Authored By:
Phani Vallabhajosyula, Ph.D., William Coleman, Ph.D., Karl Pfluke
Photo Stencil
Golden, CO, USA
Brook Sandy-Smith
Indium Corporation
Clinton, NY, USA
Summary
With advancements in board designs, and widespread application of stencils into the solar/optics fields, new challenges arise leading to new stencil solutions. Extensive work has been done in the past to print glue [1], solder paste [2, 4], and/or flux [3] into cavities using reservoir printing. The reservoir depths for these applications ranged from 75 microns to 355 microns, with aperture sizes from 65 microns to 200 microns. This paper focuses on printing solder paste into multiple cavities with depths ranging from 355 microns to 450 microns, and with varying cavity wall angles and various stencil thicknesses ranging from 100 microns to 150 microns.
Apertures varying in area ratio were placed in these cavities and experiments were conducted to analyze the print performance of the stencils. The goal was to create guidelines for future stencil designs and aperture placement in cavities. Dispense viscosities of paste and pump printing were recommended to ensure printing in deeper cavities and to obtain a good print volume. Details on stencil design, paste material selection, experimentation, and print results will be presented in this paper.
Conclusions
This experiment demonstrated reservoir printing as a successful printing process into cavities of the board as deep as 18mils (455um). For the scope of this experiment and the various paste materials and fluxes used, printing was observed as even from the aperture as close as 3mils (75μm) from the pocket wall irrespective of stencil thickness, aperture size, pocket wall angle, and pocket size.
This implies that the components can be placed as close as 75μm to the cavity wall of the substrate. For the same pocket depth and wall angles, further analysis will be conducted (a) by placing apertures much closer to the pocket wall to determine the closest component pad placement in the cavity; (b) by placing apertures with different area ratios, smaller than 0.45, to determine the smallest printable aperture; and (c) to perform quantitative analysis (print volume and any variation in the print volume) with respect to the various stencil design factors involved.
Initially Published in the SMTA Proceedings
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