If OSP process boards are not placed in nitrogen storage immediately after cutting, the organic film on the pad surface will degrade faster as ambient humidity rises. Especially when workshop RH > 60% and temperature exceeds 25°C, microscopic oxidation spots will start to appear after 12 hours of exposure. Such defects cannot be identified during stencil printing, but will manifest as local non-wetting or cold solder joints during Reflow Soldering. A batch of immersion silver boards on JJY's No. 3 SMT line was exposed for a total of 36 hours during transit because the logistics transfer warehouse was not equipped with nitrogen cabinets. After being put into production, the first article AOI missed detection rate reached as high as 18%, and re-inspection revealed continuous oxidation bands had formed along the pad edges.

The core of PCB pad treatment is not simple spraying or wiping, but matching the corresponding activation method according to the surface treatment type. For example, immersion gold boards require short-time wetting with weakly acidic ionized water with a pH of 4.2 to 4.5 before blow-drying, while OSP boards must not come into contact with any liquid, and can only be purged with positive-pressure nitrogen with an oxygen content < 100ppm combined with static elimination. Otherwise, residual moisture will instead accelerate copper surface oxidation. We once tested a domestic cleaning agent on Line 5; although it was labeled neutral, its measured pH reached 7.8. After continuously processing 300 boards, the average pad wetting angle increased by 23°, directly triggering batch rework.

Nitrogen concentration in the storage stage must be measured twice per shift with a portable oxygen analyzer, and reliance on equipment self-reported values is not allowed. This is because after the nitrogen generator filter becomes saturated, the output concentration may jump from 300ppm to 1200ppm without triggering an alarm. In July last year, JJY suffered pad oxidation on two batches of FR4 double-sided boards due to this issue. The remediation plan was to return all boards to the supplier for secondary OSP rework, the cost of which was far higher than the cost of routine spot checks. Meanwhile, all PCBs must be stored on stainless steel shelves at least 30cm above the ground to prevent bottom boards from being affected by upward seepage of ground moisture, which is especially critical during the plum rain season.

Turnover time control is more easily overlooked than storage conditions. The time from taking boards out of the nitrogen cabinet to putting them on the Solder Paste printer must be compressed within 25 minutes. Boards that exceed the time limit must be returned to the cabinet to stand for 2 hours before use. This is because the desorption equilibrium time for water molecules adsorbed on the pad surface is approximately 110 minutes, and forcing boards into production will lead to simultaneous aggravation of solder paste collapse and splashing. We compared different batches of the same model of PCB: Batch A was put into production 22 minutes after being taken out, with an average soldering void rate of 4.3%; Batch B was delayed to 31 minutes, and the void rate jumped to 9.7%, concentrated in the large-area pad areas at the bottom of QFN packages.

JJY's current work instruction clearly requires that all PCB pad treatment records must include four fields: nitrogen cabinet number, entry and exit timestamps, measured oxygen content, and operator employee ID. None of these fields can be missing. The system automatically compares overtime items and freezes the release permission for that batch. This logic has been running on eight main lines for 14 months, and pad oxidation-related defects have decreased by 82%. This shows that truly effective PCB pad treatment never relies on gut feeling based on experience, but on locking all variables in a measurable, traceable, and intervenable closed loop.