Anyone who uses solder paste knows that solder paste needs to be refrigerated, so why should solder paste be refrigerated? In fact, cold storage of solder paste is to slow down the reaction speed of FLUX and tin powder, thereby prolonging the storage time, so solder paste is usually stored in a refrigerator (2-10°C) or a freezer.
One of the necessary steps before solder paste is used is to return to temperature. The so-called return to temperature is also called thawing. It is natural thawing, not artificial heating to thaw. The tin powder is evenly distributed, so as to give full play to the various characteristics of the solder paste. The method of rewarming the solder paste is to take the solder paste out of the refrigerator and place it at a standard room temperature of 22-28°C for reheating and thawing. The standard 500g solder paste should be rewarmed for at least 2 hours, so that the temperature of the solder paste and the environment same temperature. Do not open the closed tank cover if the recovery time is not enough. If it is forced to open, the water vapor in the air will condense due to the temperature difference and enter the solder paste, which will cause the performance of the solder paste and poor soldering. The longer the opening time will lead to Solder paste dries faster.
The second is the stirring of the solder paste. There are various metal components and some fluxes in the solder paste. Due to the different densities of the substances, the main components sink below during storage, so the solder paste must be warmed up before use. Stirring, mixing methods include manual mixing and machine mixing, both methods are acceptable; mixing time: manual: about 3 minutes, machine: about 2 minutes, the time of solder paste mixing should not be too long, too long may cause Solder paste is warmer than operating room temperature, causing flow during printing. For machines with different stirring times, the stirring time required at room temperature and other conditions is different, which can be determined by testing during operation. Judgment of mixing effect: Use a scraper to scrape up part of the solder paste. When the scraper is tilted, if the solder paste can slide down smoothly, it meets the requirements.
Regarding the tools for manual mixing of solder paste, some factories use metal tools for mixing, and some use plastic mixing tools. Tools of both materials are acceptable. If a very high production process is required, plastic tools can be used for mixing. From Theoretically speaking, the mixing of plastic materials will be better than that of metal, and the damage to the tin powder in the solder paste and the stencil during printing will be much less, it will be easier to tin and solder paste to form, and the printing effect will be better. The metal mixing tools used by some manufacturers, which mixing tool to choose when mixing manually depends on the specific requirements, the operating habits of the personnel and the production process, and cannot be generalized. Conditional factories will have a special machine automatic mixer, and the operation will be more convenient.
To sum up, we can know that no matter which step the solder paste is in use, it is very important and indispensable. In the actual SMT patch application, we must follow each of the above steps. In addition, choosing the appropriate solder paste is also very important.
