During the soldering process of OSP boards, the entire production line dwell time from when the PCB completes OSP chemical film formation to before it enters the reflow oven must be strictly controlled. Because the OSP film is essentially a layer of organic complex adsorbed on the copper surface, its thermal stability threshold is only about 150°C and its moisture resistance is extremely weak. Once exposed to air for too long, especially in workshop environments with frequent temperature and humidity fluctuations, the film layer will undergo irreversible hydrolysis and oxidation reactions. This degradation process is completely invisible in appearance, and only during the Reflow Soldering stage will concentrated phenomena such as pad blackening, poor wetting, and increased bridging be exposed. We found that a customer production line of JiajinYuan once suffered from delayed line change scheduling, causing OSP boards to dwell at the SMT placement station for an average of 36 hours. As a result, the first article verification immediately showed that the solder climbing height on the end of 0402 resistors was less than 0.15mm, and the AOI missed inspection rate was as high as 8.3%.

In actual control, we cannot only focus on the total duration, but also need to break down the connection gaps between each process node. For example, if OSP boards are directly stacked on pallets without spacers after exiting the OSP line, the bottom boards will have slower heat dissipation and accumulated moisture, causing their film layer failure rate to be 1.8 times faster than that of the top layer. If the manual visual inspection station uses strong LED lights for continuous irradiation for more than 5 minutes, the local temperature rise will cause premature aging of the film layer, which has been repeatedly verified by FTIR testing on site at JiajinYuan. When the vacuum degree of the placement machine nozzle is lower than -75kPa, the impact force of picking and placing boards increases, causing microscopic scratches on the OSP film. When this type of damage is superimposed with subsequent reflow soldering thermal stress, microcracks are extremely likely to appear at the pad edges, causing the ICT test contact impedance jump to exceed 2Ω.

We require that all OSP boards be affixed with an aging label at the exit of the OSP line, with the label containing the board exit time accurate to the minute, and linked with the MES system to lock the path trajectory of each board. When a board stays for more than 12 hours after screen printing, or more than 8 hours after SPI inspection, or more than 6 hours after placement is completed and has not yet entered the oven, the system automatically triggers a red warning and freezes the release permission of this batch. JiajinYuan embedded this rule into the equipment PLC logic at its Dongguan factory, increasing the first-pass yield of OSP board soldering from 92.6% to 99.1%, and narrowing the IMC thickness CV value of solder joints from 14.7% to 6.3%. In addition, the use of chlorine-containing wiping cloths to clean OSP boards must be strictly prohibited; even a single wipe will cause local film passivation, leading to the formation of electrochemical corrosion channels between adjacent pads.

The buffer area in front of the reflow oven must be equipped with an independent constant temperature and humidity cabinet, set to 23±1°C, RH≤40%. Non-OSP boards are prohibited from being mixed in the cabinet to avoid contamination of the OSP film by Flux residues volatilized from other boards. We once found that a certain production line shared the same buffer rack for OSP boards and immersion gold boards. After one week, sampling XPS analysis showed that the Cu 2p peak intensity of the OSP boards decreased by 22%, proving that a significant oxide layer had formed. JiajinYuan recommends using transfer carts with RFID identification, with each cart limited to carrying 12 OSP boards. The cart body is equipped with a built-in temperature and humidity recording module, with data directly transmitted to the process database to ensure full-process traceability, no breakpoints, and no manual intervention.