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74 SMT Magazine • April 2014 by rachel Miller-Short PhoTo STencil llc a ShOrT SCOOp COluMn Screen printing Solutions for Small Die and precision The Short Scoop this month discusses one of our more demanding customer applications. As discussed in previous articles, when selecting stencils, the trick is choosing the right tool for the right job; but sometimes the need for tailor- ing a stencil for a specific application can take a bit more engineering time and several itera- tions to get the desired results. As a case study, consider this: Recently, a customer was trying to print solder fillets on gold Kovar tabs. The Kovar tab measured just 0.063 +/- 0.002 x 0.125 +/- 0.002 inches. The tab was finish plated with 150 µin (minimum) of nickel and then over-plated with 100 µin (minimum) of gold. When the component was attached, the solder fillet needed to align pre - cisely with the tab and the solder reflow had to be perfect. For this demanding application, there could be no gas pockets or voids any- where on the fillet, which meant that ensuring sufficient venting for flux residue from under the part was a priority. The plan was to use a stencil and a squeegee to transfer the solder onto the fillet. However, getting a successful plot was extremely difficult. Previously, the customer had been process- ing the parts by hand-soldering them onto the assembly. At this point in their manufacturing process, they decided to use machine place- ment for a more accurate and repeatable result and to speed throughput to accommodate in- creasing volumes. The stencils they had been working with had six apertures distributed over the large pad for the Kovar tab (slug). Using the initial design configuration of apertures to print the solder paste did not allow sufficient solder paste sol- vent to escape during reflow. This lack of ad- equate solvent escape was causing voids under the Kovar tab as well as poor solder fillets on the edge of the Kovar tab during reflow. Both of these defects were unacceptable conditions for the exacting assembly standards required for the end product.

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