Solder paste is a new type of welding material that comes into being along with SMT, which is mainly used in the reflow welding of electronic components such as resistors, capacitors and ics on pcb machining surfaces in SMT chip processing industry. Solder paste is formulated according to its viscosity, fluidity, and type of leakage plate at the time of printing, and can usually be classified according to the following properties:

(1) Classification by melting point of alloy solder. Solder paste according to the melting point is divided into high temperature solder paste (217℃ above), medium temperature solder paste (173~200℃) and low temperature solder paste (138~173℃). The most commonly used solder paste melting point is 178~183℃, with the different types and composition of the metal used, the melting point of the solder paste can be increased to more than 250℃, but also can be reduced to below 150℃, according to the temperature required for welding, choose different melting point of the solder paste.

(2) Classified according to the activity of flux. Solder paste according to flux activity can be divided into R grade (inactive), RMA grade (moderately active), RA grade (fully active) and SRA grade (super active). Under normal circumstances, R grade is used for welding of aerospace and avionics products, RMA grade is used for military and other high-reliability circuit components, RA and SRA grade is used for consumer electronics products, and can be selected according to the situation of PCB and components and cleaning process requirements.

(3) According to the viscosity of solder paste classification. The viscosity of solder paste varies widely, usually from 100 to 600Pa·s, up to 1000Pa·s or more. When used, it can be selected according to different solder paste processes.

(4) Classified by cleaning method. Solder paste according to the cleaning method can be divided into organic solvent cleaning class, water cleaning class, semi-water cleaning class and free cleaning class. Among them, organic solvent cleaning such as traditional rosin solder paste, water cleaning class has strong activity, can be used with difficult to braze the surface, semi-water cleaning class and no-cleaning class is the development direction of electronic products process.

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