What is Slump in Solder Paste?

Solder Paste Slump is the tendency to paste to spread after application on the PCB. Theoretically, once the paste is deposited on the circuit board, it should remain in the same state until a component is placed on it. However, in the real world, the solder paste tends to spread. This can result in various PCB defects like solder bridges between two adjacent pads which can create a short circuit. To avoid this, solder paste slump must be minimized.

There are two types of Slump:

Cold Slump: The Cold slump value depends on the height of the solder paste deposit, temperature, viscosity of the paste (slump usually occur due to the paste with low metal loading), and volatility binders (decides the speed of paste dry). 

The grade of ‘cold slump’ is determined by the following factors:

• Viscosity of the paste
• Speed with which the paste begins to dry
• Height of the paste deposit
• Temperature

Hot Slump: Hot Slump occurs in the preheat stage of the reflow process. During this stage, due to heat, the mobility of flux increases, and the heavy particles of solder can no longer remain suspended in the paste for very long. The natural act of gravity causes the heavier particles to spread causing slump.