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Microballs In The Reflow Soldering Process

Tips for analyzing the causes

As a rule, the first conclusions as to the cause of microballs can be drawn from the distribution of the microballs.

  1. Process

    Application of solder paste
    • positioning faults of print
    • smeared solder paste, dirty stencil
    • influence of humidity? (solder paste is hygroscopic)
    Reflow soldering process
    • type of heating (vapour phase has greater tendency to spilling solder than IR heating)
    • insufficient preheating process
    • heating-up for reflow peak too fast
    • dwell-time at pre-heating time too long (atmospheric condition)

  2. components, PCBs

    • poor solderability of pads and/or pins

  1. Solder paste

    number of microballs decreasing, if:
    • metal content higher
    • particle size of soldering powder larger
    • particle density lower

    number of microballs increasing, if:
    • flux activity too weak
    • oxide content of solder powder too high
    • poor stability of contours

  2. Layout

    • Wicking: Solvent with solder powder share drifts - before the soldering process -, especially in narrow, capillary gaps between component and board (small stand-off). This is favoured by thickly applying solder paste.
If
microballs reproducible as regards amount, physical distribution and chronological consistency
i.e.:
look for cause(s) in the areas
 ⇒ influence of layout
 ⇒ systematic influence on processes
 
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