Microballs In The Reflow Soldering Process

Notes on root cause analysis

From the spatial distribution of the solder beads, initial conclusions can usually be drawn about the cause of formation.

1. Process

Solder paste application

  • Print offset
  • smeared solder paste, unclean stencil
  • Influence of humidity ? (Solder paste hygroscopic)

Reflow process

  • Heating mode (vapor phase tends to solder spatter more than IR heating).
  • insufficient preheating process
  • Heating up too quickly in the reflow peak
  • Staying too long at preheating temperature (atmospheric conditions).

2. Components, printed circuit boards

  • Solderability defects for pads and/or pins

3. Solder paste

Decreasing tendency of the solder beads when:

  • Metal content higher
  • Particle size of the solder powder larger
  • Particle density smaller

Increasing tendency of solder beads when:

  • Flux activity too weak
  • Solder powder oxide content too high
  • Contour stability poor

4. Layout

  • Wicking: Solvent drifts - before the soldering process - with solder powder contents especially in narrow capillary spaces BE-LP (low stand-off). This is favored by thick solder paste application.

In the case solder beads reproducible by quantity, spatial distribution, and temporal constancy, ie:

Do you have problems with solder beads during the reflow process ?

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