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20 SMT Magazine • November 2014 style and solder connections as low as 0.4 mm. Reattaching these components often requires the use of a camera to accurately align the part to the solder pad. There are a number of factors to consider when replacing components. The operator must consider whether to apply flux to the printed circuit board or to the component (Figure 8). Liquid flux dries before the solder reflows. Gel flux, in paste form, is used during manu- facturing and should be used during rework. Applying flux to the solder balls provides little coverage. Dipping the component into gel flux provides excellent coverage as well as solder ball cleaning. The ideal flux dip is half the height of the solder ball. This transfers the exact amount of flux onto the solder balls and not on to the PCB. After reflow, less contamination is present under the BGA. Also, flux does not onto drip through the via to the other side of the PCB. Other considerations include applying solder paste to the printed circuit board, usually not possible on mobile devices due to the densely packed components; screen printing the solder onto the components is possible on parts down to 0.4 mm pitch, or the easiest solution, dipping the components into solder paste. Each type of package presents its own challenge to the re- work process. For example, component stencil printing is essential for successful rework of LGA, QFN, BTC, LLF, and PoP devices. Package on package requires the top and bottom com- ponents to be reworked simultaneously. These are processors and memory mounted on top of each other and are the most difficult to rework. Screen printing the solder paste onto the sol- der balls using a stencil is important to achieve the correct stand-off height. Flux dipping the balls of the top package is done to half the ball's height (Figure 9). The PoP device is reflowed with a solder paste reflow profile. The placement of quad flat, no-lead (QFN/ BTC) components also require a repeatable process for rework. QFNs/BTCs, also known as leadless lead frame packages (LLF), have no solder on the package, but just a thin plated surface layer of tin, and the distance be- tween the package and the circuit board is with- in 2 mils flat when soldered. QFNs/BTCs require solder paste either on the PCB or on the part to be replaced. In mobile products printing the PCB is impossible because of high density to adjacent packages; the stencil would never fit to the PCB for printing. Stencils are made to print the package with 4 or 5 mils of solder paste. The center pad is a ground pad rework cHaLLenges For smartPHones anD tabLets continues feaTure Figure 7: Micro-SMD examples. Figure 8: repeatable flux depositions on solder balls.