Tin powder and auxiliary composition
SMT manufacturing relies on quality solder paste printing. Many factors affect the quality of solder paste’s printing. It is important to know the properties of solder dust, composition of solder and ratio of flux. These parameters will determine melting points, printability, weldability, quality of solder joints, and printing abilities. The eutectic temperature of tin powder-alloy components should be as close as possible to the eutectic. It is calculated as the proportion of tin dust to flux expressed in the amount of paste. Tin powder content directly influences the viscosity, printing and durability of solder paste. In general, the tin content of tin powder is 75-90%. Tin powder content in template printing processes and in no-cleaning solder pastes is generally higher at 8590%. In contrast, drip coating process has a lower content at 7585%.
Tin powder particles are sized and arranged according to their shape.
It is important to consider the size, shape, and uniformity of the tin powder particles in order to influence the properties. It is best to use smaller particles for solder paste printing, particularly when using high density products that have a narrow spacing. About 1/5th of the opening size for a template is used to measure the particle diameter. While the pattern of solder paste printed with small particles alloy powder is very detailed, edge collapse can easily be caused by it. The potential for oxidation is greater due to small particles’ large surfaces. If the assembly density of the powder is low, it’s possible to choose a coarser powder without having an impact on the printing ability. This can improve both the weldability as well as reduce the price for solder paste. Also, the form of the alloy powder can impact the paste’s printingability and desorption as well as weldability. Spherical-particle alloy paste has good printing properties, low viscosity and easy to collab after printing. This makes it ideal for printing metal templates and narrow spacing screens. Spherical particles are small in size, have low oxygen levels, and bright welding spots, which can be beneficial for improving welding quality. At present, however, they are mostly used.
Viscosity
Solder paste, a fluid that is highly thixotropic can be made to flow by external forces. It is the most important characteristic of solder paste. Viscosity can affect the performance of printing. Viscosity determines how easy it will be to get out of the template. The percentage of tin in the paste also influences the paste viscosity. A large percentage of flux indicates a lower viscosity.
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