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Composition and characteristics of solder paste in smt patch pr

• Posted by david smithsmile March 4, 2021 Filed in Business #pcb assembly  #SMT PCB  39 views
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  1. The composition and characteristics of solder paste
  Solder paste is mainly composed of metal powder, flux, solvent, gelling agent or suspending agent, etc., which are uniformly mixed. According to different applications, the metal powder is usually a mixture of two or more metals such as tin, lead, indium, silver, and gold. The fineness of the metal powder is usually between 25 and 75 microns. Metal powder is the main component of solder paste, and it is also the residue after smt mounting and soldering. It accounts for about 90% of the weight of the solder paste. Flux is the carrier of metal powder. Its composition is basically the same as that of general flux. The activation effect can eliminate the oxide film on the surface of the welded material and the alloy powder itself, so that the flux can quickly diffuse and attach to the surface of the welded metal. The composition of the flux has an effect on the expansion, wettability, and collapse of the solder paste after printing. Cleaning and the formation of welding beads have a great influence,PCB fabrication and it is also related to its storage life.

  The rheological property of solder paste is viscosity. The viscosity of solder paste refers to a kind of anti-resistance that appears when the solder paste is flowed by external thrust. Most solvents in solder paste have low viscosity, and vice versa. The viscosity of the solder paste changes with the movement of the squeegee on the metal stencil. When the squeegee is scraped, the viscosity becomes lower. After the squeegee stops working, the solder paste returns to its original viscosity. At the same time, the viscosity also changes with the temperature, and the viscosity of the solder paste will decrease when the temperature rises. The stretchability of the solder paste after printing onto the pad is the slump of the solder paste. The slump of the solder paste is a key performance index that affects the solder paste printing. The slump of the solder paste mainly depends on the weight (percentage) of the metal components in the solder paste and the composition of the metal powder. The higher the content of the metal powder, the smaller the slump, and the smaller the slump of the solder paste, the welding effect The better.

  2. Matters needing attention in the processing and use of solder paste in SMT PCB patch

  1. The ideal working temperature of solder paste is 20~25℃, and the relative humidity is 40~60%RH. Such operating conditions are most favorable for solder paste printing.

  2. The storage temperature of solder paste is required to be 0~5℃, and it is required to maintain stability, usually in the refrigerator.

  When soldering paste is used in smt patch processing, do not open the package immediately after taking it out of the refrigerator. It should be placed in the working environment, that is, the room temperature is balanced for 1 to 2 hours before opening the package, so that the moisture in the soldering paste should be completely dry.

  3. In order to maintain good printing suitability, the editor of Zhongyan Electronics recommends that it must be fully stirred before printing, and then put into use after stirring for about 5 minutes. Because the solder paste will be separated when stored, that is, the proportion of the solder paste is relatively large. The solder powder is separated from the flux, additives and solvent mixture with a lighter specific gravity.

  4. Storage life, its validity period is generally about 3 months. So don't buy too much at one time when buying.

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