A Model Study of Low Residue No‐clean Solder Paste

As one of the major approaches to addressing the CFC issue, no‐clean solder paste has received rapidly increasing attention. Although currently the industry seems to accept full residue paste as a temporary solution, low‐residue no‐clean paste technology using inert or reactive atmosphere is advancing quickly to meet the challenge. At present no consensus has been established regarding how low a residue level could be achieved and how inert the atmosphere needs to be. In this study, a semi‐empirical model is proposed to predict the soldering performance of low‐residue solder pastes under various levels of inert reflow atmosphere. The model predicts that the soldering performance would improve rapidly, then gradually level off with decreasing oxygen content. The soldering performance vs oxygen content curves are superimposable, with the lower residue one levelling off at lower oxygen level. In general, the experimental data match this model fairly well. However, the data also indicate that, although inert atmosphere improves soldering performance, the optimum condition for bond strength performance seems to demand the presence of some oxygen. This unexpected behaviour suggests that a very tight low oxygen level control may not be required. The mechanism responsible for this phenomenon can be attributed to oxidation‐induced resin crosslinking. This slows down the flux drying rate as well as hindering the permeation of oxygen through the flux layer.