SMT007 Magazine

SMT-May2015

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52 SMT Magazine • May 2015 ENCLOSED mEDIa PRINTING aS aN aLTERNaTIVE TO mETaL BLaDES continues Feature shortcomings that include considerable mate- rial waste, which translates into significant cost, less fill volume as apertures become smaller, and unacceptable variation in the consistency of results. Newly developed enclosed print me- dia technologies have been developed as an alternative to metal blade printing. Due to recent advanc- es, enclosed media printing delivers better results overall when measured against or- dinary-use metal blades, and they excel in material savings since the print head encloses the solder paste or other me- dia from the surrounding at- mosphere, providing steady and uniform results for fine feature apertures filling due to tight process control. For these and other reasons that will be outlined in this pa- per, they constitute an attrac- tive alternative to metal blade printing. aperture Fill is Essential When printing fine pitch or fine features especially, good aperture fill volume is the key to well-formed solder joints without insufficien- cies. Aperture fill results from downward pressure on the paste by the squeegee, forcing solder paste into the apertures. Pressure is generated by compression of the bead of paste, from the squeegee blade's angle of attack as it moves across the stencil and rolls the bead of solder paste before it. Varia- tions in the blade angle (some creatively) can, with some tweaking, optimize the print pro- cess. But there are always concerns relative to the uniformity of pressure across the blade, or the quantity of paste in a given spot, which can also be affected by high-volume requirements (through-hole, paste-in-hole) on the same sten- cil adjacent to fine pitch apertures. With squee- gee blade printing, gauging the prospect of suc- cessful aperture fill has always been subject to the vagaries of area ratio. The more dispropor- tionately greater the surface area of the stencil wall to the pad area, the better the chance that, when the stencil peels away, the paste will re- main with the stencil. With fine feature devices, the pri- mary printing defect is poor or in- sufficient aperture fill. Without proper or complete fill, the sol- der paste may not adhere to the pad and pull away/remain in the aperture; or it may re- sult in a 'starved' solder joint (or incomplete joint) if unde- tected by the downstream SPI machine. Some of the causes of insufficient aperture fill are the same causes identified for other print defects (e.g., pause in printing/raised paste viscosity; squeegee speed too high or too low; squeegee pressure too low; not enough paste on stencil; and others). Proper or optimum aper- ture fill for fine feature print- ing is not only a function of mechanical setup and squee- gee blade material and param- eters, but also a function of having a precisely-controlled volume of solder paste at all times on the stencil. Control is a keyword here; the more fine-featured the printing ap- plication, the greater the degree of precise control over the process that needs to be exercised. In the words of George Babka (Assembléon): In a majority of operations, operators scoop solder paste onto the stencil without precisely mea- suring the amount applied. A few companies spec- ify the amount of solder paste to be placed on the stencil at the beginning of a production run (i.e., one full 350g jar), but most do not. Yet even these former companies fall short of full control of the volume of solder paste on the stencil when setting up the paste dispenser. when printing fine pitch or fine features especially, good aperture fill volume is the key to well-formed solder joints without insufficiencies. aperture fill results from downward pressure on the paste by the squeegee, forcing solder paste into the apertures. pressure is generated by compression of the bead of paste, from the squeegee blade's angle of attack as it moves across the stencil and rolls the bead of solder paste before it. " "

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