The essence of pre-tinning OSP pads is to enable the Solder Paste to rapidly penetrate the organic protective film during the reflow stage and form a reliable metallurgical bond with the bare copper. Therefore, OSP solder paste must have stronger acidic activation capability than ordinary solder paste, while its rosin system must not be too aggressive to avoid corroding the copper surface beneath the OSP film. This requires that the type and concentration of organic acids in the formulation must precisely match the thickness range of the OSP film (typically 0.2–0.5 μm) and its thermal decomposition temperature (130–160 °C); otherwise, if the film has not completely ruptured by the time the solder paste enters the liquid phase in the early reflow stage, the wetting of the solder paste will be severely delayed.

We have tested three solder pastes nominally compatible with OSP. When using the same stencil (opening width 80 μm, thickness 120 μm) on the same DEK printer, Model A exhibited obvious edge collapse after demolding due to a low thixotropic index; Model B met the wetting speed requirement but left semi-transparent gelatinous residues under AOI; Model C showed slight copper surface oxidation and discoloration after 24 hours. The final selected model has a solid content of 89.3%, a viscosity of 780 Pa·s at 25 °C, and a dicarboxylic acid composite activation system in its Flux. It achieves a first-article wetting yield of 99.2% under a conventional reflow profile with a peak temperature of 235 °C and a holding time of 60 seconds.

Printing parameters must be adjusted synchronously. The squeegee pressure must be controlled between 3.2 and 3.8 kg: insufficient pressure leads to inadequate solder paste filling, while excessive pressure squeezes the OSP film and causes uneven local copper exposure. A printing speed of 25 mm/s must be maintained to balance solder paste transfer rate and stencil release performance—too fast causes stringing, while too slow leads to premature flux volatilization that affects subsequent wetting behavior. These are not issues that can be solved by the solder paste alone, but rather the result of the combined interaction of OSP solder paste, stencil, squeegee, temperature and humidity, and board surface cleanliness.

Fluctuations in workshop ambient temperature and humidity directly alter the apparent viscosity of the solder paste. When humidity rises from 50%RH to 65%RH, the printing demolding success rate of the same batch of OSP solder paste drops by approximately 11%. In this case, forcibly extending the stirring time will instead aggravate the precipitation of flux components. The correct approach is to warm the solder paste at room temperature for 1 hour, then use a constant-temperature mixer at a low speed of 30 rpm for 5 minutes to homogenize the paste, which prevents both solvent volatilization and settling and stratification of the metal powder.

Incoming inspection must not rely solely on the TDS data provided by the manufacturer; wetting angle tests must be conducted using actual PCBs. Take a QFN pad with a 0.3 mm pin pitch, dispense 0.15 μL of solder paste, heat it at 3 °C/s to 220 °C in a nitrogen atmosphere and hold for 5 seconds, and record the melting and spreading process with high-speed imaging. The acceptance criterion is a wetting angle ≤28° with no shrinkage breakpoints. This data reflects the actual wetting performance of this batch of OSP solder paste on your production line more truthfully than any theoretical explanation.