Dull solder joint gloss after Solder Paste soldering occurs frequently on production lines, but most people only focus on reflow oven parameter adjustments, ignoring the essential differences caused by activity fluctuations between solder paste batches. The same solder paste may exhibit mirror gloss or a dull gray surface under different temperature zone settings, and this change is often closely related to the thermal decomposition residue of rosin derivatives in the Flux.

We once tracked the dull solder joint issue of 0402 resistors and capacitors for three consecutive days, and found that it was not caused by abnormal oven temperature, but rather that the newly arrived solder paste had a nominal metal content of 89.5%, while the actual measurement was only 88.1%. In addition, its organic acid activator ratio was too conservative, failing to fully reduce the oxide film at a peak temperature of 235°C, resulting in a micron-scale amorphous residue layer on the solder joint surface. This layer is neither conductive nor reflective, directly reducing the overall solder joint gloss.

The matching degree between stencil aperture size and pads is equally critical. When the aperture area ratio is 8% smaller than the design value, the decrease in solder paste deposition leads to insufficient spreading of liquid solder during reflow, and flux carbides tend to accumulate at the solder joint edges. This local enrichment appears as boundary blurring on AOI images, and manifests as uneven overall gloss, bright in the center and dark around the edges when observed with the naked eye.

If the oxygen concentration in a nitrogen protective atmosphere is higher than 100ppm, it will accelerate the surface oxidation of tin-lead or tin-silver-copper alloys in the molten state. Especially when the cooling rate in the cooling section is lower than 2°C/s, the oxide film solidifies before it can be re-reduced by the flux. At this time, the solder joints not only have poor gloss, but also often accompany brittle fractures in thrust tests. This type of problem is more sensitive when using bismuth-containing low-melting-point solder pastes, because their antioxidant capacity near the liquidus is inherently weak.

Excessive thickness of the OSP film on PCB pads or expired storage will also significantly weaken the wetting effect. When the OSP film thickness exceeds 0.3μm or is exposed to an environment with RH60% for more than 72 hours, the passivation degree of Cu atoms on its surface intensifies. Even if the solder paste activity is sufficient, the first-wave wetting speed still lags by more than 300ms, causing the solder to have an increased tendency to shrink into balls, and the solder joint geometry tends to be high and convex rather than smoothly spreading, which macroscopically manifests as a matte texture lacking metallic reflection.

The most easily overlooked factor in the workshop is the coupling relationship between squeegee pressure and printing speed. When the pressure is set to 5.2kg and the speed is adjusted to 45mm/s, some high-viscosity solder pastes will exhibit insufficient shear thinning, leading to edge collapse of the solder paste after demolding. After reflow, the solder joint cross-section is mushroom-shaped, with an increased top curvature radius, which naturally reduces the mirror reflection area. This structural dullness cannot be improved by adjusting the reflow curve, and must be verified by returning to the printing parameter combination.