For PCB pad finishing using the ENIG process, cold solder joints are the most common defect encountered on SMT production lines. This is primarily because an excessively thick gold layer in the electroless nickel/immersion gold structure inhibits the wetting and spreading of Solder Paste. If the gold thickness exceeds 0.1 μm, it is necessary to use no-clean, high-activity solder paste and maintain the reflow peak temperature stably at 235℃±3℃; otherwise, dull solder joints and insufficient solder climb height are likely to occur. Meanwhile, the stencil aperture must be reduced by 8–10 μm on each side relative to the pad to avoid solder ball residue.

PCB pad finishing using the immersion silver process is extremely sensitive to workshop humidity. When the ambient relative humidity exceeds 60%RH, a gray-black silver sulfide film will form on the silver layer surface within 48 hours, causing the Flux to be unable to effectively remove oxides during reflow. In this case, even increasing the preheat zone ramp-up rate from 1.2℃/s to 1.8℃/s can hardly improve poor solderability. A more effective approach is to bake the immersion silver boards in a nitrogen oven at 60℃ for 2 hours before line loading, and strictly limit the time from opening the vacuum package to completing placement to no more than 12 hours.

PCB pad finishing using the OSP process is most vulnerable to mechanical scratches and acidic contamination. The film thickness standard for JJY OSP chemical is 0.25–0.45 μm. If the stencil squeegee pressure exceeds 3.5 kgf or the number of printing cycles exceeds 15, local film rupture may occur, exposing the copper surface. This area will rapidly oxidize during reflow and cause cold solder joints. Therefore, it is necessary to use a 300-mesh stainless steel stencil with low shear force, and after every 10 boards printed, use an ionic contamination tester to spot-check whether the chloride ion content on the board surface is below 1.5 μg/cm².

For all three PCB pad finishing methods, the first-article confirmation stage must not rely solely on AOI images. A 10× magnifier must be used to inspect each solder joint side to verify whether the wetting angle is less than 30°, especially for QFN bottom pads and both ends of 0201 resistors. ENIG boards commonly exhibit good wetting in the center but shrinkage at the four corners; immersion silver boards often show full solder wetting on one side and solder dewetting on the other; while OSP boards are prone to hidden defects where individual positions in an entire row of pads have no solder wetting at all, yet no AOI alarm is triggered.

The OSP rework solution provided by JJY is only applicable to bare boards that have not undergone reflow. Once a soldering failure occurs, the board must be scrapped, because high temperature will carbonize the organic film and leave residue on the copper surface, forming a weak interface layer. In this case, re-Printing Solder Paste cannot establish a reliable intermetallic compound. Although ENIG and immersion silver boards can undergo secondary reflow, after immersion silver boards are reheated more than twice, the risk of silver migration on the pad surface increases sharply. It is necessary to simultaneously lower the furnace temperature by 5℃ and shorten the holding time by 30 seconds to reduce the rate of deterioration.