Flux paste and Solder Paste are often mistakenly assumed to be interchangeable on production lines, but they have fundamental differences in both formulation and application scenarios. Flux paste is a pure flux system with no metal powder, while solder paste is a paste formed by uniformly mixing flux paste with solder powder of specified particle size in a weight ratio. This means flux paste cannot independently form conductive connections; it only assists wetting and oxide removal, whereas solder paste must simultaneously satisfy four functions: printability, slump control, solder joint formation, and electrical continuity.

When reworking short circuits under QFN packages, we typically apply flux paste to the edges of pins first and then use a hot air gun to melt the solder. This is because its rosin-based matrix has high viscosity, strong activity, and low residue, allowing precise localized coverage without spreading. If solder paste is mistakenly used, the accumulation of solder powder will worsen pin bridging. Similarly, when cleaning pad oxide layers before BGA ball mounting, only flux paste is permitted; otherwise, residual solder powder will interfere with ball diameter positioning accuracy and increase void rates after reflow.

The metal content of solder paste is typically 88%~92% (taking Sn63Pb37 or SAC305 as examples), and its viscosity range is strictly controlled between 300~600 Pa·s to adapt to the squeegee pressure and release speed of fully automatic stencil printers. In contrast, flux paste generally has a viscosity of 800~1200 Pa·s, significantly higher than that of solder paste, so it cannot pass through stencils with fine pitches below 0.1 mm. Forcing it through printing would cause a sharp increase in clogging rates. We once encountered a factory that used flux paste instead of solder paste to print 0402 resistors, resulting in insufficient solder and open circuits in three consecutive batches. Microscopic inspection revealed that the flux paste could not effectively transfer at the stencil openings.

The reaction mechanisms during the reflow phase are also completely different. The solder powder in solder paste undergoes eutectic melting above 217°C and forms intermetallic compounds, while flux paste only activates around 150°C and reaches peak deoxidation capability at 200°C. Above 230°C, most organic acid-based carriers begin to carbonize; prolonged residence in the reflow zone can instead cause solder joint blackening or excessive ionic residue. Therefore, when setting the reflow profile, we must control the peak temperature of flux paste below 225°C for no more than 45 seconds, while solder paste requires time above the liquidus line to be maintained at 60~90 seconds.

During procurement and selection, one cannot rely solely on the "no-clean" or "halogen-free" labels provided by suppliers; compatibility with existing solder pastes on the production line must be tested. For example, a batch of flux paste was nominally compatible with SAC305, but during actual BGA rework, its residues were found to cross-link with the thixotropic agent in the solder paste, resulting in a solder ball offset rate of 12% after hot air removal. The problem was ultimately resolved by switching to the same series of flux paste from the same manufacturer. This demonstrates that even if flux paste and solder paste come from the same technical platform, they cannot be used interchangeably across processes.