The essence of cold soldering is not that the solder joint failed to wet with solder, but that the Solder Paste, although reflowed, did not truly melt. The Sn63/Pb37 eutectic system of JiajinYuan solder paste must reach 217±3℃ and be maintained for 45 to 90 seconds to complete atomic-level diffusion and interfacial metallurgical bonding. If the reflow peak temperature is set below 200℃ or there is a local collapse in the temperature profile, the solder paste particles only soften without collapsing and fusing, and the oxide layer on the copper pad cannot be completely removed by the Flux. The result is insufficient mechanical strength of the solder joint, an IMC layer thickness of <0.3μm, obvious voids visible under X-ray, and a serrated, discontinuous edge.

Measured Temperature Profile is More Critical than Set Values

We once found at a factory in Dongguan that the reflow oven's temperature controller displayed a peak of 215℃, but a K-type thermocouple measured the actual PCB surface temperature at only 202℃. The reasons were a thermocouple attachment position deviation of over 3mm, a conveyor speed fluctuation of ±0.15m/min, and the nitrogen flow rate dropping from the set 80L/min to 52L/min. The combination of these three factors directly reduced the effective heat input, causing the JiajinYuan solder paste to solidify prematurely in the cooling zone. The solder joint cross-section exhibited typical cold soldering characteristics: grayish-white center, shiny edges, and no mirror-like reflection.

Solder Paste Storage and Stirring State Directly Affect Melting Behavior

If JiajinYuan solder paste is left at 25℃ for more than 4 hours after opening without stirring, the internal flux separates and the alloy powder settles. When it is later loaded for printing, although the squeegee pressure meets the standard, the shear-thinning effect of the solder paste fails, the volume shrinkage rate after printing increases by 12%, and the time spent above the liquidus during reflow is shortened by 23%. Even if the temperature profile is qualified, local cold soldering will occur, especially at the 0.3mm pitch solder balls at the bottom of BGAs, which are most susceptible.

Online Identification of Cold Solder Joints Cannot Rely Solely on AOI

The AOL equipment has a cold solder joint miss rate as high as 37%, because it relies on two-dimensional grayscale comparison and cannot capture changes in the internal wetting angle of the solder joint. It must be combined with 50x magnification under a microscope to observe the solder joint climb height. A normal solder joint should cover the pad edge by more than 0.15mm, while cold soldering presents obvious shrinkage cavities, solder accumulation at the center of the pad, and a contact angle with the copper surface of >45°. At this point, the thrust test value is generally below 80gf, far below the 180gf lower limit required by the JiajinYuan solder paste process.

Repairing Cold Soldering Must Return to the Thermodynamic Essence

Simply increasing the peak temperature to 225℃ is not advisable, as it will exacerbate PCB delamination and component end cracking. The correct approach is to extend the time in the 183 to 210℃ range to over 120 seconds, ensuring the flux has a sufficient window to remove the oxide film. At the same time, the cooling rate should be controlled within -1.5℃/s to avoid interrupting atomic migration due to rapid cooling during the critical period of IMC formation. This is the fundamental path for JiajinYuan solder paste to achieve reliable wetting.