Authored By:
J. Li, S. Poranki, K. Srihari, Ph.D.
Watson Institute for Systems Excellence
State University of New York
Binghamton, NY, USA
R. Gallardo, M. Abtew, R. Kinyanjui, Ph.D.
Technology Development Center
Sanmina-SCI Corporation
San Jose, CA, USA
Summary
Growing demands for smaller electronic assemblies has resulted in reduced sizes of passive components, requiring the introduction of newer components, such as the 01005 devices. Component miniaturization presents significant challenges to the traditional surface mount assembly process. A successful assembly solution for these 01005 devices should be repeatable and reproducible, and should include guidelines for (i) the selection of solder paste and (ii) appropriate stencil and substrate pad design, and should ensure strict process control standards.
During the first phase of this study, different stencil types, aperture designs, pad layouts and process parameter settings were evaluated with the goal of achieving a high-yield assembly solution for 01005 components. Printed circuit board assemblies are populated, usually, with both large active devices (often surface mount packages), along with much smaller passive components (such as 01005s). Consequently, it was decided to use a 4 mil thick stencil along with a type 4 solder paste. Based on these process and design conditions, several stencil aperture shapes were evaluated using metrics, such as transfer efficiency and variance in volume of the deposited solder.
These two criteria help determine solder volume adequacy and consistency. In addition, different pad layouts were studied vis-a-vis their ability to promote (or inhibit) component self alignment during the solder reflow process. The results of the first phase of this study indicated that an electroformed stencil performed much better than a laser cut stencil in terms of transfer efficiency. However, considering the relatively higher cost of an electroformed stencil, a follow up (second) study focusing on the use of a laser cut stencil was considered essential.
The objective of this study was to develop a process that provides higher yields while ensuring cost effectiveness. Aperture designs were adjusted to increase the area ratio. The results showed that it was possible to develop a robust 01005 assembly process with the use of 4 mil thick laser cut stencil in concert with a type 4 solder paste
Conclusions
By increasing the aperture size, the performance of laser-cut stencil was significantly improved. Transfer efficiency increased from 65% to more than 85%, which is comparable to the electroformed stencil used in the first phase of the study. Based on this follow-up study, it was shown that laser-cut stencil can be used for 01005 assembling process. Among the four proposed aperture designs, Design 3, corner rounded rectangular, performed the best in terms of deposited volume, volume variance and transfer efficiency.
Initially Published in the SMTA Proceedings
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