What are the main parameters that affect the properties of SMT solder paste?
The main parameters that affect the properties of SMT solder paste are: the composition of alloy solder, the composition of flux and the ratio of alloy solder to flux; Particle size, shape and distribution uniformity of alloy solder powder; Oxygen content on the surface of alloy powder; Viscosity; Thixotropic index and slump; Working life and storage life.
1. The composition, proportion and flux content of alloy solder are generally 75% to 90% by weight of alloy solder powder, but the appropriate weight percentage of alloy solder powder should be selected according to different flux systems.
The viscosity is related to the particle size and diameter of solder paste, and mainly depends on the composition of flux system in solder paste and the proportion of other additives.
If the viscosity is too high, it is easy to cause the solder paste not to print to the bottom of the template opening, but also to stick to the scraper. If the viscosity is too low, it is not easy to control the deposition shape of solder paste. After printing, it will collapse, which is easy to produce bridging. At the same time, if the viscosity is too low, the solder paste will be scraped away from the template opening when using a soft scraper or when the pressure of the scraper is high, thus forming concave solder paste deposition, resulting in insufficient solder and false soldering. The viscosity of solder paste is usually due to the formula. If the viscosity is too low, it can be adjusted by changing the printing temperature and scraper speed. The decrease of temperature and scraper speed will increase the size of solder paste. Generally, the best viscosity range of fine pitch solder paste is 800 PA · s ~ 1300 PA · s, and that of common pitch solder paste is 500 PA · s ~ 900 PA · s
3. Melting point
The melting point of SMT solder paste mainly depends on the composition and proportion of alloy solder powder. With the different composition and melting point of SMT solder paste, different reflow soldering temperature is needed, and the welding effect and performance are also different
4. Shape, particle size and distribution of solder alloy powder
Generally, the diameter of solder particles is about 1 / 5 of the opening size of the template. For the pad with narrow spacing of 0.5mm, the opening size of the template is 0.25mm, and the maximum diameter of solder particles is not more than 0.5mm, otherwise it is easy to cause blocking during printing
5. Working life and storage period
Because the properties of SMT solder paste, especially the viscosity, change with time and room temperature, after a certain time, SMT solder paste will lose its original characteristics and cannot be used. The working life is generally required to be 12-24 hours, at least 4 hours of effective working time. If the solvent contained in SMT solder paste is too volatile, it is easy to make SMT solder paste dry and difficult to work, It is easy to lose the adhesion to the components. The storage life is generally stipulated as one year at 2-10 ℃, at least 3-6 months
6. Thixotropic index and slump
The slump of SMT solder paste is mainly related to the viscosity and thixotropy of SMT solder paste