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Stencil design. To achieve all combinations of aperture reductions and offsets in a single print, the following modifications were made to the stencil design. From left to right, of the six columns of test cells, the first two columns had apertures reduced to 70% of their pad area; the middle two were not populated, and the last two had no aperture reductions applied. Within each group of aperture reductions, the test cells were divided into four segments, with each segment having a different aperture alignment offset.

Using the design shown in Figure 3, each combination of padstack design, aperture reduction and alignment offset appears twice per board. Two replicates were used, so each combination was assembled in four test cells.

Assembly. Upon receipt of the components, samples were inspected, which included dimensional measurements and XRF screening to ensure lead was not present in the system. All TVs were assembled in the Jabil Circuit Advanced Manufacturing Technology laboratory in St. Petersburg, FL, using the following equipment:

• MPM Ultraprint 3000 stencil printer.
• Koh Young KY3030-VAL paste measurement system.
• Fuji AIM pick-and-place.
• Vitronics-Soltec XPM2 reflow oven.
• Vi Technology Vi3K2 AOI.
• Phoenix Nanomex X-ray inspection.

Stencil printing. The investigation included high and low paste volume settings. In order to achieve two levels of paste volume, the stencil apertures were reduced. To identify the most appropriate aperture reduction, a prescreening study was performed. Stencil apertures were sized at 100%, 80% and 60% of their corresponding pad areas for all four footprints. It was determined through the prescreening DoE that 70% pad area, or 30% reduction, would be appropriate for the low setting. 100% pad area and 70% pad area equate to transfer efficiencies of roughly 60% and 50%, respectively, and varied slightly with the individual solder pastes and aperture designs.

Stencils used in the study were electroformed nickel with 125 µm (0.005") foils. All solder pastes were printed at the same parameters: 2"/sec. squeegee speed, 1.25 lb./in. squeegee pressure, slow separation speed.

Paste volume measurement. The SPI was used to measure paste volume deposition in all phases of the experiment. This proved to be especially useful in the prescreening experiment to determine the appropriate aperture reduction to apply to the stencil design.

The equipment employs a proprietary Phase Shift Profilometry (PSP) algorithm using a moiré light projection technique. This technique uses opposing dual-source LED lighting to capture eight images (four from each side) for every solder deposit. Using a reconstruction algorithm, the eight images are combined to form a 3-D model of the solder deposit. The dual source lighting technique is advantageous because it reduces the shadowing effect associated with single light source systems. With a pixel size of approximately 20 x 20 µm, it is important to have the most accurate information possible, particularly with the 0201 deposits in the size range of 0.010"-0.019".

To ensure system capability, a rotational Gage R&R was performed using Minitab’s crossed ANOVA method. The Total Gage R&R figure needs to be below 30 for the system to be used as an acceptable measurement instrument. Using a process tolerance window of +/-25%, the Total GRR for the test vehicle was around 20, with a repeatability figure below three. The cycle time for the equipment to measure nearly 20,000 solder deposits was approximately 55 sec.

Placement. The two-gantry, four-head placement machine handles devices from 0201s to 74 mm square. It has 180 feeder inputs and can feed from tape and reel, strip tape, sticks, and matrix trays. Its speed is rated at 20,300 cph. The machine was installed in the laboratory and calibrated prior to the test. A GR&R study was not performed prior to the test.

Reflow soldering. Six thermocouples were used to profile the assembly. Four were attached to the topside, two to the bottom (Figure 4). The reflow profiles depicted in Figures 5 through 8 were developed on unpopulated PWBs.

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