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76 SMT Magazine • July 2014 ARTiCLE Abstract The explosive growth of personal electronic devices, such as mobile phones, tablets, and personal music devices, has driven the need for smaller and smaller active and passive electrical components. Not long ago, 0201 passives were seen as the ultimate in miniaturization, but now we have 01005 passives with rumors of even smaller sizes not far behind. For active compo- nents, array packages with 0.4 mm pitch are virtually a requirement for enabling the many features in modern portable electronic devices. To meet the challenge of stencil printing small- er stencil apertures, there is an increased inter- est in using finer particle-sized solder pastes to improve transfer efficiency. The smaller particle size results in a large surface area-to-volume ra- tio that challenges the solder paste's flux to ef- fectively perform its fluxing and oxidation pro- tection action. The potential resulting surface oxidation can lead to voiding, graping, head-in- pillow, and other defects. The combination of higher lead-free process temperatures, smaller print deposits, and tem- perature restraints on electrical components has created several challenges. Two in particular are obtaining consistent volume in the printed sol- der paste deposit and minimizing the oxidation of the solder powder in the small deposit during reflow. Solder pastes comprised of finer particle solder powders may help with stencil printing, but the increased surface oxide associated with finer powders may also reduce the reflow pro- cess window. The focus of this paper is to pro- vide a statistical comparison of the transfer ef- ficiency of different solder powder particle sizes, specifically types 3, 4, 5, and 6, and to visually observe post-reflow results in both optimal and harsh conditions. Introduction Back to the basics, or the fundamentals, is a term often heard in relation to sports teams when they lose sight of the basic foundations by Edward Briggs inDiUM CoRPoRATion Meeting Future Stencil Printing Challenges with Ultrafine Powder Solder Pastes As originally published in the 2014 International Conference on Soldering and Reliability Proceedings.

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