With the application of reflow soldering technology, solder paste
has become an important process material in surface mount technology (SMT). Connections to pads on the board. Solder paste coating is a key process in surface assembly technology, which will directly affect the soldering quality and reliability of surface assemblies. Solder paste is required to have different properties in different processes or processes of surface assembly in PCBA processing. We have collected some information to summarize the following points:
What are the rigid requirements for solder paste for surface mount and assembly processes?
1. The shelf life of solder paste is 6 months, and the solder paste should be kept within the validity period of 3 to 6 months before printing.
2. The solder paste should have excellent mold release during printing; the solder paste is not easy to collapse during and after printing; the viscosity of the solder paste is moderate.
3. The solder paste should have good wetting performance when it is heated by flow soldering after printing; the minimum amount of solder balls should be formed, and the solder spatter should be less.
Fourth, after solder paste reflow soldering, the lower the solid content in the flux, the better, and it is easy to clean after soldering, that is to say, there is little or no residue after soldering. High strength and firmness after welding.
5. Solder paste should follow four properties in soldering applications: fluidity, stripping, continuous printing, and stability.
Solder paste is a fluid and has fluidity. The fluidity of materials can be divided into ideal, plastic, pseudoplastic, swelling and thixotropic, and solder paste is a thixotropic fluid. The ratio of shear stress to shear rate is defined as the viscosity of the solder paste, and its unit is Pa.s. The percentage of solder paste alloy, powder particle size, temperature, solder amount and lubricity of the thixotropic agent are the factors that affect the viscosity of the solder paste. main factor. In practical applications, the viscosity is generally determined according to the type of solder paste printing technology and the thickness printed on the PCB.
In addition, the storage temperature of the solder paste is between 2-10°C, the ambient temperature is 20-25°C, and the relative humidity is 30%-60%. We have detailed explanations in the article, which are very important and can be searched for.
The above factors all ultimately determine the performance of solder paste. In the actual printing process, it is particularly important to have an in-depth understanding of solder paste and to standardize and rationally use it.