What are Solder Paste and Flux?

Solder Paste

Solder paste is one of the most important materials used today in the manufacture of printed circuit boards to connect surface mount components to pads on the board. It is also possible to solder through-hole components by printing solder paste in and over the holes. Solder paste helps the world to enjoy the benefits of electronic equipment.

The tin paste is called “paste” because it has a similar consistency to the toothpaste we brush our teeth with every day.  The solder paste is essential to securely weld the components to the circuit board electronic parts.  This process helps to keep all the parts in the proper position even under some vibration. The main purpose of solder paste is to weld electronic parts to the circuit board while providing the critical electronic signal connectivity needed for the electronic equipment to function as designed.

The main ingredients of tin paste are flux and tin powder.

The composition of tin paste is mainly a combination of flux and powder, the following will help explain the contents and importance of these two ingredients:

The tin powder is a metal alloy, the main use in providing weldability and welding strength, it is made up of the following metals:

Sn, Ag, Cu, Bi

There are a number of manufacturers of tin powder and the formulas have different composition ratios. In some cases, manufacturers have a special patented formula to make it different from other brands.

In the case of SAC305, which is currently the most used, tin paste with a ratio of tin (Sn, 96.5%), silver (Ag, 3%), and copper (Cu, 0.5%); SAC0307, on the other hand, uses tin (Sn, 99%), silver (Ag, 0.3%), copper (Cu, 0.7%) ratio of tin paste.

Second, there is the use of SnBi or SnBiAg alloys for cryogenic tin paste. The melting point of tin (Sn, 42%) and vanadium (Bi, 58%) of SnBi alloys are 138 degrees C, while the melting point of SnBiAg is 178 degrees C (Sn, 64%), vanadium (Bi, 35%), and silver (Ag, 1%).


The main function of “flux” is to remove oxides and dirt from the metal surface. At high temperatures, a film can be formed on the metal surface to isolate air, so that the paste is not easily oxidized. The composition of “flux” mainly consists of the following four components:

Resin rosin: 40% to 50%. Can be divided into natural resin (Rosin) or synthetic rosin, usually, lead-tin paste using Rosin, while lead-free tin paste using Resin, rosin can form a protective layer on the surface of the welded metal to prevent oxidation, with viscosity, slightly clean metal surface ability. Activator: 2 to 5%.

The main components are organic acids called halogens. These can clean the metal surface and are often used as a cleaning agent in the reflux welding process. This dissolves the oxides of the metal surface and improves the welding effect. Halogen is highly toxic. To meet today’s environmental needs, halogen-free tin paste has been a trend, but its deoxidation capacity is very efficient and less expensive, so it is still found in some tin pastes.

Contains ingredients such as ethanol. These solvents evaporate during the warm-up of the tin paste, so it does not affect the solderability of the entire tin paste. It can help dissolve and mix different chemicals in the flux so that the flux can be more uniformly coated in tin powder, improving the effect of flux, it also makes the flux easier to be used by people. Ethanol can be used to control the viscosity and flow of the solder paste. If the warm-up temperature of the return weld heats up too quickly, these solvents may boil immediately and cause problems with tin paste splashes.

Provides tactile (thixotropy) or shake denaturation to control the viscosity of the tin paste to achieve the purpose of the paste. This enhances the printability and collapse resistance of the paste so that the solder paste printed on the circuit board will maintain the original shape without collapsing causing a short circuit.