SMT007 Magazine

SMT-Aug2015

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78 SMT Magazine • August 2015 at once; a laser-cut stencil is cut one aperture at a time. If you have a wafer with a million things that need to be printed, that's going to be hours and hours instead of seconds. Speed is the fundamental difference. Other differ- ences of course are accuracy, dimensionality, and flexibility in a stencil. Like dispensing, both technologies can print different kinds of materials. It can print flux and it can print paste. The thing I see the most is just what every- one sees in terms of ever-smaller components and ever-tighter density. Right now we're look- ing at 01005 passives being quite routine, and our customers are talking about smaller passives in the relatively near future. Micro QFNs and other components are posing real challenges for people in assembly. What we're seeing are some of the companies making simpler products are using simpler stencils, but there's real value for OEMs—where the ultimate product goes into a harsh environment, like an automobile or an aircraft, or into something going into the hu- man body like a medical device—to use a bet- ter stencil to get better paste transfer, but also a better quality mechanical electrical connection there at the joint. Matties: you're talking about higher quality sten- cils. when a customer is looking at millimeter ap- plication, what should the considerations be? Weissmann: The customer should be looking at the area ratio of the stencil, the ratio of the flat side of the pad that they want to print to the walls of the stencil, and how thick it has to be. The smaller that ratio is, the harder it is to transfer those things. You want to have a sten- cil with a smoother material, and that's a pure nickel stencil rather than a steel stencil. You also would like to have a stencil that has been elec- troformed, because you end up with smoother walls on the inside of the aperture, so it's easier for the material being printed to get through. One of the things that we've added this year is a laser imaging system in place of our traditional photo process, and that produces even more ac- curate and even smoother walls for more chal- lenging printing applications. Matties: So they can go in and laser cut it? Weissmann: Well, a standard stencil has laser- cut apertures. Of course, the laser is melting and burning away the metal, so you end up with a jagged inside edge on the stencil. We are grow- ing the metal, kind of like electroplating a cheap piece of jewelry, except that the plating is the product rather than the thing that that's being plated on. We grow it, and by using pure nickel or some alloys with a high amount of nickel, we can create a very smooth surface, both on the top and bottom of the stencil, but most impor- tantly on the aperture walls. Matties: how long does it take to go through that process and create one? Weissmann: For single-level electroform sten- cils, we can normally ship them the next day if we receive an order released in the morn- ing. It's a pretty fast process. If you have a very quick-turn requirement, but which is more de- manding than a standard laser stencil, then we have a product called NiCut, where we have a pure nickel piece of foil, but we laser cut the apertures, and that is a little bit faster from a turn time perspective. The performance is bet- STENCILS: WHY THEY STILL MATTER continues IntervIeW

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