Issue link: https://iconnect007.uberflip.com/i/288471
76 SMT Magazine • April 2014 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 fail- ures." The solution included two impor- tant elements: (1) an aperture design that employs a honeycomb type structure, and (2) ground plane aperture designs for quad flatpack no-lead (QFN) stencil printing, which solves a similar problem with small part attachment with a large ground attachment. First, a hybrid electroformed stencil was chosen with a squeegee printing surface that is perfectly flat compared to woven wire knuckles used on woven screen mesh products. This flat, electroformed structure is stable and does not stretch during usage like woven wire mesh and, being an elec- troformed stencil, it can be tailored to precise thicknesses and is able to release paste from the screen more reliably than a laser cut tool. A va- riety of mesh patterns can be designed to cover large application areas. Shapes such as rectan- gles, squares, circles, ovals, and hexagon mesh shapes are used and eliminate the angular vari- ations that are seen in competing woven mesh products. In this case, the engineered mesh de- sign was central to the success of the Kovar tab attach for the customer. The next challenge was printing solder paste for QFN devices, which present several assem- bly problems. QFNs have a metal pad on the underside of the part for grounding and heat conduction. The packages are very small and light. The QFN leads and ground plane conduc- tor are flat and in the same plane on the bot- tom of the package. Printing solder paste 1–1 with the ground plane can cause the QFN to float during reflow due to the surface tension of the liquid solder which can cause miss-reg- istration of the leads on the QFN and the pads on the PCB. QFN float can be controlled by reducing the amount of solder paste printed on the ground plane. Typically a 50–60% reduction will solve the QFN float problem. However, the aper- ture reduction must be done judiciously since at some point, starving the part of solder be- comes an issue. A window pane aperture is rec- ommended for most cases of QFN attachment which allows the solder paste volatiles to easily escape during reflow without moving the QFN device. The result was an adaptation of an Accu- Screen stencil with which could be created a uniquely designed stencil with a honey-comb aperture pattern that would evenly dispense the solder paste and allow for out-gassing during re- flow, providing a uniform solder fillet around the Kovar tab. The apertures would be 6-sided and arranged in an array. SMT A ShorT ScooP rachel Short is vice president of sales and marketing at Photo Stencil llc. To read past columns, click here. figure 1: board with honeycomb pattern magnified. SCrEEN priNTiNG SOlUTiONS FOr SMall DiE aND prECiSiON continues