenario, it is necessary to increase the use of cleaning agents such as board washing water to wash the board, which is difficult to clean and increases material and labor costs. Our company's latest development and production of water washed solder paste can be washed clean with deionized water, which is low-cost, easy to use, and very environmentally friendly.
3. Composition ratio of solder paste alloy
The alloy ratio in solder paste determines the melting point of tin, which in turn determines the peak temperature of reflow. When choosing a solder paste with a certain alloy ratio, you should consider several questions: what alloy ratio of solder paste is suitable for the welding and operating temperatures in PCBA processing? Consider the ductility and adhesion of different metal components? Considering the above issues, I believe you can choose a solder paste that meets your processing technology.
