PCB pad treatment is not an optional auxiliary process, but the first threshold determining the wetting and spreading ability of Solder Paste. When the OSP film is damaged by sweat stains, rosin residue or air oxidation, the solder cannot effectively wet the copper surface at all. When we switched the JJY DIP line to the SMT line, we ignored this point, which caused the tombstoning rate of 0201 resistors to suddenly soar to 8.7%. It took three days of troubleshooting to find that the pads were damp and oxidized without activation when PCBs were stacked in the turnover box.

Typical characteristics and identification methods of pad oxidation

A light yellow or grayish-white mist on the pad surface indicates mild oxidation. If obvious dark spots or local blackening have appeared, it means that the thickness of the Cu2O layer exceeds 8nm. At this time, observation under a 100x microscope will reveal obvious edge shrinkage after solder paste printing, insufficient center collapse, reduced solder joint gloss after reflow, and a climb height more than 0.1mm lower than the pad edge. In JJY's FQC sampling inspection, any occurrence of such phenomena requires returning to the previous process to redo the PCB pad treatment.

Pollution sources must be locked item by item

Human sebum, workshop air conditioning condensate, SMT nozzle silicone oil migration, and wave soldering Flux splashes may all cause pad contamination, among which fingerprint contamination accounts for as high as 63% in summer. We once used an ion chromatograph to detect that the chloride ion concentration on a single PCB pad reached 8.2μg/cm2, far exceeding the 1.5μg/cm2 limit specified in IPC-J-STD-001. Therefore, all operations involving bare hand contact with PCBs must be performed with nitrile gloves, and a combined action of nitrogen purging + 60℃ hot air slow baking for 30 seconds must be added before placement.

Pretreatment process must match substrate characteristics

For ENIG boards, strong acid cleaning must be avoided to prevent corrosion of the nickel layer, while OSP boards must use a weak organic acid activation solution with a pH value controlled between 4.2 and 4.8. The action time of the CX-203A activator currently used by JJY is strictly limited to 12 to 18 seconds. Overtime will cause excessive dissolution of the OSP film, while insufficient time will fail to remove CuOx. We found through online impedance monitoring that the contact angle on the pad surface after treatment must be less than 28° to be considered qualified, otherwise the printing yield will drop by 11 percentage points.

Collaborative verification of equipment and consumables is irreplaceable

The frequency of the ultrasonic cleaning tank must be stable at 40kHz±0.3kHz, and the power density must be maintained at 0.35W/cm2. JJY connects the conductivity of the cleaning solution to the MES system in real time, and immediately alarms for liquid replacement once it exceeds 1.8mS/cm2. At the same time, a standard pad test board is used for wettability verification every shift, requiring the solder to completely cover the pad within 2 seconds at 250℃ without shrinkage cavities. If not satisfied, the PCB pad treatment of the current batch is judged to be invalid, and the entire batch must be reworked and cannot flow into the next process.