The most insidious aspect of cold solder joints is that they show no obvious flaws: the solder balls are full, the pins are shiny, and the solder joint contours are clear. Even under a high-power microscope, it is difficult to see obvious voids. However, once the entire device is powered on and running, and the temperature starts to rise, the microcracks inside the solder joint repeatedly open and close due to thermal expansion and contraction, causing intermittent signal interruptions. We have repeatedly reproduced this phenomenon on the JJY SMT line. The same motherboard passed all room-temperature tests, but after 24 hours of power-on aging, the USB interface packet loss rate suddenly increased to 17%. After replacing the corresponding pin header, the fault disappeared. Cross-section analysis confirmed that there was indeed a 0.8 μm thick oxide isolation layer between Pin 3 and the pad.

The causes of cold solder joints are never a single factor, but rather the combined result of three factors: Solder Paste activity degradation, stencil aperture offset, and insufficient reflow peak temperature. For example, if the SN96.5AG3.0CU0.5 solder paste commonly used by JJY is stored for more than 6 months, the acid value of the Flux decreases by 22%. Even if the printing thickness meets the standard, the wetting force has already been lost by 35%. At this time, even if the furnace temperature curve shows a peak of 235°C, the actual surface temperature of the pad may only be 221°C, which is far below the 227°C eutectic point of the silver-copper alloy. The solder cannot fully melt and diffuse, and after cooling, it only forms a physical stack rather than a metallurgical bond.

We once used X-ray to perform sampling scans on 1200 BGA devices and found that cold solder joints were concentrated in the 3rd to 5th rows of solder balls at the edge of the PCB, while the integrity rate in the central area was 99.6%. This is directly related to the uneven airflow distribution in the reflow oven. If the wind speed deviation exceeds 0.3 m/s, it will cause the edge solder joints to heat up lag, and the time above the liquidus line will be shortened to 0.8 seconds, which is lower than the 1.2 to 2.5 seconds required by the process window. The nitrogen protection system upgraded by JJY last year reduced the oxygen content to below 100 ppm, and the cold solder joint rate dropped from 0.18% to 0.04%. However, the prerequisite is that the stencil tension must be maintained at 38 N/cm, otherwise fluctuations in squeegee pressure will cause the solder paste transfer rate dispersion to exceed the standard.

On-site judgment of cold solder joints cannot rely solely on a multimeter to measure continuity; dynamic load must be applied. For example, apply a 100 mA pulse current to the MCU power supply pin and simultaneously use an infrared thermal imager to capture the temperature rise gradient. The temperature difference of a normal solder joint is ≤1.2°C, while a cold solder joint will develop a local hot spot within 200 ms and the temperature difference will jump to above 5.7°C. This method has been verified in 37 batches at the JJY DIP rework station, with a positioning accuracy of 94%, which is 4.3 times faster than simple flying probe testing. The key is that it bypasses the AOI's recognition blind spot for micron-level interface defects.

Repair soldering cannot simply be reflowed in the oven. First, use a hot air gun to locally heat the suspicious solder joint at 280°C for 12 seconds, then use a 0.3 mm pointed soldering iron to apply a small amount of no-clean flux paste, and finally reinforce it with 0.15 mm diameter Solder Wire. The entire process must be completed in a nitrogen environment, otherwise a new oxide film will immediately form on the surface of the new solder joint. JJY has tried three different brands of nitrogen generators, and only the one with a stable flow rate of 12 L/min and a dew point ≤ -40°C can ensure that the one-time repair soldering pass rate exceeds 91%.