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74 SMT Magazine • March 2014 SummarY: this article will highlight the dif- ferences between the water-soluble and no-clean families of solder pastes to help guide technologists in their choice. abstract Looking back 25 years ago, the solder pastes residues had to be cleaned after reflow due to their corrosive nature. Two ways of cleaning were possible, either using solvent or water, with or without detergent. Now the assembly world is mainly no-clean: Paste formulation is safer in terms of chemical reliability and process costs are reduced without cleaning. However, some applications (i.e., military, aerospace, high-frequency, and semiconduc- tors) require a perfect elimination of the residue after reflow. There are several options to achieve this result: the use of a no-clean paste whose residue can be removed with the most suitable cleaning method, or the use of a paste designed to be cleaned, as a water-soluble solder paste. The water-soluble solder pastes generally show great wettability because of their strong activation, but they are also known to have shorter stencil life and to be more sensitive to working conditions such as temperature and humidity, compared to the no-clean pastes. Ad- ditionally, with components' stand-off getting smaller and smaller, washing residues with wa- ter only is becoming more challenging due to its high surface tension. Often, the addition of detergent becomes necessary. The purpose of this article is to highlight the differences between these two families of solder pastes to guide users in their choice. This will be achieved through the comparison of sev- eral recent water-soluble and no-clean formu- lations for reliability. First the printing quality will be evaluated (viscosity, tack, cold slump, printing speed according to pressure, stencil life, idle time, printing consistency). Then the reflow properties will be compared (hot slump, solderballing, reflow process window, wetting feaTure by Emmanuelle Guéné and Steven Teh invenTeC perForManCe CheMiCalS Reliability Assessment of No-Clean & Water-Soluble Solder Pastes, Part I

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