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The Short Scoop: Screen Printing Solutions for Small Die & 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 tailoring a stencil for a specific application can take a bit more engineering time and several iterations 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 precisely with the tab and the solder reflow had to be perfect. For this demanding application, there could be no gas pockets or voids anywhere 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 processing the parts by hand-soldering them onto the assembly. At this point in their manufacturing process, they decided to use machine placement for a more accurate and repeatable result and to speed throughput to accommodate increasing 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 solvent to escape during reflow. This lack of adequate 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.

In the customer’s own words, “We had set extremely high standards because the fillet of the solder on this part needed to be perfect. We had tried many stencils and made many plots, but they all resulted in failures.” Read the full column here.Editor's Note: This column originally appeared in the April 2014 issue of SMT Magazine.