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64 SMT007 MAGAZINE I SEPTEMBER 2026 Filtration can reduce lower-energy radiation that contributes to device dose without always providing a proportional improvement in image quality. However, the effectiveness of a filter depends on the X-ray system, tube settings, detector sensitivity, sample construction, and required image detail. Any filtration method should be validated before being incorporated into a production inspection process. The distance between the sample and the X-ray source remains one of the most important factors affecting dose rate. Increasing the focal-point- to-sample distance reduces radiation exposure according to the inverse-square law, although it also reduces magnification. For example, increasing the focal-point distance from approximately 2.0 mm to 6.9 mm can reduce the dose rate by approximately 90%, while still providing sufficient image detail for many routine inspection tasks. The objective should not be to use the lowest possible X-ray settings at the expense of inspection quality. Instead, the goal is to obtain the necessary image quality to make a reliable inspection decision while avoiding unnecessary magnification, power, exposure time, and repeated inspection. Conclusion X-ray inspection remains an essential and gener- ally safe method for evaluating hidden solder joints and internal features within printed circuit board assemblies. However, assemblies containing radi- ation-sensitive semiconductor devices or requiring repeated high-magnification inspection may need additional dose controls. Users should understand how source-to-sample distance, tube settings, exposure time, filtration, and repeated inspection cycles affect the total radiation dose delivered to the assembly. When a device is known or suspected to be radiation-sensi- tive, the proposed inspection procedure should be reviewed and, when necessary, the actual dose should be measured. A controlled inspection process allows users to obtain the analytical information needed to verify assembly quality while minimizing unnecessary radi- ation exposure to semiconductor devices. SMT007 Nash Bell is president of BEST, Inc. To read past columns, click here. K N O C K I N G D OW N T H E B O N E P I L E Distance (mm) Dose Rate (rads/min.) Dose Rate (Gy/min.) Time to 100 Rads (min.) 2.0 100.0 1.000 1.0 3.1 41.6 0.416 2.4 4.4 20.7 0.207 4.8 6.9 8.4 0.084 11.9 11.1 3.3 0.033 30.3 17.5 1.3 0.013 76.9 31.4 0.4 0.004 246.0 Table 3: Effect of focal-point-to-sample distance on radiation dose rate