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What is solder paste in SMT?

Solder paste is a new welding material that comes into being because of surface mount technology. And solder paste is so complex, which is a mixture of solder powder, flux and other additives. Solder paste has certain viscosity at ordinary temperature and is able to mount the electronic components on initial positions. At the temperature of soldering, the printed circuit pad and components will be permanently connected with the volatilization of solvent and some additives.


Solder paste is mainly composed by flux and solder powder.


Solder Paste

Flux

The main elements of flux as follow:

1. Activation:

Activation plays a role in getting rid of the oxide on the pad surface and components soldering part. Besides, it is able to reduce the surface tension.

2. Thixotropic

It can adjust the viscosity and printing performance, which plays the role in avoiding the trailing and adhesion in solder paste printing.

3. Resin

Resin is able to enhance the adhesion of solder paste, with the aim to protect and avoid re-oxide of PCB after soldering. It is very important in components mounting.

4. Solvent                                                            

Solvent can help dissolve the different chemical substances mixed in the flux and make flux coating smoother, achieving the promotion of flux effect. Also, it makes it easier to control the paste viscosity and fluidity.


Solder Powder

Solder powder is usually mixed by Sn, Ag, Cu, and Bi. The smaller size of solder powder can achieve better result that solder powder can be complete attached on the surface. However, one the other hand, the smaller size of solder powder means the higher probability to be oxide.


In solder paste printing, it is very important to make a choice about solder paste for the difference of solders, which will have great influence on the PCB manufacturing.


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