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.
Linear Ramp to Spike
Temperature Gradient
Reflow profiles often employ a continuous thermal trajectory from ambient temperature directly to the liquidus threshold. The linear ramp to spike methodology simplifies furnace setup by removing the flat soak zone, thus reducing the total thermal exposure of sensitive packages. This straight-line heating profile is commonly used for boards with low thermal mass.
Flux Activation
Chemical cleaning and wetting must occur dynamically when the temperature rises without a pause. In a linear ramp to spike profile, the flux vehicle must activate and clean the solder pads while the board is actively heating. If the slope is too steep, the flux will boil off before doing its job, which leads to solder balling.
Maintaining a steady rate of rise ensures that the liquid paste is cleaned and prepared for the liquidus transition.
Thermal Stress
Thermal management of the assembly becomes critical when employing a fast ramp without a soak period to equilibrate temperatures. Using the linear ramp to spike approach can cause large temperature differences between light and heavy components on the same board. When some areas of the board melt sooner than others, the risk of tombstoning or skewed parts increases.
To reduce this gradient, operators must limit the ramp rate to less than two degrees per second. This balance allows the board to heat up more evenly while avoiding the excessive thermal exposure of a long soak.