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54 SMT Magazine • May 2015 ENCLOSED mEDIa PRINTING aS aN aLTERNaTIVE TO mETaL BLaDES continues Feature Most paste dispense processes are suited to a larger volume of paste (75–150g), and dispensed at infrequent intervals (every 40–50 prints). As a result, the roll size can still change significantly during the course of the shift…a 100g change in solder roll size will cause approximately a 7% change in maximum paste filling pressure. This is a significant change. It is enough to dramati - cally alter the solder paste print quality, especial- ly on small apertures…A larger solder paste roll will have higher applied pressure, as well as a longer time that the pressure is applied. Too large of a roll size for a given speed applies too much pressure to the aperture for too long of a time, increasing the potential for bridging of fine pitch devices…True solder paste volume control can be accomplished in one of two ways: via an enclosed print head, or to dispense more accurately and more often… [1] The smaller the pad/aperture, the more dra- matic the effects of poor aperture filling, due to the more precise (and smaller) volume of paste required to create a successful solder joint. The absence of a specific small volume of paste from a large pad print, with no deleterious effect on the solder joint ultimately formed, could, for a fine feature component, be the entire amount of paste required for that connection. Enclosed media Print Head Technology Enclosed media print head technology was designed to provide the same required pres- surization of the solder paste as the squeegee blade's angle of attack, but uniformly across the length of the print head while printing, re- gardless of the amount of paste in the enclosed chamber. The key to consistency and unifor- mity in printing results, where the stencil or print application requirements were anything but uniform, was determined to be direct, fast- response control of paste pressure. This would have to be closed-loop controlled so that pres- surization could respond to such demands, for example, as are made by through-hole fills that require a very high volume of paste, rather than a pattern of fine pitch pads. Additionally, with optimum aperture fill, the more likely the paste will cover the entire pad area, and that its tack will be sufficient to keep it on the pad when the stencil peels off. Efforts to improve the printing performance of enclosed media resulted in a change in the way that the paste in the paste chamber is pres- surized. Rather than indirectly or pneumati- cally, pressurization is accomplished mechani- cally by a motor-driven plunger or piston that is separated from the paste by a membrane. The plunger applies pressure directly to the Table 1: volume and standard deviation comparison with squeegee blades.