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Which Soldering Defects are Related to the Incorrect Setup of the Reflow Profile?

As shown in the figure below, there are four different zones in a typical refow profile – pre-heat, pre-heat and soak, reflow, and cooling zone. The incorrect setup with zone temperatures and times will cause different soldering defects. different zones in typical refow profile
  1. Pre-heat zone (from room temperature to 150°C)
    • Solder paste collapse -- ramp up too fast
    • The solvent in flux cannot vapour thoroughly, so the viscosity of the solder paste will be decreased and cause the paste collapse
    • Soldering ball -- ramp up too fast
    • The solvent in flux vapours out too fast, causing some tiny paste areas to separate from the major paste area and forming individual solder balls around the component.
    • Solder splash – ramp up too fast
    • Same as soldering ball.
    • Poor wetting – ramp up too slow
    • Solder paste is oxidizing if the pre-heat too long.
  2. Pre-heat and soak zone (from 150°C to 180°C)
    • Tombstone -- ramp up too fast
    • The PCB and components didn’t get an even heat up.
    • Void -- ramp up too fast
    • The solvent in flux escaped too quickly.
    • Poor solder joint fillet -- ramp up too slow
    • The flux lost its activity once it stayed soaking too long.
  3. Reflow zone (above 217°C ~220°C for SAC305 solder paste)
    • Component damaged – peak temperature is too high
    • Once the peak temperature is over 260°C and stays there for more than 10 sec, most of components will be damaged.
    • Cool solder -- peak temperature is too low or time too short
    • The peak temperature should be 20~25°C higher than the solder liquid point and remains there for 30~90 seconds.
  4. Cooling zone (below 217~220°C after reflow)
    • Components crack -- cooling down too fast
    • Components experienced thermal shock outside of their specifications.
    • Increase component’s fatigue risk –cooling down too slow
    • The slow cooling can easily lead to an excess amount of intermetallic compounds and produce larger crystal particles together in soldering joint, reducing the fatigue strength.
 
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