Issue link: https://iconnect007.uberflip.com/i/1137649
44 SMT007 MAGAZINE I JULY 2019 All Type 3, 4, and 5 solder pastes gave accept- able solder balling results. The no-clean Type 5 solder paste gave borderline acceptable/unac- ceptable results. All Type 6 solder pastes gave unacceptable results showing rings of solder balls and clumps of solder balls (Figure 18). Printing Area Ratio Limits The printing patterns on the PR test circuit board are challenging for solder pastes. The aperture sizes in the print patterns range from 6 mils (0.152 mm) up to 10 mils (0.254 mm) and the stencil is 5 mils (127 μm). This cor- responds to aspect ratios of 1.2 up to 2.0 and area ratios of 0.30 up to 0.50 respectively. The guidelines given in IPC-7525 [2] for acceptable solder paste printing are aspect ratios above 1.50 and area ratios above 0.66. The 0.30 and 0.35 area ratio apertures have aspect ratios of 1.2 and 1.4 respectively, which are below the recommended guideline of 1.50. All of the area ratios are below the industry guideline of 0.66. The printed solder paste transfer efficiency (TE%) values for these patterns typically range from 5% up to 60%. Figure 19 shows the transfer efficiencies for the printed solder paste broken out by area ratio. These transfer efficiency numbers are well below the generally accepted industry limit of 80%, which is normal for this stencil design. Table 7: IPC solder balling results for each solder paste. Figure 19: Transfer efficiency box plots for each solder paste by area ratio. Figure 18: IPC J-STD-005 solder balling pictures of acceptable and unacceptable results.