Design007 Magazine

PCBD-Mar2014

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48 The PCB Design Magazine • March 2014 long-shaped pad will ensure enough surface for consistent and even via plating, even though the recommended pad ring diameter has been violated. For both of the packages discussed and for almost every surface mount component, it is recommended that the paste mask openings match the pin etch and that the stencil thick- ness is appropriately thick for the pitch to insure proper fillets are developed at soldering. For the devices discussed, 0.1 mm is recommended, but for larger pitch parts, thicker stencils may be re- quired. Another aspect that influences yield is the selection of the solder paste. Normal solder pastes are specified according to sphere size, volumetric count and the cleaning process. For small pitches and pins hidden by the packages, it is often necessary to use highly active fluxes to be sure the solder will properly wet and form a solid bond between the pin and the PCB. At the same time, it is also important that the flux is easily clean-able so caustic residues do not re- main on the final PCB. In Figure 3, CSP and DFN (SC-70) packages and final footprints are shown. It should be not- ed that the green solder mask is present in be- tween every solder connection, guaranteeing a robust and consistent assembly when produced in low or high volumes, designed with a consid- eration of manufacturing cost-adders and yield. The final product when implementing such de- signs will be durable, high yields produced at the lowest possible cost. PCBDESIGN References 1. Application Notes for Surface Mount As- sembly of Amkor's Dual Row MicroLeadFrame Packages, Amkor Technology, August 2005; Web, April 2009. 2. SiA30DJ - N-Channel 20 V (D-S) MOSFET, Vishay Siliconix, 13 January 2013; Web, Febru- ary 2014. Figure 3: Final assembled CsP and DFn (sC70) footprints in use on PCBs. nicholaus smith is an applica- tions engineer at integrated Device Technology inc. article PROPERLy DESIGNING PCB FOOTPRINTS continues

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