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April 2015 • SMT Magazine 39 specifications. On the contrary, all no-clean sol- der pastes showed an ionic contamination level below the specifications. Some pictures of the boards are given as examples of poor and good cleaning in Figure 10 and 11. Discussion Wettability on copper coupons was gener- ally worse for water-soluble pastes using long preheat on hotplate and in reflow oven. Nev- ertheless opposite results were obtained when using FR-4 substrates with cleaned copper fin- ish: larger wetting diameters were found for WS pastes and a slight de-wetting for no-clean paste D was observed. These results were confirmed using the wetting test board with OSP finish. On the other hand, the same wetting test board with ENIG finish has led to more mixed results: wetting performance was dependant on solder pastes and on pattern types. Pastes A, E and F gave similar results, C and B were behind; the only constant was the poor performance of paste D for all patterns and graping on small deposits. WS solder pastes have demonstrated more predispositions to generate tombstoning than no-clean solder pastes. In terms of residue corrosiveness, due to their formulation, WS pastes exhibited high corrosivity as far as cop- per mirror, corrosion on copper and surface in- sulation resistance tests are concerned. And due to their chemistry, their residues were also eas- ily cleaned with water only, paste A being the best, then C and B. However, in case of incom- plete cleaning, the ionic contamination was very high for water-soluble solder paste while Table 9: Cleanliness according to cleaning method (visual/ionic contamination). Figure 10: Poor/good cleaning on wetting H pattern: (left) Paste B/poor; (right) Paste B/good. rEliaBiliTy aSSESSMENT OF NO-ClEaN aND WaTEr-SOluBlE SOlDEr paSTES, parT ii continues FeAture