In actual production lines, Solder Paste and liquid Flux are never selected based on parameter tables; instead, they are solutions driven by specific working conditions. For example, when we worked on Mini LED backlight boards with 0.25mm pitch IC packages and stencil apertures of only 0.12mm, we had to use solder paste with a viscosity of 750Pa·s and a metal content of 90% to ensure complete release. If liquid flux were used for direct spraying instead, capillary action would cause the flux to climb and contaminate the solder mask around the pads.

Solder paste printing before Reflow Soldering is extremely sensitive to squeegee pressure, speed, and stencil tension. When edge whiskering occurs after solder paste printing or the void rate exceeds 8%, the first step is to check whether the thixotropic index of the solder paste is below 0.65, rather than simply increasing the release speed. This is because solder paste with poor thixotropy cannot recover its structural strength in time during the scraping and printing process, easily forming paste buildup on the sidewalls of stencil apertures, which in turn causes solder bridging or insufficient solder.

In the spraying process of liquid flux before wave soldering, the balance between atomized particle size and coverage is more critical. When we were debugging a certain rosin-based liquid flux, we found that when the nozzle air pressure was set to 0.18MPa and the moving speed was 45mm/s, the flux coverage on the bottom pads of the PCB reached 92%, but the residue at the bottom of the BGA exceeded the standard. Later, we switched to 0.12MPa with double-head offset spraying, which maintained the coverage while reducing the residue thickness to within 3.5μm. This shows that the dosage control of liquid flux must be comprehensively adjusted in combination with conveyor speed, nozzle height from the PCB, and PCB component layout.

The application of solder paste in rework stations is very typical. For example, when replacing a QFN48 package chip, first use a hot air gun to blow off the old component, then use a non-woven cloth dipped in a small amount of solder paste to wipe the oxide layer on the pad surface. At this time, liquid flux cannot be used as a substitute, because liquid volatilizes too quickly and dries up and fails during the preheating stage, while the active agent in the solder paste can continuously act until the reflow peak temperature, ensuring sufficient solder wetting when the new component is soldered.

Customers often ask whether solder paste can be diluted into liquid for use. The answer is no. The thickeners in solder paste, such as hydrogenated castor oil, are simply insoluble in ethanol or isopropanol. Forced stirring will only cause the system to delaminate. After standing for two hours, the upper layer is clear liquid and the lower layer is agglomerated. Such a mixture will cause nozzle clogging after being put into production, and the active ingredients will be unevenly distributed. On the same board, some solder joints turn black and others have cold soldering, making it impossible to trace the cause.

The flux supply method of selective soldering tooling fixtures also reveals the essential difference between the two. When we performed selective soldering on a certain automotive ECU motherboard, we used a nitrogen-driven quantitative solder paste dispensing system, with each dispensing amount accurate to ±0.02mg. On the same line, the connector through-hole soldering used ultrasonic atomized liquid flux, with the median droplet diameter D50 controlled at 42μm. The former relies on the paste's shaping ability to prevent flux overflow, while the latter relies on micron-level droplets to penetrate dense pin gaps, with completely different functional positioning.