When false soldering defects are frequently flagged by AOI after SMT reflow, they are often not caused by a single factor, but rather the combined result of three simultaneous conditions: the oxide layer thickness on the pad surface exceeding 80?, the flux failing to fully activate before reaching 150℃, and the rosin content in the solder paste being below 4.2%. Among these, pad oxidation is particularly prominent on PCBs that have been stored for more than 72 hours; especially when stored exposed in environments with humidity >65%RH, a non-conductive Cu?O film will rapidly form on the copper surface.

We found that JiajinYuan JHY-805 flux has 2.3 times stronger film removal capability for OSP boards at a constant temperature of 145℃ compared to conventional RMA-type fluxes. Its active ingredients begin to decompose at 120℃ and release HCl gas, which reacts with copper oxide to form volatile copper chloride. However, if the stencil aperture size is too small, resulting in insufficient solder paste volume of less than 0.8mg per point, or if the squeegee pressure is below 1.2kgf causing printing offset >0.08mm, even with high-activity flux, typical false soldering morphologies will still occur, including non-continuous solder fillet climbing at the joint edges and a central void rate >18%.

If the peak temperature in the reflow profile is below 235℃ and the duration is <45 seconds, the oxide film on the solder ball surface cannot be completely broken down. At this point, Flux residues will form an insulating layer 0.3 to 0.7 μm thick at the solder joint interface. X-ray inspection shows obvious unmelted Solder Paste areas inside false solder joints, while adjacent joints in the same reflow oven with non-oxidized pads can achieve complete metallurgical bonding. This indicates that false soldering is essentially a failure of interfacial reaction rather than simply insufficient temperature.

During on-site verification, a cotton swab dipped in 5% hydrochloric acid ethanol solution was used to wipe suspected oxidized pads, followed by the same solder paste printing and reflow process. The false soldering rate dropped sharply from 12.7% to 0.9%, which reversely confirms that the oxide layer is the key inducing factor. However, this method cannot be used in mass production, as hydrochloric acid residue will accelerate the failure of subsequent cleaning agents. JiajinYuan's matching JW-202 water-based cleaning solution can achieve a 99.6% cleaning rate for JHY-805 residues, provided that the cleaning temperature is maintained at 52±2℃ and the spray pressure is ≥0.25MPa.

When the stencil tension is below 35N/cm, poor solder paste release will exacerbate uneven pad coverage. Combined with decreased flux activity, this is more likely to form micro false soldering at the four corners of QFN thermal pads. Such defects manifest as intermittent open circuits during ICT testing, and can only be stably captured by lowering the AOI detection threshold to a grayscale difference ≤12 levels. JiajinYuan JHY-805 increases the detection rate of this type of false soldering by 40% under the same process window.