As aerospace companies consider the shift to lead-free solder alloys and glues, concerns have been raised about whether their current X-ray inspection and quality-control procedures will still be valid. With lead solder, joints are easily interpreted by the operator or the system imaging software because lead provides excellent image contrasts due to relatively high X-ray absorption compared to that of PCB and component materials. Will this hold true as they shift to lead-free solder compounds?
In this paper, we study how the physics of X-ray requires the change of settings and procedures for an accurate inspection. The atomic number of a material (Z) drives its X-ray absorption. Materials with high Z absorb more X-rays than materials with low Z. Tin (Z = 50), for example, is a common material found in lead-free solders. Lead, on the other hand, has atomic number 82. This difference in atomic numbers—and the different alloys present in the market today—may drive the need to change the settings and modes of the X-ray inspection system to match the chemistry of the solder joints. An accurate X-ray technique can only be achieved once the imaging system can produce similar contrast for both lead and lead-free solder inspection.
It has been over a decade now since environmental concerns over contamination due to lead waste led the electronics industry to migrate solder processes away from the usage of eutectic tin-lead solder and towards utilization of lead-free compounds. Such a process change had several ramifications. The first major impact in the industry was the initial investment in equipment and materials to adapt to RoHS requirements. Another critical impact was the necessity to redesign reflow processes to bring the new assemblies with lead-free solders to an equivalent yield as the industry experienced prior to the migration to lead-free.
Although most companies have successfully moved to lead-free SMT lines, a considerable portion of manufacturers working in aerospace and military programs are still required to use lead solder. These companies routinely use X-ray inspection systems to assess and validate the quality of their assemblies. However, an X-ray inspection system will only be useful in a lead-free world if the changes in solder compounds do not have a significant deleterious effect on its imaging characteristics.
To mitigate the concerns related to the inspection migration from lead to lead-free solder, in this paper we provide theoretical and practical studies of some of the effects of lead-free solder on an X-ray imaging capability. This is done primarily by comparison of X-ray attenuation properties for eutectic tin-lead solder and various lead-free solder compounds. We additionally show both actual images of solder joints made with lead and lead-free solder pastes. The conclusion of this analysis is that we found little contrast variation when using lead-free solders, which implies little change in inspectability.
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