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A Quick Primer on SMT Solder Paste

Jul 27, 2018 10:47:54 PM

Soldering in an automated surface mount technology (SMT) machine isn't the same as hand soldering. There is a process that's used to make all of the components connect to the correct layers and connect to each other. The solder paste is the magic that makes this part of the process work. 


Solder paste is, just like the name implies, a paste-like substance formed by mixing tin powders with flux , which has the ability to print bits.

SMT Solder paste pack SMT Solder pasteSMT Solder Paste flux medium

Solder paste is an alloy compound that's generally composed of two or more kinds of metal.  The metals in solder paste are generally composed Tin, Lead, Silver and/or Copper. 


2. Common Particle size of tin powder

Common size of a tin powder

Tin powder has spherical or oval shapes. Spherical printing should be a wide, small surface area with low oxidation and bright solder points. The oval shape is generally poorer in each of these qualities. The smaller the particle diameter, the easier the oxidation. 


3. Temperature Categories of Solder Paste According to the temperature can be divided into three major categories

Solder paste can also be categorized by the melt temperatures. These are: 

1) High Temperature (SN-AG-CU alloy series, melt point 217-227 ℃)

2) Medium temperature (such as sn64-bi35-ag1, melt point 178 ℃)

3) Low temperature (e.g. sn42-bi58, melt point 138 ℃)

Note: Adding bismuth (BI) can reduce the melting point of solder paste

4. Storage of solder paste

The preservation of the solder paste should be controlled in the industrial refrigerator between 0-10 ℃. The validity period of solder paste is 6 months (unopened). It also needs to be kept away from sunlight as this breaks down some of the basic properties of the paste.

Industrial refregirator

5. use of solder paste

Before opening the solder paste, the temperature needs to be brought back to the ambient temperature 25±3℃. This rise in temperature should be gradual and last about 4-6 hours. High heat heaters should not be used to raise the temperature more quickly. The machine pictured below is one used to gradually raise the temperature to usage temperature. 

SMT solder paste heater 

After the return to an ambient temperature, a stirring machine (pictured below) is used to stir the compound for about 1-3 minutes. 

solder paste mixer 

PCB plate printing with solder paste needs to be completed within 2 hours of the patch and reflow soldering. (Interested in SMT Solder Paste Printing? Make sure to have look at this article)

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6. selection of solder paste products

According to your product requirements, the characteristics of the product, and finally in the quality of welding, we will choose the appropriate solder paste of your product. Some components, for example, cannot handle very high temperatures in the reflow part of the SMT process, so lower temperature solder paste must be used. It's very important to know all of the characteristics of the project before choosing the correct paste.