Issue link: https://iconnect007.uberflip.com/i/1137649
JULY 2019 I SMT007 MAGAZINE 41 The QFN thermal pad stencil design was a window-pane type with nine panes. The web width of the QFN window panes was 0.51 mm (20 mils). Four QFN components were placed on each of five circuit boards for a total of 20 QFNs and 20 void area measurements per sol- der paste. Statistical analysis was used to com- pare voiding performance for each solder paste. Standard Solder Paste Tests Several industry standard solder paste tests were run to compare and contrast the perfor- mance of the solder pastes. Viscosity using both T-bar spindle and spiral pump, slump, and solder balling were all run per J-STD-005 [1] . The data from these industry standard tests was compared and contrasted for each solder paste, mainly to illustrate basic differences in the solder pastes. Stability: Tack Force Tack force testing was run per JIS Z 3284 [11] on freshly prepared coupons. Additional tack force coupons were printed and placed in a controlled environment at 21–24°C (70–75°F) and 50–55% relative humidity. The tack force coupons were stored, and tack force was mea- sured after hold times of 24, 48, and 72 hours. Changes in tack force over time give some information about reactivity or stability of the solder paste. Ideal performance is no change in tack force over 72 hours. Stability: Heat Aging The solder pastes were placed in sealed jars in an oven at 50–55°C (122–131°F) for 72 hours. This temperature is much higher than recom- mended for solder paste storage. The normal recommended storage temperature for most solder pastes is 5–10°C (40–50°F). Elevated temperatures tend to speed potential reactions within the solder paste. This may result in a loss of activity, thickening of the solder paste, and overall degradation in performance. After heat aging, the solder pastes were printed on the PR test board and reflowed. Viscosity, solder balling, and tack force mea- surements were taken. These results were compared to the results from the fresh solder pastes (before heat aging). Stable solder pastes tend to show very little change in performance while more reactive solder pastes tend to show drops performance. Statistical Analysis Tukey-Kramer honest significant difference (HSD) testing was done on the voiding data sets to compare and contrast the data. Tukey- Kramer HSD analysis determines whether multiple data sets are significantly different or Figure 15: Stencil design for QFN thermal pads.