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80 SMT Magazine • March 2014 test, the abandon time on the stencil is about four hours for 0.4 mm pitch and 120 µm stencil thickness at room temperature. Slump The slump was tested according to IPC- TM-650 method 2.4.35. The pastes were printed on alumina substrates through the IPC-A21 0.2 mm thick stencil (Figure 3). For cold slump the substrates were observed after 10–20 minutes at ambient conditions (25 +/-5°C and 50+/-10% relative humidity): the minimum spacing at which the bridges are formed is noted. The paste shall show no evi- dence of bridging when spacing is 0.56 mm or greater (for 0.63x2.03 pad size) and 0.25 mm or greater (for 0.33x2.03 pad size) to conform to J-STD-005 paragraph 3.6.1. No bridge was seen on 0.63x2.03 pad size whatever the paste. Bridges with spacing of 0.06 to 0.10 mm was observed for 0.33x2.03 pad size. None of the pastes are prone to cold slump. Pastes A and C results are shown in Figure 4. For hot slump, the substrates were observed after 10–15 minutes at 150°C preheating and cooling to ambient temperature. The paste shall show no evidence of bridging when spacing is 0.63 mm or greater (0.63x2.03 pad size) and 0.30 mm or greater (0.33x2.03 pad size) to con- form to J-STD-005, paragraph 3.6.1. The general trend is to obtain better results with no-clean solder pastes than with water-soluble (WS) sol- der pastes: The no-clean are all below the limits. Among the WS, Paste A is more prone to slump than the other pastes and exceeds the limits. Be- cause a quick preheat at 150°C does not really reflect the reality of a thermal profile, a slower preheat was evaluated: First, the substrate is placed on a hotplate at 100°C during one min- ute, then immediately on another hotplate at 140°C for one minute and finally on a hotplate at 160°C for one minute. The slump behavior was improved for all the pastes and only paste A was still above the limit. Pictures of pastes A and C are gathered in Table 3 and all the results are summarized in Figure 5 and 6. rELIaBILITY aSSESSmENT OF NO-cLEaN aND WaTEr-SOLUBLE SOLDEr PaSTES, ParT 1 contin ues feaTure Figure 3: ipC a-21 stencil (extracted from ipC TM-650 2.4.35). Figure 4: Cold slump a) paste a, and b) paste C.