There are various classifications of solder paste performance in SMT processing, including melting point, flux activity, viscosity, cleaning method, lead content, and tin powder particle size. Choosing the appropriate solder paste is crucial for welding quality and production efficiency. As an important soldering material, the performance of solder paste directly affects the quality and reliability of soldering. Solder paste is mainly composed of metal powder, flux, and other additives, and its performance classification is mainly based on the following aspects:
1、 Classification by melting point of alloy solder
Solder paste can be divided into high-temperature solder paste, medium temperature solder paste, and low-temperature solder paste according to their melting points.
High temperature solder paste: The melting point is usually above 217 ℃, suitable for occasions that require higher welding temperatures. The common alloy compositions include Sn Ag Cu series, such as Sn96.5Ag3Cu0.5, with a melting point above 217 ℃.
Medium temperature solder paste: with a melting point between 173~200 ℃, commonly used alloy compositions include Sn Bi Ag and Sn63Pb37, suitable for soldering at medium temperature requirements.
Low temperature solder paste: The melting point is usually between 138~173 ℃, suitable for thermal sensitive components or occasions that require low-temperature soldering. Common alloy compositions include Sn Bi series, such as Sn42Bi58, with a melting point of 138 ℃.
2、 Classified by the activity of solder flux
Solder paste can be classified into R grade (inactive), RMA grade (moderately active), RA grade (fully active), and SRA grade (super active) based on the flux activity.
R-grade: Non reactive solder paste, commonly used for soldering aerospace and aviation electronic products, with extremely low requirements for flux residue.
RMA grade: Medium activity solder paste, suitable for soldering military and other high reliability circuit components.
RA grade: Fully active solder paste, suitable for soldering consumer electronic products, with good wettability and soldering effect.
SRA grade: Super active solder paste, suitable for occasions with extremely high requirements for welding effect.
3、 Classified by viscosity of solder paste
The viscosity of solder paste varies greatly, usually ranging from 100 to 600 Pa · s, with the highest reaching over 1000 Pa · s. The choice of viscosity mainly depends on the different application techniques.
High viscosity solder paste: suitable for fine pitch printing and situations where the shape of solder paste needs to be maintained for a long time.
Low viscosity solder paste: It falls quickly during printing and is suitable for high-speed printing and dispensing processes.
4、 Classified by cleaning method
Solder paste can be divided into organic solvent cleaning, water cleaning, semi water cleaning, and non cleaning categories according to the cleaning method.
Organic solvent cleaning: such as traditional rosin solder paste, which needs to be cleaned with organic solvents after welding.
Water cleaning class: After welding, it can be cleaned with water, with strong activity, suitable for surfaces that are difficult to solder.
Half water cleaning: Between organic solvent cleaning and water cleaning, it is the development direction of electronic product technology.
Non cleaning category: No cleaning required after welding, minimal residue, suitable for highly automated production lines.
5、 Classified by whether it contains lead or not
Lead solder paste: containing lead components, it poses a significant threat to the environment and human health, but has good welding effect and low cost. It can be applied to some electronic products that do not require environmental protection.
Lead free solder paste: with environmentally friendly ingredients and minimal harm, it is applied to environmentally friendly electronic products and is a development trend in the SMT processing industry.
6、 Classified by the diameter of tin powder particles
Solder paste can be classified into grades 1, 2, 3, 4, 5, and 6 based on the particle diameter of tin powder, with grades 3, 4, and 5 being the most commonly used. The more precise the product, the smaller the tin powder selection needs to be, but the smaller the tin powder, the larger the corresponding oxidation area of the tin powder. Circular tin powder helps improve printing quality.
