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JULY 2019 I SMT007 MAGAZINE 37 Print Performance Print performance was measured using area ratio limit patterns, which have stencil aper- ture area ratios ranging from 0.30 up to 0.50 in 0.05 AR steps. The stencil is made of 5-mil (127 microns), fine-grain (2–5 μm) stainless steel without nano-coatings. The aperture sizes range from 6–10 mils in the area ratio limit pat- terns. These small area ratios show the print- ing limit of the solder paste (Figure 8). Print performance was also measured using 0.4-mm pitch ball grid arrays (BGAs), which have a stencil aperture area ratio of 0.50. The apertures are 10 mils square with 2-mil radi- used corners ("squircles"). Solder paste vol- umes were measured in the area ratio limit, and 0.4-mm BGA patterns using a solder paste inspection system (SPI) and statistical analy- sis performed to compare solder paste per- formance. Lower limits of area ratios are sug- gested for each solder paste. Stencil Life and Response to Pause A print-and-pause test was conducted on each solder paste to measure stencil life and response to pause. The process used is shown in Figure 9. The solder paste volume data for the area ratio limit patterns and the 0.4-mm BGA pat- terns was compared for each time. During this test, significant drops in solder paste volume indicate that thickening or drying of the solder paste has occurred. This data is used to give a suggested stencil life for each solder paste. Table 4: Solder paste metal concentrations. Figure 8: Printed solder paste in the area ratio (AR) limit patterns. Figure 7: PR test board patterns for quantifying print, reflow, and voiding performance.