Reflow Profile Deltas across a Fully Loaded Panel
Thermal deltas across fully loaded panels require profile soak expansion and localized copper balance to keep peak temperatures within a five degree window.
Solder Paste Volatile Evaporation
Material Loss
Chemical components within the solder paste matrix transition from liquid or solid states into gas phase during the thermal ramp of a reflow oven. Solder paste volatile evaporation represents the reduction in mass of these solvents and binders as temperatures rise toward the liquidus point of the alloy. This process relies upon the vapor pressure of individual flux vehicles to determine how quickly components leave the deposit.
Stable paste chemistry requires that these constituents depart in a predictable sequence to prevent premature crust formation that might trap gas pockets.
Process Influence
Flux density changes because of these escaping vapors modify the viscosity and tackiness of the printed deposit before the solder reaches a molten state. High rates of moisture or solvent loss lead to insufficient wetting because the remaining flux chemistry loses its ability to remove surface oxides from copper pads and component leads. Operators monitor reflow profiles to ensure that the heating rate allows adequate time for these substances to vacate the joint area before the alloy reaches its solidus temperature.
Excessive removal of volatile agents creates voids within the solder fillets while inadequate removal results in internal pressures that force the molten metal to bridge between adjacent features.
Measurement Method
Thermogravimetric analysis quantifies the percentage of weight lost from a sample of paste under controlled heating conditions that replicate actual production ramp rates. Laboratory technicians measure the mass of a solder sample at the start of a thermal cycle and compare that value to the final mass remaining after the volatile components depart. This standard test ensures that the formulation of a solder paste matches the specific requirements of the reflow environment and the intended assembly geometry.
Higher evaporation rates correlate with increased tendencies for solder balling or spattering during rapid heating cycles.