Thermogravimetric Analysis of Flux Solvent Evaporation in Reflow Profiles
Thermogravimetric solvent mass loss below half a percent prior to liquidus eliminates outgassing macro-voids under bottom-terminated components.
Outgassing Pressure
Vapor Generation
Entrapped solvents and volatile organic compounds in solder pastes exert physical forces as they evaporate within a confined joint during heating. High outgassing pressure can push liquid metal out of the joint, creating solder balls or leaving empty spaces inside the finished joint. This pressure is generated by the rapid expansion of gas that cannot escape.
Defect Formation
Solder joints that contain large cavities or voids are often the result of trapped gas escaping late in the reflow cycle. When outgassing pressure increases during the liquidus phase, the gas forces its way through the molten alloy, creating paths that freeze into empty spaces. These voids reduce the structural strength of the joint and lower its electrical conductivity.
Ensuring proper venting and choosing the right paste chemistry helps prevent these issues.
Mechanical Consequence
Managing the rate of heating during reflow minimizes the rate at which gas is generated within the joint. Lower outgassing pressure prevents the liquid solder from spattering across the board, which can cause short circuits between adjacent pads. Solder masking and board design also play a role in allowing gases to escape before the alloy melts.
By keeping the preheat rate below two degrees per second, the solvent evaporates gently, allowing the gas to disperse without damaging the joint. This gentle release keeps the joint dense and free of large voids.