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Meeting Future Stencil Printing Challenges with Ultrafine Powder Solder Pastes
September 17, 2014 |Estimated reading time: 1 minute
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 components, 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 smaller stencil apertures, there is an increased interest in using finer particle-sized solder pastes to improve transfer efficiency. The smaller particle size results in a large surface area-to-volume ratio that challenges the solder paste’s flux to effectively perform its fluxing and oxidation protection 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 temperature restraints on electrical components has created several challenges. Two in particular are obtaining consistent volume in the printed solder 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 process window. The focus of this paper is to provide a statistical comparison of the transfer efficiency 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 of playing the game. In the SMT process, this may not necessarily be due to losing sight of the basic fundamentals, but because the game itself continues to evolve. As the trend toward miniaturization continues, a process that was successful in the past may suddenly present unacceptable results due to the decreasing size of the stencil apertures required to print ultrafine solder paste deposits. Read the full article here.Editor's Note: This article originally appeared in the July 2014 issue of SMT Magazine.