Classification and composition of solder paste
The solder paste in the SMT processing is a cream solder which is uniformly mixed by the alloy powder and the paste flux carrier, and is a necessary material for the SMT surface assembly reflow soldering craft. Below, FASTPCBA mainly introduces the classification and composition of solder paste.
Classification of SMT solder paste:
1,According to the composition of the alloy powder can be divided into: high temperature, low temperature, lead or lead-free.
2,According to the particle size of the alloy powder, it is divided into: general spacing and narrow spacing.
3,According to the composition of the flux can be divided into: free ofcleaning, can not cleaning, container cleaning and water cleaning.
4,According to rosin activity: R (inactive), RMA (medium activity), RA (full activity).
5,According to the viscosity can be divided into: printingusageand dispensing usage.
The composition of SMT solder paste:
The alloy powder is the main component of the paste. The composition, particle shape and size of the alloy powder are the key factors determining the characteristics of the paste and the amount of solder joints.
At present, the metal components of the most commonly used solder paste in SMT processing are Sn63Pb37 and Sn62Pb36Ag2.
The composition and ratio of alloy solder powder are the main factors determining the melting point of solder paste; the shape and graininess of alloy solder powder directly affect the brushability and viscosity of solder paste: the surface oxidation degree of alloy solder powder has great influence on the solderability of the paste.The surface oxide content of the alloy powder should be less than 0.5%, preferably control below 80ppm. The fine powder in the alloy solder powder is one of the factors that produce the solder ball. The content of the fine powder should be controlled below 10%.
Flux is a key material for purifying metal surfaces, improving wettability, preventing solder oxidation, and ensuring solder paste quality and superior craft.
Different flux ingredients can be formulated into solder pastes for free-clean, organic solvent cleaning and water cleaning. The composition of the flux has a great influence on the wettability, slump, viscosity, cleanability, solder ball splash and storage life of the solder paste.
3,The ratio of alloy solder powder withflux content
The ratio of alloy solder powder with flux content is the main factor determining the viscosity of the solder paste. The alloy solder powder ‘ content is high and viscosity will high: the flux percentage is high and the viscosity is small. Generally, the percentage of alloy solder powder is 75-90.5%. The free-clean solder paste and the solder paste for stencil printing have a higher alloy content , about 90%.
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