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May 2015 • SMT Magazine 41 B. New Materials Although the methodology was mainly de- veloped to ensure quality in the solder paste printing process, it has also proven to be an ef- ficient and standardized way to evaluate new materials that are introduced into the process. This section describes some of the tests that have been performed in order to secure and evolve the solder paste printing process. The first test was a solder paste printability test. During a printability test, solder paste is printed through stencil apertures with the same size and shape as the board pads. The volume of all printed solder paste deposits are then mea- sured by a SPI machine and the solder paste de- posit volumes from the same size apertures are then sorted out and the volume distributions are compared for different solder pastes. Figure 11 shows the solder paste volume dis- tributions for two different solder pastes print- ed through the reference stencil's 0.4 x 0.4 mm square apertures. Both solder pastes had Type 4 particle size, but they had different tackiness, rheology and viscosity properties. The center of the reference board has an area intended for the step stencil test. Around this area are pads of different sizes and with different distanc- es towards the central area (Figure 12). The test board pads are squares with rounded cor- ners with side-lengths of 0.25 mm, 0.35 mm, 0.45 mm, 0.7 mm, 1.1 mm, 1.7 mm and 3.0 mm. The pitches be- tween the same pad sizes, in the direction away from the center are given in Table 2. A test was performed where test stencils with a 1:1 aperture ratio to the test board pads were used. Five laser cut stencils with a base thickness of 5mil were used in this test. Each of the sten- cils had just one step height and the step heights for the different stencils were 1 mil, 2 mil, 3 mil, 4 mil and 5 mil. An example of a print result is shown in Figure 14. After each printed solder paste deposit had been measured by the SPI, the solder paste vol- ume distribution relationship to the distance from the step edge were analyzed. Examples of distributions for the same aperture size at differ- ent distances from the step edge and with dif- ferent step heights are shown in Figure 15. Another production tool that was investi- gated was the difference between two manu- facturing methods for stencils (laser cut and etched). The two test stencils (as described in the methodology section) were ordered and verified by measuring the area of several dif- ferent stencil apertures. The stencils were then used to perform ten solder paste prints each and the results were analyzed and are depicted in Figure 16. Manufacturing method one was laser cut and manufacturing method two was etched. All of the above described tests are the re- sult of the project site's used machines, envi- ronment, operators and consumables and the presented results should not be considered as generally true. Instead, they are provided here to simply give an understanding of how SOLDER PaSTE PRINTING: QUaLITy aSSURaNCE mETHODOLOGy continues Feature Figure 11: Solder paste volume distributions from two different solder pastes printed through 0.4 x 0.4 mm square stencil apertures.