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54 SMT007 MAGAZINE I JUNE 2018 Article by Anna Lifton, Paul Salerno, Jerry Sidone and Oscar Khaselev ALPHA ASSEMBLY SOLUTIONS Bottom terminated component packages, such as QFN, are becoming increasingly rele- vant due to their ability to carry high-power dies in a small form factor. With increasing reliability performance requirements, power management components in packages like QFNs are critical to optimizing thermal and electrical performance. Additionally, low void- ing is important for decreasing the current path of the circuit to maximize high-speed and RF performances [1] . The market demand for void reduction under thermal pads of QFN components due to shrinking package sizes and increasing power requirements has gener- ated the need to evaluate key process factors that contribute to voiding to design an optimal solution. The addition of a micro-fluxed preform in conjunction with a low-voiding solder paste and process knowhow is seen to create ideal solder volume with minimal voiding. As IPC 7093 specification acknowledges, one of the key concerns with bottom termination compo- nents (BTC) such as QFNs is achieving the solder volume required for a high-reliability solder joint [2] . A multitude of processing factors such as reflow profile, reflow atmosphere, pad finish, and stencil design have been assessed in this study to develop a solution for achiev- ing a high-reliability solder joint with low void- ing for QFN packages. Experimental Procedure A full factorial DOE was designed based on key factors contributing to voiding under bottom termination components. The use of a solder preform was investigated compared to a solder paste only benchmark sample. The key factors in this DOE were identified and selected by subject matter experts from a leader in semi- conductor manufacturing, an OEM of special- ized test and measurement equipment for radio communications, and a solder manufacturer. A custom single layer 1.6 mm PCB test vehi- cle was designed specifically for this investi- gation that encompassed numerous variables Novel Approach to Void Reduction Using Microflux Coated Solder Preforms

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