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January 2017 • SMT Magazine 19 these units in shear testing were similar, with several consistent differences from the initial shear testing. The force required to shear the parts increased slightly for each surface finish, the standard deviation of values went down slightly in each group, and there was a signif- icant reduction in the number of lifted pads. The incidence of pad lifting dropped from 50% to 3%. These values are based on the number of individual pads lifted rather than the locations indicated in Tables 1 and 2 (identified by high- lighted cells). It is expected that this is a result of further curing of the PB to improve pad ad- hesion. Remaining failures on the HASL sam- ples were exclusively ductile failures in the bulk solder again. When the solder type is considered, Sn- PbAg solder had a lower force at failure relative to SnPb (11.8 vs. 13.4 lbs). After thermal aging, the average shear values relative to solder alloy both increased, but SnPb still demonstrated a slightly higher strength (15.1 lbs. for SnPbAg vs. 15.7 lbs. for SnPb). In both the pre-thermal aging samples and the post-thermal aging samples, there were some instances of brittle facture on the ENEPIG units. Some of the brittle fractures were associ- ated with each of the three ENEPIG surface fin- ishes. The proportion of this failure mode was very low at about 3% regardless of thermal ex- posure, although the occurrence of brittle fail- EVALUATION OF THE USE OF ENEPIG IN SMALL SOLDER JOINTS The second factor impacting the shear test results is observed in a comparison of the cross sections from an ENEPIG sample and a HASL sample. On the ENEPIG units, the excess thick- ness of the solder mask previously identified prevented the parts from sitting as low as they would have otherwise. This caused the solder joints to form an hourglass shape (Figure 7). The shape provided a smaller cross-section at the middle, which reduced the overall strength of the solder joint. This reduction was in the range of 10–20%. Figure 8 shows a typical 15% reduction in solder joint cross sectional area at the fracture point. A reduction in fracture cross- sectional area due to the solder joint geometry contributed a considerable portion of the dif- ference between HASL and ENEPIG in shear strengths. The combination of thinner solder mask and additional solder from the HASL pro- cess resulted in a different solder joint geometry on the HASL units. A second set of test units for all surface fin- ishes was aged at 100°C for 10 days. Results for Table 2: Shear test results (lbs.) after isothermal aging. Figure 9: Shear force of failure (lbs.) for each surface finish prior to and after thermal aging. The size of each bubble indicates the standard deviation of the data for that group.