The core contradiction in OSP board soldering is that the Solder Paste Flux must have sufficient activity to break through the OSP film, but excessive activity will leave strong acidic ions that cause solder joint corrosion, abnormal ICT test impedance, or reduced long-term reliability. We tracked a customer's production line in Dongguan for three consecutive months and found that after using Solder Paste with an activity level above REL0.8, the BGA solder joint black edge rate increased from 1.2% to 4.7%, while after switching to JJY JH-602 (REL0.55), this problem completely disappeared. This shows that OSP board soldering does not require the strongest possible activity, but rather a closed-loop match with the OSP film thickness, storage duration, and reflow peak temperature.

The physicochemical properties of the OSP film determine the difficulty of film removal

The film thickness of mainstream OSP solutions on the market ranges from 0.2 to 0.4 μm, and the film density is directly affected by the deposition time, pH value, and drying temperature. The measured film impedance of OSP boards from the same supplier but different batches can vary by up to 30%, which requires the organic acid dissociation rate of the solder paste Flux to cover this fluctuation range; otherwise, some pads will have delayed wetting while others will be over-corroded. We once used flying probe testing to compare the tinning speed of 12 pads at different positions on the same batch of OSP boards, and the maximum time difference reached 0.8 seconds, which is a direct manifestation of the mismatch between uneven film thickness and flux response.

The type of organic acid in the solder paste is more important than the total amount

JJY JH-602 uses a compound system of azelaic acid and hydroxybenzoic acid: the former provides mild initial etching power, while the latter accelerates decomposition above 150°C and releases secondary activity. This two-stage activation mechanism can avoid the premature activity burst of traditional single rosin-based solder paste in the 140°C soaking zone, which would cause local breakdown of the OSP film. We used a hot stage for real-time observation on a Suzhou customer's production line and found that this solder paste starts to break the film significantly at 165°C, which falls exactly in the overlap zone between the OSP film thermal decomposition onset temperature (162±3°C) and the solder paste main melting range (179 to 217°C), ensuring wetting synchronization while eliminating the risk of copper surface oxidation.

Printing and reflow parameters must make room for activity

Even if the correct solder paste is selected, if the stencil opening is too small or the squeegee pressure is too high, resulting in a solder paste volume below 0.08 g/cm², the flux will not be able to form a continuous liquid phase coverage layer at the initial stage of reflow, and local areas of the OSP film will remain unbroken due to lack of activator. We once used XRF to inspect the cross-section of cold solder joints and found that a complete OSP residual layer of 0.03 μm thickness still existed on the copper surface, while the solder paste printing volume at the corresponding position was only 0.062 g/cm². Therefore, the solder paste printing volume must be controlled within the range of 0.085 to 0.095 g/cm², and the reflow soaking zone should be set at 150 to 160°C for 90 seconds to ensure that the flux has enough energy to complete the film stripping across the entire board.