Flux paste cannot replace Solder Paste to complete SMT placement soldering, because solder paste is essentially a uniform mixture of solder alloy powder and flux, which must not only provide the metal skeleton that forms reliable solder joints after melting, but also perform multiple functions such as wetting, deoxidation, and surface tension reduction. In contrast, flux paste only contains highly active fluxing components and completely excludes metal powders such as tin-silver-copper, so it cannot form conductive paths or mechanical connections after reflow. Forcing the use of flux paste for printing followed by placement will inevitably result in floating components, cold solder joints, dry solder joints, or even zero solder joints.

We have actually verified the direct printing replacement solution using flux paste multiple times on the production line. Even with a 0.1mm thick stencil, 30μm particle size flux paste, and squeegee pressure adjusted to 0.8MPa, AOI still reported 100% missing components after reflow. X-ray inspection showed no metal bridging on the pads, only charred flux residue remained. This indicates that flux paste lacks the melt spreadability and metallurgical bonding capability required for solder. Although its viscosity is close to that of solder paste, its weight loss rate is as high as 92%, and without metal skeleton support, it cannot maintain the positioning stability of components after placement.

Some engineers mistakenly use flux paste for pre-coating before BGA underfill, or secondary fluxing reinforcement for QFN thermal pads. The premise for this usage to be feasible is that solder paste has already completed the main solder joint bonding, in which case flux paste only plays an auxiliary activation role. If used independently without solder paste, even with nitrogen reflow, it cannot produce solder joint morphology that meets the IPC-A-610 Class G standard. Under a microscope, severe solder balling, wetting angle greater than 90°, and IMC layer thickness less than 0.3μm can be observed.

The procurement side often considers replacement due to the lower unit price of flux paste compared to solder paste, but the comprehensive cost must be calculated clearly, including the hidden losses caused by a 3-fold increase in the first article debugging scrap rate, a 45-minute per shift increase in AOI re-inspection man-hours, and a 27% post-reflow rework rate. More critically, flux paste cannot pass the REL test of J-STD-004B, resulting in extremely high long-term reliability risks. A customer once suffered batch failure and rework of automotive T-BOX due to this issue, with the root cause being the use of flux paste instead of solder paste to solder the 0201 crystal oscillator pads of the MCU.

What truly needs optimization is solder paste selection and process matching. For example, choose low-residue, high-thixotropy SN63/PB37 solder paste for fine-pitch QFP, or switch to nickel-containing SN96.5/AG3.0/CU0.5 solder paste to improve wettability in lead-free soldering, rather than hoping that flux paste can overstep its role to do the main job. Its only compliant role in the SMT production line is an "assistant" used in conjunction with solder paste, not the "main attacker".