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Tips for Successfully Using Vacuum Reflow Ovens in SMT Soldering

  • Solder Paste
  • Voiding

  • The tremendous increase in the use of bottom terminated components (BTCs), some estimates are at more than 50 billion per year, have heightened the concern for minimizing voiding between the thermal pad and the BTC. It is not uncommon for more than 40% of the thermal pad area to experience solder voiding after reflow, increasing the risk of over-heating of the BTC in field operation.

    In response to this concern, vacuum reflow ovens have emerged. These ovens all but eliminate voiding, however, if not used properly, they can cause solder splatter/splash.  Here are some critical recommendations to successfully reduce both voids AND solder splatter:

    1. The pressure in the oven will be reduced to about 200 mbar (1 atmosphere is 1000 mbar). The steeper the slope, in going from 1000 to 200 mbar, the more likely to produce splattering. So minimizing the slope will help to reduce solder splatter.
    2. The vacuum should be pulled about 30 seconds after the solder becomes molten.  This parameter is important with some fluxes needing more time and others needing less time. The time delay before the vacuum is pulled is to maximize the chances of solvent volatilization in the flux and prevent solder spatter.
    3. When possible, keep the vacuum on until after the solder has solidified.  Some ovens only have a vacuum chamber in the peak zone and in this case the vacuum should be kept on as long as possible. 

    In some cases, what people have thought was solder splatter was actually caused by the stencil printing process. If the stencil under-wipe process or the board gasketing to the stencil is not adequate, solder can be transmitted to the board in areas where it is not intended to be transferred. After reflow, this offending solder can look like solder splatter. To avoid the defect, assure effective under-stencil cleaning and board gasketing.

    It should also be pointed out that some solder pastes have been developed to reduce voiding to around 10% without requiring vacuum reflow. Contact Indium Corporation’s Technical support team to discuss these lower-voiding solder paste options.