Temperature Drift: The Hidden Culprit Behind Cold Solder Joints

After the JJY Dip Soldering machine operates continuously for 4 hours, the solder pot temperature often fluctuates by ±8°C. At this point, the PCB pads cannot receive sufficient energy to activate the Flux, and the surface tension of the solder cannot be reduced in time. The result is dull solder joints, insufficient solder climb height of less than 0.3mm, and obvious flux residue remaining at the root of the pins. Such defects are often misjudged as Solder Paste printing offset during AOI inspection, but are actually caused by the failure of the temperature compensation mechanism.

Preheating Zone Temperature Control Accuracy Determines Wetting Success or Failure

If the temperature is below 90°C when the PCB enters the preheating zone, the moisture absorbed by the FR-4 substrate will not be fully discharged before it directly contacts the high-temperature solder. The water vapor instantly vaporizes and forms micro-explosions, resulting in an internal porosity rate of over 15% in the solder joints and preventing the solder from climbing upward along the pins. If the temperature is above 110°C, the body of the resistive and capacitive components will age prematurely, leading to tombstoning or cracking in the subsequent Reflow Soldering stage. We once found during production line testing that a 2cm deviation of the preheating zone sensor probe from the center of the heating plate was sufficient to cause a ±6°C reading deviation.

Dip Soldering Time and Temperature Must Be Dynamically Matched

For every 5°C increase in solder pot temperature, the standard dip soldering time must be shortened by 0.3 seconds. Otherwise, the solder joints will suffer from a series of problems such as copper foil peeling, aggravated solder oxidation, and excessive dissolution of the pin plating due to overheating. When processing 0.4mm pitch QFP packages, we once failed to reduce the dip soldering time synchronously, resulting in 32% of the pins exhibiting dewetting. After recalibration, this proportion dropped to below 0.7%.

Daily Inspections Must Cover Three Critical Checkpoints

Before starting work every day, use a calibrated infrared thermometer to measure the temperature 10mm below the surface of the solder pot, use a thermocouple to verify whether the temperature difference between the inlet and outlet of the preheating zone is less than ±2°C, and use an electronic stopwatch to recheck whether the dip soldering cylinder action cycle is stable between 2.8 seconds and 3.3 seconds. If any item exceeds the tolerance, the machine must be shut down for adjustment. Do not rely on the values displayed on the equipment panel, as some batches of JJY controllers have an inherent offset error of 0.5°C.

Temperature Pitfalls in Solder Replacement Operations

When mixing new and old Solder Bars, if the pot temperature is not first lowered to 220°C before adding them in batches, the cold solder blocks will locally lower the temperature of the molten pool, causing uneven heating of the PCB pads that have just entered the pot. At this time, even if the instrument displays 255°C, the actual temperature in the effective soldering area may only be 242°C, directly inducing batch cold solder joints. We require that each solder replacement must follow the five-step method of 'lowering temperature, batch feeding, constant temperature soaking for 30 minutes, recalibration'.