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84 SMT Magazine • July 2014 or experience and attributed to an increase in surface area for the solder paste deposit to ad- here to when solder mask define pads are used. However, the challenge with solder mask at these small apertures is the registration of the solder mask to the pad. Achieving this precise registration can become difficult for the board supplier. Reflow Profile Improving the stencil printing process with finer particle solder pastes affects the reflow process, often minimizing the reflow process window for acceptable results. As the particle size decreases, the total surface area for the same volume of solder paste increases. This increase in total surface area also means an increase in total surface oxide and increases the demand on the flux to remove them. The demand on the flux is further chal- lenged as the reason for selecting a finer particle solder paste to print into a smaller stencil aper- MEETING FuTuRE STENCIl PRINTING CHAllENGES continues ARTiCLE figure 10: Relationship of surface area to available flux as aperture size decreases. figure 11: optimal reflow profile.