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SI8425DB Datasheet(PDF) 10 Page - Vishay Siliconix |
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SI8425DB Datasheet(HTML) 10 Page - Vishay Siliconix |
10 / 11 page AN824 Vishay Siliconix Document Number: 71990 06-Jan-03 www.vishay.com 3 Board pad design. The landing-pad size for MICRO FOOT products is determined by the bump pitch as shown in Table 3. The pad pattern is circular to ensure a symmetric, barrel-shaped solder bump. TABLE 3 Dimensions of Copper Pad and Solder Mask Opening in PCB and Stencil Aperture Pitch Copper Pad Solder Mask Opening Stencil Aperture 0.80 mm 0.30 " 0.01 mm 0.41 " 0.01 mm 0.33 " 0.01 mm in ciircle aperture 0.50 mm 0.17 " 0.01 mm 0.27 " 0.01 mm 0.30 " 0.01 mm in square aperture ASSEMBLY PROCESS MICRO FOOT products’ surface-mount-assembly operations include solder paste printing, component placement, and solder reflow as shown in the process flow chart (Figure 5). FIGURE 5. SMT Assembly Process Flow Stencil Design IIncoming Tape and Reel Inspection Solder Paste Printing Chip Placement Reflow Solder Joint Inspection Pack and Ship Stencil design. Stencil design is the key to ensuring maximum solder paste deposition without compromising the assembly yield from solder joint defects (such as bridging and extraneous solder spheres). The stencil aperture is dependent on the copper pad size, the solder mask opening, and the quantity of solder paste. In MICRO FOOT products, the stencil is 0.125-mm (5-mils) thick. The recommended apertures are shown in Table 3 and are fabricated by laser cut. Solder-paste printing. The solder-paste printing process involves transferring solder paste through pre-defined apertures via application of pressure. In MICRO FOOT products, the solder paste used is UP78 No-clean eutectic 63 Sn/37Pb type3 or finer solder paste. Chip pick-and-placement. MICRO FOOT products can be picked and placed with standard pick-and-place equipment. The recommended pick-and-place force is 150 g. Though the part will self-center during solder reflow, the maximum placement offset is 0.02 mm. Reflow Process. MICRO FOOT products can be assembled using standard SMT reflow processes. Similar to any other package, the thermal profile at specific board locations must be determined. Nitrogen purge is recommended during reflow operation. Figure 6 shows a typical reflow profile. 0 50 100 150 200 250 0 100 200 300 400 Thermal Profile Time (Seconds FIGURE 6. Reflow Profile PCB REWORK To replace MICRO FOOT products on PCB, the rework procedure is much like the rework process for a standard BGA or CSP, as long as the rework process duplicates the original reflow profile. The key steps are as follows: 1. Remove the MICRO FOOT device using a convection nozzle to create localized heating similar to the original reflow profile. Preheat from the bottom. 2. Once the nozzle temperature is +190_C, use tweezers to remove the part to be replaced. 3. Resurface the pads using a temperature-controlled soldering iron. 4. Apply gel flux to the pad. 5. Use a vacuum needle pick-up tip to pick up the replacement part, and use a placement jig to placed it accurately. 6. Reflow the part using the same convection nozzle, and preheat from the bottom, matching the original reflow profile. |
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