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16 SMT Magazine • January 2017 soldered using Sn63Pb37 solder of type 3 with RMA flux. The remainder of the assemblies were soldered using Sn62Pb36Ag2 solder of similar type and flux content. Results Fabrication of the test vehicles was complet- ed and it was determined that the solder mask thickness was greater than desired (approxi- mately 3.5–4.5 mil compared to 1.0 mil pad thickness). This was discovered during PB fabri- cation, as the thickness of the solder mask inter- fered with board electrical testing and the HASL process. Additional boards were fabricated with thinner solder mask to enable the HASL process on the control units. PBs with ENEPIG surface finish were not replaced. XRF measurements were taken on the PBs at 26 locations on each board and at 20 loca- tions on additional test coupons. These mea- surement locations were located on both sides of the PB, and it was found that some of the Pd layer thicknesses did not meet the IPC-4556 specification requirements. In a typical produc- tion run, each board may not be measured, but a sample would be taken. Samples are collected over time and a standard deviation is calculat- ed; this standard deviation is used to determine if the process is in control per the standard, along with the individual measurements. In our testing, multiple locations on every board were measured, so the data itself is compared to the specification limits without consideration of standard deviations. Assembly of the test units was completed with SnPb or SnPbAg solder, then each com- ponent was removed by applying force to the component edge at a speed of 0.5 in/min (Fig- ures 3 and 4). The force required to remove the test parts is given in Table 1. The shear strength EVALUATION OF THE USE OF ENEPIG IN SMALL SOLDER JOINTS Table 1: Shear test results. Figure 4: Sample after components have been removed through shear test. Figure 3: Test sample containing LGA36 (4 components per test sample) prior to shear test.