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56 SMT Magazine • May 2015 volume of paste in the chamber, and a series of transducers sense the pressure and are part of a closed-loop system of very tight pressure control, which maintains chamber pressure ac- tively at +/- 0.1 psi from set point. In comparative printing tests against squee- gee blades, these expectations proved correct, and surprisingly, the finer the feature, the bet- ter the results that were obtained with enclosed media. For example, whereas with metal squee- gee blades the finer the aperture the less per- centage of desired volume could be depended upon, with enclosed media, the trend was just the opposite, with print volumes over 50% for 150 micron apertures with a 0.375 AR. Table 1 shows the results of comparative tests between printing with an enclosed media print head and metal blade squeegees. Note that the finer the aperture, the more the quality and consistency of squeegee blade printing deterio- rates compared to the performance of enclosed media. Print Head Design and Operation The enclosed media print head maintains solder paste in a closed print chamber that can ENCLOSED mEDIa PRINTING aS aN aLTERNaTIVE TO mETaL BLaDES continues Feature be pressurized to force the solder paste out of the chamber and onto the substrate. Conven- tional squeegee blades are replaced with metal blades that function as both scraper and seal. These blades are aligned in a leading edge con- figuration at 45 degrees, as opposed to a typi- cal trailing edge squeegee configuration. The inwardly-inclined blades are 10mm apart at the point of contact with the stencil. When the head is lowered onto the stencil, the stencil foil will close the entire print chamber during op- eration and allow a positive pressure to be gen- erated inside. Silicone dams seal the opposite ends of the chamber. The pressure inside the chamber is generat- ed by a plunger, or piston, that is motor-driven downwards into the upper part of the cham- ber to generate pressure. A flexible membrane separates the plunger from the solder paste. Three pressure transducers measure the inter- nal chamber pressure and feed this information back to a control system that drives the plunger to the required height to actively maintain the desired pressure to +/- 0.1 psi from set point. With every print cycle, more solder paste trans- fers out of the head onto the substrate while the figure 1: chamber pressure consistency plot.