Critical Threshold Limit for Effective Solder Void Size Reduction by Vacuum Reflow Process for Power Electronics Packaging

In recent years, power electronics packaging industry has tightened the tolerance criteria for acceptable solder void size to below 5% in the power packages. Vacuum reflow technology has been introduced to reduce the solder void size in applications, where there is a large contact area coupled with low bond line thickness (BLT). Three critical vacuum reflow parameters which are the pressure level, the pressure pump-down rate, and the vacuum dwell time were studied by using the full-factorial design of the experimental method. The pressure level is shown to be the dominant significance factor, followed by the pressure pump-down rate to a much lesser extent, whereas the vacuum dwell time has minimal effects. The critical threshold of the pressure level required to ensure the achievement of 100% yield in meeting the 5% criteria is estimated to be 8 kPa. Effective solder void size reduction has been consistently demonstrated for a range of solder alloys from high lead Pb95Sn5, eutectic PbSn, to lead-free BiSnAg by using nonclean and low residue flux in post reflow as part of solder paste selection.