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18 SMT Magazine • January 2017 values for the samples with ENEPIG finish were lower than those on HASL. For all finishes, there was a mixture of bulk solder fracture and pad lifting. For HASL, the fracture occurred in the bulk solder 18% of the time (the remaining por- tion resulted in pad lifting), while for ENEPIG pad lifting occurred about 35% of the time, with the remaining failures predominantly ex- hibiting bulk solder failure, and some instanc- es of brittle fracture (approx. 3%) at the PB pad. The average shear strength for ENEPIG in this test was 11.5 lbs. and the average shear strength for the HASL samples was 16.0 lbs. While the difference in these results initial- ly appear to be significant, two factors are im- portant to comparing these values. The first is that in some cases, there was an extra solder joint that formed outside of the intended 4 x 4 array. This only occurred on the HASL boards, and is attributed to an excess amount of sol- der from the HASL process on the pad, which was enough to solder to the part. These con- nections varied from relatively little connec- tion between the PB and component to 'full' solder joints similar to those within the 4 x 4 array. Table 1 indicates the locations where an extra solder joint occurred with values in red. Cells of this table highlighted in orange indi- cate that some or all of the pads lifted at that test site. The average of the shear strength val- ues for HASL samples that did not contain any extra solder joints was 15.2 lbs., but this signifi- cantly limits the sample size. EVALUATION OF THE USE OF ENEPIG IN SMALL SOLDER JOINTS Figure 5: Example of HASL unit removed in shear test with 'extra' solder joint. Figure 7: Cross-section of solder joint on ENEPIG. Figure 8: Shear fracture surface of ENEPIG sample with ductile fracture surface. Figure 6: Cross-section of solder joint on HASL.