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PCB-Sept2017

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54 The PCB Magazine • September 2017 Similarly, fabricators take special precau- tions with thin materials. Often special carrier panels are used to transport the product from location to location and operators are trained to handle materials—picking up from opposing corners to eliminate flex in the material. Any dents or dings in the copper will have a high probability of creating scrap as the circuit pat- tern is created. Because automated equipment is often not specifically designed for thin mate- rials, leader panels are often taped to the man- ufacturing panel to provide additional support moving through automated equipment. Stencil Design "The screen printing process is one of the most critical steps in the SMT process," says Kevin Buffington, manufacturing engineer for Electronic Systems. "The combination of a good stencil and solder paste inspection is vital to the outcome of placement and reflow." The screen printing process begins with a well-designed SMT stencil. Proper volume and placement of solder paste is crucial to the reduction or elimi- nation of solder defects such as insufficient sol- der, shorts, and solder balls. This is achieved by choosing the right foil thickness and aperture size for the mix of components on the printed circuit board. Stencil design parameters are de- veloped based on aspect ratio and area ratios of components. These ratios are a calculation of the size of the stencil opening and the stencil thickness that allows the solder paste to release. While not preferred, in limited cases where some very large components are included on a design with very small, fine-pitch components, step stencils may also be used to ensure that enough solder paste is deposited for the larg- er components. Step stencils, as the name sug- gests, step up the stencil thickness in a specific area to place a greater amount of solder paste. Framed stencils can either be fixed frame or universal frame. Stencils will typically range from 15" x 15" to 23" x 23" and use either a sol- id or hollow aluminum frame. As the name sug- gests, the fixed stencil is permanently fixed to the frame. With the universal frame, the stencil is held within the frame which makes it possi- ble to make changes as they are needed. While IPC specifies best practices for com- ponent spacing in design, today's designers are expected to do more with less space and more often need to push the envelope of standard processing. Fabricators and EMS providers are continually refining their processes to manu- facture tighter pitch trace and space and assem- ble tighter pitch components. Involving your PCB fabricator and EMS provider early in the design helps to ensure that manufacturability is designed into the product. Even better, plan a facility tour and gain a little insight into what goes on behind the curtain when manufactur- ing PCBs and PCBAs! PCB Tara Dunn is the president of Omni PCB, a manufacturer's rep firm specializing in the printed circuit board industry. To read past columns, or to contact Dunn, click here. THE MAN BEHIND THE CURTAIN A research team led by Professor Kyung Che- ol Choi at the School of Electrical Engineering presented wearable displays for applications including fashion, IT, and healthcare. Integrating OLED (organic light-emitting diode) into fabrics, the team has devel- oped some of the most high- ly flexible and reliable technology for wearable displays in the world. Recently, information displays have become increasingly important as engineers construct the external part of smart devices for the next genera- tion. As world trends are focusing on the Internet of Things (IoTs) and wearable technology, Choi's team has made great progress towards commer- cializing clothing-shaped 'wearable displays.' Highly Flexible, Wearable Displays

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