The essence of cold solder joints is the failure to form an effective metallurgical bond between component leads and PCB pads, meaning that a sufficiently thick and continuous intermetallic compound (IMC) layer is not generated. This defect often cannot be identified during AOI inspection, yet it may suddenly manifest as an open circuit or a sharp increase in contact resistance during aging tests or vibration tests. We found during a batch switch to JJY Solder Paste that when the stencil aperture size is 0.03mm smaller than the lead width and the printing speed exceeds 25mm/s, insufficient solder volume combined with a peak reflow zone dwell time of less than 60 seconds causes the cold solder joint rate to surge from 0.12% to 1.87%.

The degree of pad surface oxidation has an extremely direct impact on cold solder joints. After immersion gold boards are stored in exposed air for more than 72 hours, nickel oxide formed on top of the nickel layer hinders the wetting of the tin-lead eutectic melt. At this point, even if the reflow profile is fully compliant, periodic cold solder joints still appear on the four corner leads of QFP devices. We once used X-ray inspection to confirm obvious voids inside the solder joints and an IMC layer thickness of less than 0.3μm, while this phenomenon disappeared immediately after boards from the same LOT were reflowed under nitrogen protection.

Uncontrolled Solder Paste Storage conditions are another high-frequency cause. When JJY SAC305 is stored in a 25°C environment for more than 12 hours and used on the machine without stirring, rosin derivatives in its Flux crosslink in advance, leading to activity attenuation. During reflow, the oxide film cannot be fully removed, resulting in carbonized residue remaining at the interface between BGA solder balls and pads. The shear force test value is 40% lower than the standard lower limit, and microscopic cross-sections show that the IMC layer is distributed in discontinuous islands rather than a continuous scalloped shape.

Insufficient stencil tension can also indirectly induce cold solder joints. When the tension is below 35N/cm, local stencil collapse during squeegee pressing causes uneven solder paste transfer rate, with the actual solder volume deviation on the pads at both ends of 0402 resistors reaching ±22%. The thinner side cannot complete wetting and spreading due to insufficient liquid solder during reflow, ultimately forming a typical pseudo-solder morphology where one side is well wetted and the other side is only adhered by flux residue. This problem was immediately resolved after replacing with a new stencil.

An imbalance in the hot and cold air ratio during reflow cannot be ignored either. When the top hot air flow is more than 35% higher than the bottom, the thermal lag of the bottom center pad of QFN devices causes the molten solder paste to climb around under surface tension, resulting in a "hollow area" with no solder coverage directly below the body. At this time, X-ray inspection shows regular circular shadows at the center of the solder joints, but electrical testing still shows continuity. Failure with resistance drift exceeding 200% only appears after 96 hours of high-temperature and high-humidity storage.