Issue link: https://iconnect007.uberflip.com/i/989774
72 SMT007 MAGAZINE I JUNE 2018 Statistics calculated from the output read- ings included: • Means (or averages) • Standard deviations • Coefficients of variation (CV, or the standard deviation divided by the mean and expressed as a percentage). CV is preferred over Cpk when comparing different SPI datasets because it normal- izes the variation with respect to the mean without the influence of control limits • Using generally accepted industry practices, the acceptability criterion was TE at least 80% of theoretical aperture volume and a CV of less than 10% Stencil under wipes were performed before each set of five prints, but not between prints. SPI readings were taken immediately after each print. The full runs that included 0, 30-, 60- and 90-minute pauses took approximately four hours from start to finish. The tests were nested as follows: Time 0 1. Install stencil and squeegees 2. Stir solder paste and apply to stencil 3. Print 5 boards 4. Start timer for 30 minutes 5. Remove stencil and squeegees, leave paste on stencil At 30-minute mark on timer run WVD wipe 1. Print 5 boards 2. Start timer for 60 minutes 3. Remove stencil and squeegees, install stencil and squeegees 4. At 30-minute mark on timer, run WVD 5. Print 5 boards At 60-minute mark run WVD wipe 1. Print 5 boards 2. Start timer for 90 minutes 3. Remove stencil and squeegees, install "A" stencil and squeegees 4. At 60-minute mark on timer run WVD wipe 5. Print 5 boards At 90-minute mark on timer "F" run WVD wipe 1. Print 5 boards 2. Start timer for 120 minutes 3. Remove stencil and squeegees, install stencil and squeegees 4. At 90-minute mark on timer, run WVD wipe 5. Print 5 boards 6. Remove stencil and squeegees Table 2 shows the data management work- sheet that indicates variable, run order, board labeling and SPI file tracking information. Results and Discussion There is myriad facets to consider when analyzing the collected data. To streamline the analysis: 1. data has been compartmen- talized by component type or padstack 2. only best- and worst-case scenarios are reviewed 3. paste transfer quality is interpreted with respect to the main input variables of nanocoating, paste particle size, and foil tensioning BGAs 0.4 and 0.5 mm BGA aper- tures are the first features consid- ered, as these devices are becom- Table 2: Data management spreadsheet.