We all know that
lead-free solder paste is one of the indispensable solder materials in chip soldering. Leaded solder paste is particularly prominent in QFN, FPC, LED and other fields, and there are special solder pastes for SMT patch printing.
Soldering paste by material composition
(1) Organic series flux (OA)
The fluxing effect of the organic series flux is between the inorganic series flux and the resin series flux, and it is also an acidic and water-soluble flux. The water-soluble flux containing organic acid is based on lactic acid and citric acid. Since its welding residue can remain on the soldered object for a period of time without serious corrosion, it can be used in the assembly of electronic equipment, but it is generally not used in In the solder paste of SMT, because it does not have the viscosity of rosin flux (to prevent the movement of SMD components).
(2) Resin series flux
The resin-based flux is the most widely used in the soldering of electronic products. Because it can only be dissolved in organic solvents, it is also called organic solvent flux, and its main component is rosin. Rosin is inactive in solid state and active only in liquid state. Its melting point is 127°C and its activity can last up to 315°C. The optimal temperature for soldering is 240-250℃, so it is in the active temperature range of rosin, and its soldering residue does not have corrosion problems. These characteristics make rosin a non-corrosive flux and are widely used in electronic equipment. of welding.
(3) Inorganic series of fluxes Inorganic series of fluxes have strong chemical effects and very good soldering performance, but they have a large corrosion effect and belong to acid fluxes. Because it dissolves in water, it is also called water-soluble flux, which includes inorganic acids and inorganic salts.
The main components of fluxes containing inorganic acids are hydrochloric acid, hydrofluoric acid, etc., and the main components of fluxes containing inorganic salts are zinc chloride, ammonium chloride, etc. They must be cleaned immediately after use, because any residues The halides on the welded parts can cause severe corrosion. This type of flux is usually only used for the soldering of non-electronic products, and it is strictly forbidden to use this type of inorganic flux in the assembly of electronic equipment.
Then, the temperature of the solder paste with lead must be raised to the ambient temperature (25±3°C) before unsealing, and the temperature recovery time is about 3-4 hours, and it is forbidden to use other heaters to make the temperature rise instantaneously; After returning to temperature, it must be fully stirred. The manual stirring time is 3-5 minutes, and the stirring time using the mixer is about 1-3 minutes, depending on the type of the mixer.
How to use lead-free solder paste after rewarming and stirring:
(1) Add about 2/3 of the amount of solder paste to the steel plate, and try to keep the amount of solder paste on the steel plate with no more than 1 can;
(2) Depending on the production speed, supplement the amount of solder paste on the steel plate by adding a small amount of times to maintain the quality of the solder paste;
(3) Solder paste that has not been used on the same day cannot be placed with the unused solder paste. It is recommended that the solder paste be used within 24 hours after opening;
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