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76 SMT Magazine • September 2015 sample materials Analysis The solder composition of all the assem- bled sample vehicles, Design 1, Design 2, and the MCM, was verified using both X-ray fluo- rescence (XRF) for all solder terminations, and scanning electron microscope energy dispersive spectroscopy (SEM-EDS) for selected solder ter- minations that were prepared by mounting and cross-sectioning (destructive physical analysis). Generally, two terminations were analyzed on each component, and between one and three locations per termination. If the component had wrap around leads, measurements could only be taken at the top of the lead, but if the component had "J" style leads three locations were measured, as discussed next. These factors resulted in multiple data points per component and multiple repetitions of the components on the sample vehicles. The XRF and SEM-EDS data produced agreed within measurement accuracy for the two techniques at the surface locations. The XRF data was typically taken at three locations on formed terminations at the egress from the part body, just before a knee in the "J" style lead bends, and at the solder foot in all cases. The SEM-EDS data was taken in cross-section views, not on the top down views used in XRF. The SEM-EDS data was taken at near the surface of the solder (most comparable to the XRF mea- surements), at a mid-point representing the bulk of the solder, and then near the interface between the solder and the lead substrate mate- rial. This sample location plan took advantage of the speed of measurement and non-destruc- tive nature of XRF, while also taking advantage of the relatively small spot size of the SEM-EDS measurement to verify the depth of mixing of the lead into the pure tin solder terminations. A. XRF Quantitative Analysis Data Typical of Designs 1 and 2 A sample of the design 2 XRF data is shown in Figure 7. The legend in Figure 7 shows white for self-mitigating, pink for not mitigating, blue for physically impossible measurements that were excluded from consideration in the analysis, and green for locations where we verified no part was present. The blue highlights usually occurred on locations at the knee of a lead bend, and XRF data is usually not valid on a surface parallel to the direction of the X-ray beam. The X-ray beam must strike a flat surface perpendicular to the beam for typical X-ray detector function in the materials analysis. The blue highlights in this data are typically present on the second of three rows of data for a part, which corresponds to the vertical portion of the lead bend area. However, a few locations with physically impossible compositions were excluded, as it is not possible to get 41.8 Wt % tin when the part starts out as 100 Wt% tin, and the solder paste was 67 Wt% tin. These would indicate locations where the beam alignment to the pad was off, perhaps due to pick and place irregularities. B. XRF Quantitative Analysis Data for MCM Samples A sample of the MCM XRF data is shown in Figure 8. The legend in Figure 8 shows white for self-mitigating, pink for not mitigating, blue for physically impossible measurements that were ArtiCle tIN WHIsKer seLF-mItIGAtION IN surFACe mOuNt COmpONeNts continues figure 7: Design 2 xrf data sample.