Advantages of Protective Atmosphere Control for No‐Clean Solder Pastes*

Abstract

Reflow soldering in a controlled O2 atmosphere will lead to practical benefits in the wetting behaviour of solder pastes. The required O2 level and its control depend on the solder paste used and the solderability of the materials to be joined. For a typical application with either fine‐pitch technology, multiple soldering or low residue solder paste, a lack of O2 control can cause soldering problems. Fine‐pitch solder pastes today still do not show an optimum slump behaviour, which is absolutely necessary to avoid bridging between IC leads and to reduce the formation of solder balls during reflow. Using a low O2 atmosphere to reduce these defects can make a difference, but an improved slump behaviour through predrying of the applied solder paste at 50°C or in a low pressure environment will be more effective. Spread of solder is in general better at lower O2 levels, but substantial improvements are only achieved if low residue pastes with low activation levels are reflowed. Even when the low residue paste is compatible with reflow in air, there is still a significant difference found between 20 and 200 ppm O2. Reflow systems which show variations up to 500 ppm in the preheat zones during loading, while maintaining 50ppm in the peak zone, will show non‐wetting or dewetting when a 100 ppm O2 solder paste is used. The visual cleanliness of solder joints is significantly improved at lower O2 levels because the flux residue will be distributed differently over the solder joints. Bridging is initially determined by the slump behaviour of the paste and the alignment of the paste pattern to the pads and the leads, but when the paste of adjacent pads slumps together before reflow, the tendency for the solder to pull back is influenced by the oxygen level. Wetting and spread of the solder on the lands are then an essential factor. All the proven benefits of a low O2 level are not worth the money spent on nitrogen if the consumption necessary to obtain the properly controlled O2 level is very high. When solderability or the solder defect level not an issue the benefits may not represent a value higher than the cost of the nitrogen consumed.