SMT007 Magazine

SMT-Feb2016

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70 SMT Magazine • February 2016 In order to duplicate the original manufac- turing process as closely as possible, the rework of complex devices can be accomplished by paste printing solder paste in a select location. Ever since the advent of widespread usage of surface mount devices such as fine pitch QFNs or BGAs, this practice has continued to grow. Today's pitches are commonly 0.5–0.4 mm with packages of tiny outline sizes 510 mm square, making the rework of such devices a challenge. Spacing to neighboring components continues to be compressed so the rework techniques are getting more challenging from a practical point of view. The process of printing consistently has giv- en rise to a more modern technique for printing solder paste in a given location for fine pitch de- vices without there having to be poor yields or highly skilled technicians to perform the work. A soon-to-be-reported study compares the older miniature metal stencil printing process to the more modern plastic film with adhesive ap- proach. This report quantifies the differences in performance for the first time. The miniature metal stencil (Figure 1), while matching the original SMT printing sten- cil, has several shortcomings with respect to the rework printing process. This technique, while perfectly capable for most SMT boards, has fallen out as preferred and capable method for several reasons. With the tighter spacing be- tween components these "mini" metal stencils are too large for modern board layouts as there needs to be an oversized area compared to the print area for the holding and supporting of the stencil. In addition, the finer pitches and pad sizes makes it difficult, even for the most skilled rework technicians, to perfectly print in a single pass the correct volume of solder paste onto the PCB. With ever-thinning stencils due to lower paste volume requirements, these metal sten- cils are easily bent or damaged during cleaning Figure 1: Adhesive-backed plastic film stencil used for rework. Figure 2: Mini metal used for rework. by Bob Wettermann BeSt iNc. rework site Printing using mini stencils—Plastic adhesive vs. metal kNocki Ng doWN tH e boN e Pi le

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