The phenomenon of Solder Paste generating solder balls during the Reflow Soldering process occurs frequently on production lines, but directly attributing responsibility to defective solder paste batches often masks the true causes. For example, we once encountered dense solder balls on Line A with three consecutive batches of the same brand of solder paste, while the same batches showed no abnormalities on Line B. It was ultimately found that fluctuations in nitrogen purity on Line A led to elevated oxygen levels, which intensified the side reaction of Flux decomposition, thereby promoting the oxidation and agglomeration of solder powder into balls.

The metal content, particle size distribution, and thixotropic index of solder paste do indeed affect the tendency to form solder balls, but these parameters are fixed at the time of manufacture. Unless there is transportation damage or expired storage, their stability is far higher than on-site process variables. Therefore, when a new batch of solder paste produces solder balls immediately after being put into use, it is necessary to simultaneously verify whether the stencil aperture size matches the pads, whether the squeegee pressure fluctuates within the 0.4～0.6MPa range, and whether the dwell time after printing exceeds 30 minutes. This is because once the rosin-based flux in the solder paste forms a surface film, it will burst and spray due to a sudden increase in internal vapor pressure when subjected to reflow heating.

If the peak temperature of the reflow soldering temperature profile deviates by more than ±5℃ from the nominal melting point of the solder paste, or if the heating ramp rate exceeds 3℃/s in the 120～180℃ range, the solvent will enter the liquid phase zone before it can escape, forming tiny bubbles that carry solder powder upward. After cooling, these become typical solder balls. This phenomenon is particularly noticeable around packages smaller than 0201, as their pads have low thermal mass and heat up quickly, making them more prone to local overheating and flux carbonization.

When the workshop environmental humidity is consistently above 60%RH, the risk of solder paste absorbing moisture increases sharply. Although most solder pastes claim to be tolerant of 40%～60%RH, in actual production lines, if the air conditioning dehumidification unit is shut down for two hours and the humidity rises back to 65%, even with newly opened solder paste, slight edge stringing can be observed after 15 minutes of standing after printing, and the number of solder balls after reflow increases by approximately 2.3 times. This data comes from our cross-validation records on six SMT lines in Q3 last year.

Unreasonable PCB pad design is also a hidden contributing factor. For example, solder mask misalignment leading to increased exposed pad area, or adjacent pad spacing less than 0.3mm, makes the solder paste prone to bridging, tearing, and splattering during reflow collapse. In such cases, even replacing three different brands of solder paste cannot completely solve the problem; it is necessary to collaborate with DFM engineers to adjust the pad aperture ratio and solder dam width.