If Flux paste volatilizes too quickly during high-temperature soldering, the main flux component escapes in large quantities before it can fully remove the oxide film, causing re-oxidation of the solder particle surface, a sharp drop in wetting force during melting, and further leading to cold solder joints, dry solder joints, and an increase in solder joint void rate. Our production line once experienced consecutive batches of three 0201 resistors with non-wetting on one side, which was ultimately identified as being caused by the peak Reflow Soldering temperature rising from 235℃ to 245℃, leading to premature vaporization of low-boiling-point rosin derivatives in the flux paste and insufficient residual active components.

An accelerated volatilization rate will also significantly increase the risk of solder bead formation, because the local vapor pressure generated by rapid vaporization of the flux paste will spray incompletely melted solder powder particles from the edge of the pad, especially forming dense micro solder beads around QFN bottom pads or fine-pitch ICs. These solder beads often cause false short circuit judgments in subsequent ICT testing, while the nitrogen protection environment further inhibits the oxidative precipitation of thermal decomposition products of the flux, making the problem more concealed.

The printing process is also affected by this. When the stencil aperture size is too large and the flux paste viscosity is too low, during the squeegee pressing process, the flux paste is affected by the superposition of shear heat and ambient temperature rise, and the surface solvent evaporates rapidly, causing the paste edge to dry out and the center to collapse. The result is distortion of the Solder Paste pattern and poor volume consistency, which is particularly prominent when the workshop temperature exceeds 28℃ in summer without constant temperature control.

We once conducted a comparative test on the performance of three flux pastes with different Tg values at peak temperatures above 240℃, and found that the model with Tg below 95℃ lost more than 70% of its soldering activity within 60 seconds after entering the reflow soldering temperature zone, while the model with Tg of 108℃ could still maintain an effective wetting time of 95 seconds. This shows that the thermal stability of flux paste is not only determined by the boiling point, but the crosslink density of its resin network and the solvent-carrier ratio are the key factors restricting high-temperature soldering adaptability.

The production line adjustment strategy must be carried out around the actual process window. For example, reducing the heating rate of the preheating section from 2.5℃/s to 1.8℃/s can extend the activation time of the flux paste, and reducing the nitrogen concentration from 800ppm to 300ppm can moderately increase the oxidation reaction rate to balance the film removal efficiency. However, such adjustments must simultaneously monitor the measured data of the furnace temperature profile, and cannot rely solely on theoretical speculation, otherwise it is easy to cause wetting lag or even local non-wetting at the solder joints at the bottom of the BGA.