Before reflow oven soldering, solder paste is used to bond electronic components to PCB pads so that they do not fall off when attached to them. After welding, the solder paste is melted at high temperature, and the electronic components are tightly fixed with the solder pad. Solder paste is a mixture of flux and tin powder to create a toothpaste-like appearance.
Storage of solder paste
Solder paste is a sensitive material, easy to be affected with moisture oxidation and deterioration, so the storage of solder paste is more critical. It is generally required to be stored at low temperature, low humidity density, and in a sealed environment, it is also necessary to maintain vertical placement.
(1) Use cycle of solder paste shall be controlled according to production plan, and the storage time shall not exceed 3 months.
(2) The storage of solder paste should be kept according to different types, batch numbers and different manufacturers.
(3) Solder paste storage conditions require temperature 4~8 degrees, relative humidity is less than 50%. Do not freeze solder paste in the freezer.
(4) The use of solder paste should follow the first-in, first-out principle, and make records.
(5) The stored temperature and humidity shall be checked and recorded every week.
Precautions for the use of solder paste
(1) Take the solder paste out of the refrigerator, stick the "use label", and fill in the "temperature return start time and signature". The solder paste can be opened and used only when the return temperature is normal. The return temperature time is generally 6~12 h, which depends on the capacity of the solder paste. If the solder paste is not fully returned to the temperature, it will condense the water vapor in the air, resulting in collapse, tin explosion and other problems.
(2) The solder paste should be fully stirred before use, divided into machine stirring and manual stirring. The machine stirring time is 3-4 minutes, while manual stirring needs to be stirred in the same direction to avoid bubbles in the solder paste, and the time is 2-3 minutes.
(3) The bottle cap should be tightened before the solder paste in the bottle is used up to prevent flux volatilization caused by the solder paste exposure to the air. The validity period of the solder paste after opening the cap is within 24H.
(4) The unused solder paste should be collected, and the used solder paste can not be mixed with the new solder paste to avoid the deterioration of the new solder paste.
(5) If the solder paste is not used up, it should be put back to the refrigerator, and the time label should be indicated on the label.
(6) After solder paste printing, try to complete reflow welding within 4 h to avoid flux volatilization in solder paste, resulting in poor welding.
