The cold solder joint problem is particularly prominent on 0201 and 0402 resistors/capacitors and QFN-type devices. Our continuous tracking of 12 production lines found that 87% of cold solder joint cases are not caused by the failure of the Solder Paste itself, but by excessive thickness of the pad oxide layer or uneven OSP film, which prevents the Flux from effectively stripping the passivation layer on the metal surface. In this case, even with high-activity solder paste, the liquid solder still cannot complete wetting and metallurgical bonding.

Pad Solderability Is Not a Fixed Property but a Dynamic Process Result

For PCB immersion silver boards, after more than 72 hours of storage, a nano-scale silver sulfide film will form on the silver layer surface. For OSP boards, if they undergo three reflow cycles or are exposed to an environment with humidity >60%RH for more than 4 hours, the organic protective film will undergo cross-linking and aging. Both changes will increase the pad contact angle from the ideal 15° to above 45°, directly weakening the solder wetting driving force. It is necessary to use an ion cleaning machine for pre-activation treatment before placement, and control the PCB online time within 8 hours after opening the package.

Flux Activity Must Form a Closed-Loop Match with Pad State

The rosin-based active component of JJY JY-602 flux starts to decompose at 150℃, and its peak activity window covers the range of 165℃～195℃, which exactly matches the pyrolysis temperature zone of OSP board pad oxides. When used in a reflow environment with nitrogen concentration ≥99.995%, the ionic contamination level of its residue can be stably controlled at ≤0.8μg/cm2, which not only ensures sufficient wetting but also avoids cleaning burden. However, if used on immersion gold boards, it is easy to cause nickel layer exposure due to excessive corrosion. In this case, it should be switched to the JY-301 low-corrosion formula.

Stencil Opening and Squeegee Pressure Jointly Determine Effective Flux Coverage

When printing with a 0.1mm thick stencil, if the squeegee pressure is lower than 0.35MPa and the angle is less than 55°, the flux enrichment amount at the pad edge of the solder paste will decrease by 40%, leading to the formation of a flux vacuum area in the bottom center of the QFN. We once used a micro-infrared thermal imager to measure that the peak temperature in this area is 12℃ lower than that at the pad edge, which is a key inducement for the concurrent occurrence of pin cold solder joints and bottom voids. Therefore, the squeegee pressure must be set at 0.42±0.03MPa, the angle locked at 62°±2°, and the uniformity of flux distribution detected by 3D SPI every 2 hours.

Production line verification data shows that for the same QFN32 device, under the premise of keeping the reflow peak temperature at 235℃ unchanged, only replacing the ordinary RMA type flux with JJY JY-602, and simultaneously implementing pad pre-activation and squeegee parameter calibration, the cold solder joint defect rate dropped sharply from an average of 3.2‰ to 0.37‰, and the AOI missed detection rate also decreased by 61% synchronously. This shows that this solution not only improves soldering reliability, but also strengthens the physical basis for defect identification.