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82 SMT Magazine • March 2014 rELIaBILITY aSSESSmENT OF NO-cLEaN aND WaTEr-SOLUBLE SOLDEr PaSTES, ParT 1 contin ues The last revision of IPC-TM-650 method 2.4.35 procedure mentions a change in the pre- heat temperature: The samples shall be tested at a temperature of 35°C below the melting point which means 180°C for SAC 305 solder pastes. The test was performed at 180°C: A, B, C and D were above the defined criteria. Stability at Elevated Temperature The stability of the paste being part of its reliability, the influence of storage at elevated temperature was examined: The pastes were stored at 40°C and checked after four days and seven days. A decantation phenomenon oc- curred for paste A after four days: After mixing many lumps were found in the paste. Paste C surface looked dry and dull after seven days but resumed its shiny appearance after mixing. The feaTure other pastes did not exhibit any change. A sim- ple printability test was done: The pastes were printed on an alumina substrate using a 0.250 mm thick stencil with round openings of 5 mm diameter. Only paste A was difficult to print correctly, resulting in a non-planar surface. The tackiness was also measured on each paste after seven days. Paste A showed a higher tackiness but the decrease was fast; it lost completely its tackiness after eight hours. There was no signifi- cant change for the other pastes neither in val- ue nor in duration. Slump trials showed an "im- provement" for paste A at 150°C but no change for pastes B, C, D, E and F. The modification of paste A properties is due to an internal reaction due to the exposure to temperature: This paste is sensitive to temperature. Pictures and results are given in the Table 4 for paste A, B, C and E. Table 4: pastes a, b, C, e after 7 days at 40°C.