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100 SMT007 MAGAZINE I SEPTEMBER 2019 But this rework method has drawbacks, including the handling of stencils when very small packages are involved, the "messiness" related to solder paste printing, and the time it takes to have the stencils on hand (24–48 hours) all limit the usefulness of stencil print- ing solder paste for rework. Dispensing Dispensing, along with jet printing, offers an alternative to both stencil printing as well as hand soldering. There are several advan- tages for dispensing when it comes to the rework of high I/O count and/or very small package components (Figure 3). The biggest differentiator of this technology is the pre - cision of the dispensing technique. Modern Archimedes screw-type dispensers can dis- pense 20,000 dots per hour down to 800-um sizes. A single jet printing setup can print at a constant speed of up to 300 Hz, or 1,000,000 dots per hour, with the dots getting down to 200-μm in size. One of the other big advantages of jetting is that multilevel boards and odd-shaped lands— such as RF shields with the same deposited sol- der volume—offer high repeatability. In addi- tion, this method is more automated than the stencil or hand solder rework methods. While this method offers many advantages, it is not for every scenario due to its capital-intensive nature, the programming required, and the rel- atively slow speed of deposition. Dispensers that can be used for solder paste deposition start at several thousand dollars and range up to over $100,000 U.S. dollars. And each of the package footprints take time and skill level to program. This method, even when finely tuned, is very slow compared to the methods. Finally, any process that involves a liquid and dispens- ing like this requires expertise so that lines, nozzles, and other elements of the dispensing system are kept clean and optimized. This is rheological "tightrope to walk" with dispens- ing technology and the associated precision deposition upside. Conclusion The process engineer has to take each rework situation and consider the turn time required, the reliability of the assembly in the end-use operating environment, the skill level of the operators, the available funds, and the eco- nomics to make the right decision. SMT007 Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. For more information, contact info@solder. net. To read past columns or contact Wettermann, click here. Figure 2: Flexible stencil paste print. Figure 3: Dispensed solder paste on ultra-fine-pitch SMT component.

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