One of the most important properties of solder paste is its ‘Stickyness’ or Tack. The stickiness ensures that components placed on the solder paste can be held in position before the solder joint is made.

This property is a measure of how long the solder paste can maintain its stickiness over time. It is important to know this as there can be significant time gaps between when the paste is deposited on the board, the component is placed on it and then the board goes through the reflow process. When the used solder paste is fresh, it usually has adequate tackiness to serve its function. 

However, ‘tackability’ tends to decline after the paste has been exposed to the atmosphere for some time. This is primarily due to solvent evaporation. Therefore, tack can be defined as the maximum amount of time that solder paste can remain exposed to the atmosphere without a significant change in its stickyness. A solder paste with long tack life is more likely to provide the user with a consistent and robust printing process.

The natural ‘stickiness’ of rosin is a frequent contributor for the tack in paste.

Pastes with high metal loadings and reduced flux vehicle content are more difficult to formulate with high tack qualities. On the other hand, they are more resistant to slump, and the retention of tackiness must be a compromise with the limit of slump specified. Solder paste is designed to have a proper holding force over a period of time. This time varies based on the assembly process. This ability to hold the component can be expressed in tack force and tack time.