Cold solder joints often appear grayish-white without metallic luster under AOI inspection, with obvious edge burrs, local depressions or residual solder balls. Under a microscope, incomplete fusion of solder particles and a clear boundary at the interface can be clearly observed. Such joints occasionally show open circuits in ICT testing and undergo batch fracture during the high-temperature aging stage after functional testing. The root cause is that the actual peak temperature of the reflow oven fails to meet the melting redundancy requirement of 20℃ above the Solder Paste alloy eutectic point. For example, the theoretical eutectic point of JIAJINYUAN SN96.5AG3.0CU0.5 solder paste is 217℃, but the production line measured temperature profile shows that the end of the third temperature zone only reaches 214.2℃, and the holding time is less than 45 seconds, leading to Flux residue wrapping around unmelted particles and forming a false mechanical interlock.

Insufficient stencil tension in the printing process exacerbates the risk of cold soldering. When the stencil aperture size is 0.25×0.4mm and the tension is lower than 38N/cm, the fluctuation range of solder paste transfer rate expands to 62%～79%, and the deposition amount on some pads is lower than 0.08mg. Even if the reflow parameters meet the standards, it is difficult to support complete metallurgical bonding. We tracked 12 batches of cold solder defects on a customer's production line and found that 9 of them corresponded to a daily stencil cleaning frequency exceeding 3 hours per time, the squeegee pressure dropped from 4.2kg to 3.6kg, and the insufficient solder paste warm-up time of less than 4 hours caused the viscosity to increase by 18%. The combination of these three factors reduced the solder paste collapse amount by 0.015mm, directly weakening the self-alignment capability during reflow.

Insufficient nitrogen protection concentration is also a hidden cause. When the O? content in the furnace is higher than 150ppm, the growth rate of the SnAgCu alloy surface oxide film in the 210℃ range reaches 0.8nm/s, far exceeding the reduction capability threshold of the flux. At this time, even if the peak temperature meets the standard, the solder joint still presents a typical cold solder morphology. We used XPS to conduct component analysis on the failed solder joints and confirmed that their SnO? proportion reached 12.7%, while that of normal solder joints was only 2.1%. This indicates that the blockage of the nitrogen flowmeter was not detected in time, and the actual N? flow dropped from 25L/min to 16.3L/min, yet the original value was still maintained in the process record sheet.

The shear force test data of cold solder joints is highly deceptive. When tested with a Dage 4000+ at a rate of 10μm/s, the initial reading can reach 12gf, but after continuous loading for 0.8 seconds, it suddenly drops to 3.4gf accompanied by a brittle fracture sound. This nonlinear attenuation characteristic is directly related to the absence of an IMC layer. The XRD pattern confirms that the intensity of its Cu6Sn5 diffraction peak is less than 31% of that of the standard sample, and the EDS surface scan shows that the Cu/Sn atomic ratio deviates from the ideal 3.8:1 ratio by ±0.9 units, indicating that the reaction kinetics are severely hindered.

After replacing with a new batch of JIAJINYUAN solder paste of the same model, the cold solder rate did not improve. Reverse investigation found that the calibration deviation of the reflow oven thermocouple reached ±1.9℃, and carbon accumulation on the track chain caused a 7.3% decrease in heat transfer efficiency. Without adjusting the temperature zone settings, simply cleaning the chain and recalibrating the thermocouple reduced the cold solder rate from 1280PPM per million pieces to 210PPM. This reminds us that equipment status monitoring must be embedded in the daily inspection sheet, rather than relying on solder paste batch switching to mask hardware degradation issues.