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July 2014 • SMT Magazine 79 MEETING FuTuRE STENCIl PRINTING CHAllENGES continues ARTiCLE settings utilized for the experiment. Each sol- der paste was printed at 50 mm/s with a blade pressure of 4.4kg, 5 mm/s separation speed, and separation distance of 2 mm. The underside stencil-wipe method was used only during the one-hour pause W/D/V (wet/dry/vacuum). A test vehicle (Figure 3) was selected with fo- cus on 6, 7, 8, 9, 10, 12 mil circles and squares, as well as 0201 pads in both solder mask defined (SMD) and non-solder mask defined (NSMD) pads. Four powder types were evaluated: types 3, 4, 5, and 6 in no-clean and water soluble flux formulations. Response to pause was measured by continuous printing 20 times, pausing for 1 hour, and printing 6 more times. A wet/dry/ vacuum stencil wipe was incorporated imme- diately after printing the first 20 PWBs. A Koh Young KY-3020T laser scanning system was used to measure the volume of the stencil printed de- posits. In all, 1,386,500 data points (solder paste deposits) were collected. Figure 4 shows the component identifica- tion and the area ratio associated with it. Printing Results Gauge Variability Chart for Volume (%) Figure 5 represents a broad view of the re- sultant test data. The y axis of the upper graph indicates the transfer efficiency in volume %, with 100% being the target. The bottom graph indicates the standard deviation of the process; a smaller standard deviation indicates a more consistent process. Included in the test were a number of 0201 and 12 mil chip scale package (CSP) test pads that are not depicted. The solder paste deposits on these pads were well-printed regardless of powder and flux type and were therefore excluded from further investigation. The data in Figure 5 reveals that the smallest test aperture (6 mil) did not print well for all powder types or flux vehicles. The area ratio is quite small (0.375) for the 6 mil aperture and the results were inconsistent. Variability Gauge Component ID=S07_SMD Variability Chart for Volume(%) A dramatic improvement occured with 7 mil apertures that deserved further investigation, notably between the water soluble and no-clean flux formulations. Figure 6 shows the improve- ment in more detail by expanding the view of the 7 mil aperture and breaking out the data be- fore and after the 1-hour pause. Note that a stan- dard deviation <10%, with the exception of a few outliers which exhibits a controlled process, was achieved specifically with the Indium8.9 no-clean flux chemistry. The area ratio for the figure 3: Test vehicle. figure 4: Pad labels, description, and associated area ratios.