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16 SMT Magazine • December 2017 by Stephen Las Marias I-CONNECT007 The solder paste printing process has always been considered a major contributor to yield loss. According to many studies, solder paste printing accounts for up to 70% of all PCB as- sembly defects. For this month's issue of SMT Magazine, we interviewed experts in the solder paste printing process to learn more about the key issues lead- ing to this huge percentage of defects, and the technology developments that are addressing these challenges and improving yields in the process. These experts, including Tim O'Neill, technical marketing manager for AIM Solder; Edward Stone, sales manager at Manncorp; and Lenthor Engineering's Dave Moody and Matt Kan, director of sales and marketing, and EMS manager, respectively, provided their insights from the perspective of a solder paste supplier, an equipment manufacturer, and a PCB assem- bler. Lenthor Engineering's Kan says 99% they mostly deal with flex and rigid-flex assemblies, so it's a whole different environment than rigid: "I can only speak from my personal experience. When I tried to deploy process steps that I used for rigid, I found out quickly most of the time, it didn't work for the flex and the rigid-flex world. With flex and rigid flex, any kinks from one area to the next, you get misprints, even if your fixture your board correctly. Another option is to array with support tabs to keep the board as flat as possible. Since we design, fabricate and assemble under one roof, we have the advan- tages of deciding which method is the most cost effective," explains Kan. "With the flex and the rigid-flex, you have to tweak your printing pa- rameters because everyone knows a flat surface is crucial; and dealing with flex, you have dif- ferent thicknesses, so your typical 5 mils stencil for rigid doesn't always apply in the flex circuit world. Our standard is 3 mils; we change up or down in foil thickness as we see fit, step down, back etched, etc. Printing problems that we en- counter is either not enough or too much paste. As things get smaller and smaller, we would switch from Type 3 to Type 4 for the smaller 0201 or 01005 components." FEATURE

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