A study of no-clean Pb-free flux behavior weight change as a function of time and temperature

This paper proposes the requirement of a controlled atmosphere to reflow no-clean solder pastes. The requirement of nitrogen reflow atmosphere as related to the solder paste solids and halide content is discussed. Low residue solder pastes (solids content less than 35%) require a nitrogen atmosphere below 1000 ppm oxygen to reflow. The effect of solder paste halide content on the maximum oxygen level a no-clean paste can tolerate is not obvious. A low halide, no-clean paste achieved better copper wetting when reflowed in nitrogen with less than 1000 ppm oxygen. A minimum halide content is needed to overcome surface oxides existing before reflow. Otherwise, nonwetting will occur regardless of atmosphere. Thermogravimetric analysis of a solder paste quantifies the amount of flux vehicle left after soldering. The residual flux vehicle defines the solids content of the solder paste before soldering. Solder pastes with a low solids content are less able to protect solder powder from oxidation when soldering in air. The lower the solids content a solder paste has the more it needs an inert atmosphere during the reflow soldering process. The advantage of low solids content solder paste is less flux residue left on the circuit board after soldering.