What is Solder Paste? What are the Functions, Compositions, and Properties of Solder Paste?


Solder paste definition and use:

What is solder paste? Solder paste is composed of tin-based alloy powder (generally, powder with a particle size of 30~50 um is used, and ultra-fine solder paste is used with spherical powder with a particle size of 1~15 um), flux, and some additives. The solder paste is a stable mixture with a certain viscosity and good thixotropy. The solder paste can initially adhere electronic components to a given position at room temperature. After heating, the solvent and additives are volatilized, the tin powder melts, and the soldered parts are connected to the pads. After cooling, permanently connected solder joints are formed.


Solder paste composition and characteristics:

Table 1 shows the main components and functions of the solder paste. Table 2 is the mass and volume ratio of the solder alloy powder and flux in the solder paste.

Table 1. Solder paste compositions and functions.


Component Name


Solder Alloy

Spherical Solder Alloy Powder
(Ultra-Fine Powder)

Bonding with base metal



Rosin, Abietin, Epoxy Resin

It has the necessary adhesion to fix components and purifies the soldering surface


Amine, Amine Hydrochloride, etc.

Cleaning the soldering surface

Thixtropic Agent

Hydrogenated Castor Oil,
Carnauba Wax, etc.

Preventing separation of solder alloy powder and flux, and imparts thixotropy to solder paste


Ethylene Glycol, Glycol, etc.

Adjusting the solder paste to have the proper viscosity

Table 2. Mass and volume ratios of solder alloy and flux.


Mass Ratio (wt%)

Volume Ratio (%)

Solder Alloy







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