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58 SMT Magazine • May 2015 ENCLOSED mEDIa PRINTING aS aN aLTERNaTIVE TO mETaL BLaDES continues Feature plunger travels further down into the chamber. A low-level sensor detects when the plunger reaches the lowest position, thus triggering the refilling of the head with solder paste. The paste is provided from two industry-standard cartridg- es that are mounted onto the front of the head. Air pressure is applied to the cartridges to force the paste out of the cartridges and into the chamber. The plunger is kept at its upper position dur- ing this fill routine. Once the pressure transducers read a defined fill pressure inside the chamber, the air pressure to the cartridges is relaxed and the plunger moves back down onto the membrane. The en- closed media print head is ready to continue printing. Figure 1 is a plot of cham- ber pressure during a simu- lated production run. Inter- board pressure data illustrates how tight, closed-loop cham- ber pressure control is main- tained consistently board to board regardless of paste con- sumption requirements (large apertures & small). Operation Prior to the first print, the head chamber must be filled completely with solder paste. The filling operation is support- ed by a dedicated software routine that ensures that the machine is in the correct status for this operation. During the fill routine, the plunger is positioned at a fixed 'fill position' above the membrane, for the purpose of lim- iting the expansion of the membrane upwards while solder paste is being forced out of the car- tridges and into the chamber. At the end of the fill routine, the plunger is moved to the relaxed distance relative to the fill position. The enclosed media print head differs from similar types of print heads in that pressure is applied to the solder paste mechanically, not through air (pneumatic) pressure. The print head consists primarily of a paste chamber that is filled with solder paste from standard indus- try paste cartridges. These replaceable cartridges are mounted onto the print head assembly. Air pressure is used (software-controlled) to drive the paste out of the cartridges, through a mani- fold, and into the paste chamber. Pneumatic pressure is employed only to fill the chamber; it plays no role in pressurizing the paste for printing. Print pressure is supplied through a plunger (piston). Three robust, highly sen- sitive pressure transducers ensure tight control of pres- sure inside the head dur- ing filling, as well as during printing. These sensors are mounted directly inside the paste chamber and constantly monitor the chamber to en- sure a constant 'full' status. The chamber is considered full when the fill pressure is averaged across the selected transducers. During printing, the head is designed to keep the chamber full, and is thus programmed to initiate an "in production fill" between cy- cles. A sensor reliably senses a paste empty condition in the cartridges. Blade assemblies on either end (front and rear) of the print head control the paste distribution and excess re- moval from the stencil. Blade assem- blies consist simply of specially-designed blades attached to a round shaft. materials Savings Customer data indicates that the materi- als and solder paste savings achievable with enclosed media can be in excess of 50%. Sol- der paste is saved because it does not dry out, does not need to be discarded at the end of a shift, and there is overall less throwaway and loss through cleaning. Every scenario will be different, but the one represented in Figure 2 (actual example) realized a payback in a little more than 28 weeks. Clearly, higher volumes of a low-level sensor detects when the plunger reaches the lowest position, thus triggering the refilling of the head with solder paste. the paste is provided from two industry-standard cartridges that are mounted onto the front of the head. air pressure is applied to the cartridges to force the paste out of the cartridges and into the chamber. the plunger is kept at its upper position during this fill routine. " "