SMT007 Magazine

SMT-Apr2015

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26 SMT Magazine • April 2015 Note: Part 1 of this article published in the March 2014 issue of SMT Magazine, available here. abstract Twenty-five years ago, solder paste residues had to be cleaned after reflow due to their cor- rosive nature: two ways of cleaning were avail- able, either with solvent or by using 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-fre- quency, and semiconductor) require a perfect elimination of the residue after reflow. Several options can achieve this result: a no-clean paste which allows residue to be removed with the most suitable cleaning method, or a paste de- signed to be cleaned, such as a water-soluble solder paste. Water-soluble solder pastes generally show great wettability because of their strong activa- tion but they are also known to have shorter stencil life and to be more sensitive to working by Emmanuelle Guéné and Steven Teh InVEnTEC PERFoRMAnCE CHEMICAlS Reliability Assessment of No-clean and Water-soluble Solder Pastes, Part II conditions as temperature and humidity, com- pared to the no-clean pastes. Additionally, with the components stand-off getting smaller and smaller, washing residues with water only is more and more challenging due to its high sur- face tension: the addition of detergent becomes often 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 as far as reliability is concerned. First, the printing quality will be evaluated (viscosity, tack, cold slump, printing speed according to pressure, stencil life, idle time, printing consis- tency). Next, the reflow properties will be com- pared (hot slump, solderballing, reflow process window, wetting ability on different finishes). Finally, the residue cleanability will be assessed. The IPC SIR will be also done to conclude the study. Both standardized tests and production tests will be used to evaluate the performance of these two kinds of solder pastes. introduction Six lead-free pastes were extensively stud- ied, three being water-soluble and three being FeAture

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