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88 SMT Magazine • March 2014 minutes at 160°C, respectively class 4 and class 5 instead of class 1 for both initially. The sum- mary is presented in Table 11 with images in Figure 13. Discussion After checking the initial properties of the pastes, which were all in their specification in terms of metal content and viscosity, several tests were done to evaluate their printing perfor- mance. In terms of viscosity, spiral pump type viscometer results and spindle type viscom- eter results showed the same tendency, except for paste A and C. These two pastes also had a different behavior and a different curve shape, compared to B, D, E and F as far as viscosity ac- cording to time was concerned. Paste C was far the worst in terms of high printing speed abil- ity. Regarding tackiness, adhesion of paste C started to decrease before the other pastes, the idle time was also the lowest. Hot slump was generally worse for the WS pastes than for the no-clean pastes. Paste A was extremely sensitive to storage at 40°C and lost its printing properties after four days only. The printing definition as well as the anti-bridging ability all along the prints was acceptable for all the pastes and, although C and A ranked last, the difference was not significant enough to draw any conclusion. The exposure to ambient condi - tions (21°C, 50%RH) between printing and reflow led to a degradation of solderballing performance for all the water-soluble solder pastes whereas the no-clean pastes were not affected at all. Figure 13: a) Class 1, b) class 4, c) a, b, C without preheat after 4 hours at ambient temperature and d) explosion. feaTure rELIaBILITY aSSESSmENT OF NO-cLEaN aND WaTEr-SOLUBLE SOLDEr PaSTES, ParT 1 contin ues

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