A Quick Primer on SMT Solder Paste

Soldering in an automated surface mount technology (SMT) machine isn't the same as hand soldering. This is a process that connects all of the components to the correct layers and to one another. The solder paste is the magic that makes this part of the process work. 


A solder paste is, like its name suggests, a paste-like substance formed by mixing tin powders with flux and has the ability to print bits.

SMT Solder paste pack SMT Solder pasteSMT Solder Paste flux medium

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

2. Common Particle sizes of tin powder

Common size of a tin powder

Tin powder has either 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 Pastes

Solder pastes can be categorized by its melting temperatures. These are: 

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

2) Medium temperature (eg. sn64-bi35-ag1, melting point 178 ℃)

3) Low temperature (eg sn42-bi58, melting 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 of between 0-10 ℃. The validity period of solder paste is 6 months (unopened). It also needs to be kept away from sunlight to prevent some of the basic properties of the paste from breaking down. 

Industrial refrigerator

5.  temperature regulation of solder paste

Before opening the solder paste, the temperature needs to be brought back to the ambient temperature of 25±3℃. This rise in temperature should be gradual and done for about 4-6 hours. High temperature heaters should not be used to raise the temperature rapidly. The machine below is one used to gradually raise the temperature to the usable temperature. 

SMT solder paste heater 

After returning to the ambient temperature, a stirring machine 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 look at this article)

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6. selection of solder paste products

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

If you are needing consultation on choosing the appropriate solder paste for your product and other related issues, feel free to contact us or send us your project request and our team of experienced engineers will be happy to help you out. 



Posted by Liang Guo

Liang Guo

Liang is a skillful Electronics Engineer with an ability to solder almost anything. By anything, we mean 15+ year old shattered HDD Circuitry to retro gaming consoles .