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34 SMT Magazine • April 2015 Wettability on Test Board using reflow Oven The pastes were printed through a 100 µm stencil on a test board designed for wettabil- ity (Figure 6). In order to pre-oxidize the test boards, they were submitted to one prior re- flow. P5 was used. Organic solderability preser- vative (OSP) and electroless nickel immersion gold (ENIG) finishes were tested. Wetting per- formances were checked. Pictures of wetting on patterns A and G are given in Table 6 and Figure 7. On OSP finish, the ranking was similar to the test previously made on the FR-4 copper substrate whereas the ranking on ENIG finish was dependent on paste and on pattern type. The average results are, in decreasing order of wettability: A, E and F almost equal, then C, B and D. Only the wetting properties of D were constant and always the worst. Tombstoning The water-soluble solder pastes being usu- ally more prone to create tombstoning defects due to their high activation, a test was per- formed: Forty 0603 capacitors were placed with an offset on the wetting test board comprising several aperture designs. Special reflow condi- tions were used, in order to observe a significant tombstoning percentage. The conditions will not be described in this paper. The experiment was repeated three times for each paste. The ta- ble below (Table 7) indicates the percentage of tombstoning observed. figure 6: Wetting test board. Table 6: Wettability on test board pattern a. rEliaBiliTy aSSESSMENT OF NO-ClEaN aND WaTEr-SOluBlE SOlDEr paSTES, parT ii continues FeAture