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March 2014 • SMT Magazine 83 stable paste leads to a sudden increase of vis- cosity after a few hours only (Figure 6b). The overall appearance of the curve gives additional indications of the paste behavior. The viscosity curves as a function of time are summarized in Figure 7. No sudden in- crease was seen. However, pastes A viscos- ity increased during the three first hours before decreasing. Pastes A and C curves are irregular while B, D, E and F curves are smoother. According to these results, it is likely that A and C have a lower printability performance. viscosity over Time A more drastic viscosity test was conducted on each solder paste: The test consists in the fol- low-up of viscosity over time using a Malcom spiral pump viscometer at 25°C and 10RPM. The test simulates solder paste ageing when it is sub- mitted to a permanent mixing and reproduces the shearing it undergoes during printing. The measure of viscosity evolution versus time dur- ing the stirring allows predicting the premature solder paste ageing by correlation. A stable paste keeps a constant viscosity over time (Figure 6a). On the contrary, an un- Figure 6: a) Stable viscosity and b) sudden increase of viscosity after seven hours. figure 7: viscosity curves over time. rELIaBILITY aSSESSmENT OF NO-cLEaN aND WaTEr-SOLUBLE SOLDEr PaSTES, ParT 1 contin ues a b feaTure