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March 2014 • SMT Magazine 87 Table 9: viscosity after cycling test. Table 10: Solderballing classification (nF-C-90550). Table 11: Solderballing test. Solderballing Solderballing performances were checked on a hotplate using our internal procedure. The pastes are printed on alumina substrates through a 0.250 mm thick stencil with two round opening of 5 mm diameter, with a dis- tance between centers of 25 mm. The tempera- ture of the hotplate was set at 250°C and several temperatures and times of preheat were tested. The goal was to classify the paste according to preheat. The solderballing classification ranks from class 1 to class 5, the balls being counted with 30x magnification (Table 10). Solderballing was class 5 for paste B with- out preheat. Otherwise, the pastes gave good re- sults whatever the preheat, except for the most demanding condition (180°C, 2 during 2 min- utes) where solderballing ranked from class 2 to class 5. After four hours at ambient conditions (21°C/50%RH), pastes A and B have exploded upon contact with the hotplate and a signifi- cant degradation occurred for B and C after 2 feaTure rELIaBILITY aSSESSmENT OF NO-cLEaN aND WaTEr-SOLUBLE SOLDEr PaSTES, ParT 1 contin ues