SMT Magazine

SMT-Dec2017

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40 SMT Magazine • December 2017 by Ken Horky PETERSON MANUFACTURING Many engineers are leaving the editing up to the stencil fabricator these days, describing the PCB array, aperture undersize/oversize, shape conversions, etc., for the fabricator to then edit. From the outside, this may appear as a time sav- er for all of us overworked process engineers, but considering how many stencil redos have been required and how many processes that have run "sort of OK," there's a tremendous amount of scrap and rework that could be saved from just a little more attention paid to stencil tooling. I prefer to make my own mistakes rather that receiving a stencil (with the project due tomor- row) and finding that someone else didn't do something right, or misunderstood my design intent. It's a huge advantage for the experienced en- gineer to be able to produce good, first run re- sults most, if not all, of the time. You can learn what works best for your equipment in your environment. There are several Gerber editing tools available that will let you create a library and apply changes semiautomatically or auto- matically to your data. One of the most common edits used for resistors and capacitors is the "home plate" design. The left aperture below (magenta) is a typical aperture reduction. The right aperture below (cyan) is the home plate pattern. Black being the component outline and terminals, and red being the original pads. This is a very useful aperture design for miti- gating solder balling and tombstoning. It can also be a difficult shape to describe, and convey where to apply it, to a stencil fabricator. Some large connector pins may benefit from oversiz- ing the aperture to improve mechanical inter- connect and ease inspection. Large heatsinks, such as those for DPAKs (Decawatt Package), can be divided into multiple apertures to reduce squeegee scooping and voids. Bowtie patterns can reduce solder balling on large gull wing pins. I position the print image within my foil perimeter in the data, depending on the as- sembly type, and indicate the location for the stencil ID. My operators like the image forward FEATURE

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