SMT007 Magazine

SMT-Feb2016

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72 SMT Magazine • February 2016 or squeegeeing, meaning that it is difficult to re- tain a co-planarity with the PCB. The lack of co- planarity causes solder paste to squirt out from underneath the stencil. These problems make the miniature metal stencils less suitable today for the deposition of solder paste during rework. Plastic film stencils (Figure 2) have taken over where the mini metal stencils have left off. They offer up some distinct advantages based on the user complaints over their mini metal stencil counterparts. Their repositionable adhe- sive backing allows the user to move the stencil around creating micro-fine alignments on the PCB after the macro adjustments have been made. In addition, the adhesive helps ensure co-planarity with the PCB even if the board is slightly warped. The sticky backing also allows the user to use multiple swipes with the squee- gee, thus ensuring the apertures are "filled" completely. The flexible nature of the stencil allows for the stencil to be used in very tight areas, which need to be printed prior to rework. These stencil designs even allow for "flaps" to be built into the stencil design and prevent sol- der paste from being pushed outside of the re- work area requiring further clean-up. For these and other reasons the adhesive-backed plastic film stencil is now the preferred rework stencil. In a recent study, the results of which will be published at the APEX conference in March of 2016, the printing consistency between these two types of rework paste printing stencils will be revealed. In the meantime, select data from the study as well as the results of that study are included in this discussion. In this study, a solder paste inspection mea- suring system measured the solder paste volume and solder paste "brick" height at each of the pad Figure 3: Box and whisker plot for plastic film adhesive-backed rework stencil for solder paste height trials. reWork site PriNtiNg usiNg miNi steNcils—Plastic adHesiVe Vs. metal

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