Appearance and Physical Inspection: Observe the solder joint with a magnifying glass. A qualified solder joint is smooth and full, while a cold solder joint will show dark gray rings, fine cracks, and incomplete solder wetting of the thermocouple wire surface.
Gently shake the thermocouple wire near the solder joint. A cold solder joint will feel noticeably loose, while a qualified solder joint will not feel loose at all.
Electrical Performance Testing: Use a multimeter in continuity/resistance mode to measure the thermocouple circuit. A cold solder joint will show high impedance, with a resistance value much higher than a qualified thermocouple of the same specification, and may even show intermittent open circuits.
With the power on, gently tap the area near the solder joint. A cold solder joint will show a jump in thermoelectric potential and momentary fluctuations in the temperature display value, while a qualified solder joint will have a completely stable signal.
Hot-State Verification and Troubleshooting: Locally reheat the solder joint with a soldering iron or hot air gun. If the temperature signal suddenly returns to normal after heating, but drifts again after cooling, it can be determined as a cold solder joint.
During the hot runner heating process, the temperature display of a poorly soldered thermocouple will exhibit irregular jumps and will not rise smoothly in sync with the actual hot runner temperature.
By combining the above three steps for troubleshooting, the hidden faulty solder joint of the thermocouple can be quickly located, preventing inaccurate hot runner temperature control caused by the poor solder joint.
