As the name suggests, solder paste is also kind of solder that is used during a PCB assembly workflow and process and in association with the new infrared reflow machine. The solder paste provides for a connection between the board’s pad and the different components of surface mount.  

Composition and Texture
The solder paste consists of the minute and small spheres of solder that is held within a flux. As the total composition has a paste texture to it, it is termed as solder paste. The flux is added to act like a temporary adhesive, which holds all the components together until the solder is completely melted and the parts are fused with each other. Solder pastes are usually composed of the tin-lead alloy. Other kinds and varieties include the-tin antimony combination that has more shear and tensile strength.  

There are also various grades of the paste. The type-1 grade may have a particle size of up to 150 mm while the Type 8th grade has a particle size of up to 8 mm. Apart from the particle size and other aspects, the viscosity of the flux is also important. Some formulations may need to be stored before they are actually applied so that they can assume the right viscosity.

Types of solder paste flux  
The solder paste may have a different kind of fluxes and hence can be classified as under:

Solder paste with water-soluble flux- This paste contains flux having glycol bases and organic materials. A number of cleaning agents can be used for this paste.

Solder paste with rosin flux- The rosin flux consists of rosin, which is a natural extract that is obtained from the pine trees. These fluxes can also be cleaned once the soldering process is over.

No clean flux- The fluxes are made entirely of resins and the solid residues. No cleaning is required for a solder paste having this flux, but special workflows and equipment may be required. 

Using a solder flux
In the initial stage, the solder paste is applied to the PCB or printed circuit board. Use of a soldering pencil provides for specific and precise application of the solder paste to the given and required areas.  

After the application of the solder paste, the PCB is subsequently moved to the machines where more components can be added. The paste can hold these newly added components together easily, but the board should be handled very carefully. The fixation of components should not be delayed as the solder paste may start to lose on its quality and functionality if it is not melted in time.  

Solder Paste Inspection
The solder paste inspection method is used commonly for measuring the precise volume of solder paste before the components are added to the PCB and the solder is melted. The SPI method is quite useful towards removing the errors and ensuring that the defects related to solder are non-existent or not significant statistically.  
The various solder paste grades and varieties are now also sold online, at affordable costs and reasonable solder paste price.