Solvent Vaporization
Liquid mass transfer rates determine the speed at which volatile compounds escape from solder pastes or flux solutions during the initial stages of thermal reflow. Understanding evaporation kinetics is essential for optimizing the preheating zone of a reflow oven. The rate of solvent removal is governed by the solvent vapor pressure, local temperature, and the velocity of convective airflow.
If the rate is too low, residual solvents can cause solder spattering and voiding during the liquidus phase.
Solder Chemistry
Volatile solvents are formulated into paste systems to keep the flux active and the metal particles suspended. Rapid solvent escape occurs as the temperature climbs toward the flux activation window. This process must be controlled to prevent dryout or premature crusting of the paste deposit.
When the paste surface dries too quickly, a crust forms that traps the remaining solvent underneath, leading to outgassing and solder balling as the assembly reaches reflow temperature.
Thermal Profile
Heating rates must be set to allow steady outgassing without causing thermal shock to the board. Inadequate ramp-up times can result in incomplete solvent removal before the solder alloy melts. Excessively long preheat cycles degrade the fluxing action by depleting the active agents too soon.
