In PCBA technology, the selection of solder paste is a crucial link, which directly affects the welding quality and product reliability. The following are some key factors to consider when choosing solder paste:
1. the choice of alloy welding powder
- Composition: solder paste is mainly composed of alloy solder powder and flux, in which alloy solder powder accounts for the main part.
- Melting point: Solder pastes with different melting points can be selected according to the temperature required for welding. The melting point of the most commonly used solder paste is 178~183℃, but depending on the type and composition of the metal used, the melting point of the solder paste can be increased to 250℃ or higher or lower to 150℃.
- Shape and particle size: The shape and particle size of alloy solder powder have great influence on the properties of solder paste. Spherical alloy powder has small surface area and low oxidation degree, and the prepared solder paste has good printing performance.
2.The choice of flux
- Activity level: choose the activity level of flux according to the situation of PCB components and cleaning process requirements. Generally, it can be divided into three levels: inactive (R), towel equivalent activity (RMA) and active (RA). Products with high reliability (such as aerospace and supplies) can choose R grade; KJRMA level is usually adopted; PCB and components are stored for a long time, and the surface oxidation is serious. RA grade should be adopted and cleaned after welding.
- Cleaning process: Choose solder paste according to the cleanliness requirements of PCBA and different cleaning processes after reflow soldering. When using the no-cleaning process, choose the no-cleaning solder paste without halogen and strong corrosive compounds; When using solvent cleaning process, choose solvent to clean solder paste; When water cleaning process is adopted, water-soluble solder paste should be selected.
3.Other considerations
- Viscosity: The viscosity of solder paste has a great influence on the printing effect. The viscosity of solder paste is selected according to the solder paste coating process and assembly density of PCBA processing. High-density printing requires high viscosity and drop coating requires low viscosity. Generally, the viscosity of liquid distributor is 100 ~ 200 Pa s, that of screen printing is 10 ~ 30 Pa s, and that of stencil printing is 200 ~ 600 Pa s..
- Welding requirements: For example, when welding heat sensitive components, low melting point solder paste containing bismuth should be used; For high-value and versatile PCBA products, high-quality solder paste must be selected.
- Storage life: Solder paste should be stored for 3 to 6 months before printing to ensure its stable performance.
To sum up, when selecting solder paste processed by PCBA technology, it is necessary to comprehensively consider the composition, melting point, shape and particle size of alloy solder powder, the activity level of flux and the adaptability of cleaning process, as well as other factors such as viscosity, welding requirements and storage life. By comprehensively considering these factors, the most suitable solder paste for PCBA processing can be selected, thus ensuring the welding quality and product reliability.
