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44 The PCB Design Magazine • March 2014 that is used to mechanically and electrically connect the package to the PCB. Once the PCB has been etched, laminated, and the vias have been electroplated according to design, the next process step involves solder mask application to the outer surfaces of the PCB using print- ers, photo-masking, and radiation hardening. The role of the solder mask is to keep the sol- der on the pin during reflow. The solder paste is applied during assembly and is controlled by a layer called the paste mask. Regarding the final PCB, all of these lay- ers are geometric shapes that are drawn dur- ing footprint generation and are intended to facilitate connecting the pins of the device to the PCB. At this point the board would be blank with openings in the solder mask webbing at each pin. It would then be placed into a solder paste application machine where a 4-mil-thick stencil with laser-cut openings matching the pin etch is placed over the board, and then sol- der paste is applied by a squeegee sliding across the stencil, forcing solder paste onto each pin. The stencil is then removed, leaving sol- der on the pins of each component. The sol- der paste is comprised of a slurry of flux and lead-free solder spheres, and it sticks to the PCB pins and not to the stencil due to the sticki- ness of the flux and surface tension. Finally, a pick-and-place machine puts each part in the defined locations and rotations; then the PCB is conveyed through a reflow oven. After assem- bly, the boards are tested and any failures are categorized; the root cause analysis is conduct- ed assuming the percentage exceeds permissible yield limits. Now a CSP will be considered and footprint design for high yield will be the focus. The most important aspects of any component are the pin geometries, pitch, and the package keepout. The pitch is the spacing from the center of a pin to the next adjacent pin. The keepout is the area defined around the component; it is intended to force the designer to provide sufficient clear- ance between components, so that they do not interfere with each other during placement and reflow. When designing the footprint for the package drawing shown in Figure 1 with a pitch of 0.4 mm, key factors will be: a) specifying the solder mask so that it is guaranteed to be applied in between each pin without overlapping any pins b) specifying the pin diameter for consistent even soldering Figure 1: 36-pin CsP packaging diagram used to hold an iDTP902x wireless power receiver iC. article PROPERLy DESIGNING PCB FOOTPRINTS continues