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Figure 12: Oblique view of a BGA assembled using lead-free solder. Note the size and shape of the balls in the first row. The balls on the left are correctly connected, while the balls on the last column in the right are either open or head-in-pillow (HIP). This side view indicates a planarity problem with this assembly, which means that the BGA must be removed and reworked.
Figure 13: Lead solder BGA with several of its balls showing excessive voiding which indicate that this part should be reworked. The irregular shape of some of the balls suggests irregular reflow.
Figure 14: Lead-free solder BGA with several of its balls showing excessive voiding which indicate that this part should be reworked.
CONCLUSIONS
In this paper, we demonstrated some effects on an X-ray inspection system the imaging characteristics for a few of the lead-free solders. A theoretical analysis based upon X-ray attenuation characteristics showed that there is almost no difference between eutectic tin-lead solder and 42Sn/58Bi. We also determined that predominantly Sn based solders lead to a contrast about 88% of that for eutectic tin-lead solder. Consequently, we expect little variation in the inspectability of joints made with these alternative solders. We additionally provided some images of real solder joints constructed with these different compounds. These images showed the expected contrast variations.
Although the analysis performed in this paper does not constitute a rigorous proof, it indicates that usage of a number of lead-free solders should not seriously degrade the X-ray inspection imaging capability.
REFERENCES
1. Hupfer, M., Nowak, T., Brauweiler, R., Eisa, F., and Kalender, W.A. (2012). “Spectral optimization for micro-CT”. Med. Phys.39, 3229–3239.
2. Attix, Frank H. “Introduction to Radiological Physics and Radiation Dosimetry”. John Wiley & Sons, New York, 1986.
3. Cullen, D. E., et. al. “Tables and Graphs of Photon-Interaction Cross Sections Derived from the LLNL Evaluated Photon Data Library (EPDL), Z=1-50”. Lawrence Livermore National Laboratory, Livermore, CA, Vol. 6, Part A, Rev. 4, 1991.
4. Cullen, D. E., et. al. “Tables and Graphs of Photon-Interaction Cross Sections Derived from the LLNL Evaluated Photon Data Library (EPDL), Z=50-100”. Lawrence Livermore National Laboratory, Livermore, CA, Vol. 6, Part B, Rev. 4, 1991.
5. Bastecki, Chris. “A Benchmark Process for the Lead-Free Assembly of Mixed Technology PCBs”. Technical Brief, Alpha Metals Inc., January 1997.
Editor's Note: This paper was originally published in the proceedings of SMTA International, 2016.
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