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Causes and Rectification Plans of SMT Cold Solder Joints & Dull Solder Joints
Cold solder joints and dull solder joints are hidden quality problems easily occurred during reflow soldering. The solder joints look dim without luster with weak internal combination, which will easily cause poor contact and signal interruption and seriously affect long-term product reliability.

1. Cold Solder Joints

Phenomenon: Insufficiently filled and dim solder joints, solder is not fully melted with weak bonding strength.
Main Causes

  1. Too low peak temperature fails to reach solder paste melting point.

  2. Insufficient soaking time leads to incomplete flux activation.

  3. Excessively fast conveyor speed shortens heating time.

  4. Thick multi-layer PCB absorbs too much heat resulting in insufficient actual temperature.

  5. Mismatched solder paste with high melting point.

Solutions

  1. Raise reflow peak temperature to meet melting requirements.

  2. Extend soaking time to remove oxide thoroughly.

  3. Slow down conveyor speed properly for sufficient heating.

  4. Make exclusive temperature profile for thick and multi-layer boards.

  5. Select suitable solder paste according to product features.

2. Dull Solder Joints

Phenomenon: Solder joints turn gray and foggy with poor wettability.
Main Causes

  1. Insufficient nitrogen protection leads to high-temperature oxidation.

  2. Low-activity flux cannot remove oxide effectively.

  3. Severe oxidation on PCB pads and component pins.

  4. Too slow cooling speed causes continuous oxidation.

  5. High workshop humidity brings moisture influence.

Solutions

  1. Stabilize protective gas concentration inside oven.

  2. Adopt high-activity matched flux.

  3. Reject severely oxidized incoming materials strictly.

  4. Speed up cooling process to keep bright solder appearance.

  5. Control workshop temperature and humidity well.

These hidden defects need temperature testing, first article strength test and aging sampling inspection for early detection. Full control on temperature, atmosphere and materials is essential.
Globalwell tests actual reflow temperature for every new product and sets different parameters for different circuit boards. We effectively avoid cold and dull solder joints and guarantee stable performance of finished PCBA products.


 

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