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60 SMT007 MAGAZINE I SEPTEMBER 2026 These effects may occur at radiation levels much lower than those required to cause visible physical damage. They can also be random, meaning that exceeding a threshold does not guarantee that an individual device will fail. Instead, the probability of a failure occurring somewhere within a production lot may increase. Radiation dosage is cumulative. A device inspected during initial production, after rework, during failure analysis, and again during customer- return evaluation receives exposure during each inspection cycle. Typical Radiation Thresholds Not all commercial-off-the-shelf semiconductor devices have the same sensitivity to radiation. In fact, most devices will not be affected by the levels normally experienced during routine X-ray inspec- tion. However, maximum radiation values published by semiconductor manufacturers can vary widely, and the dose at which a random event may occur can be difficult to define precisely. These ranges should be treated as general refer- ence values rather than universal acceptance limits. Device-specific information provided by the semi- conductor manufacturer should take precedence whenever it is available. Because radiation-related effects can be random, exceeding an approximate threshold does not mean that every semiconductor device will fail. It may instead increase the statistical probability of failure within a larger production population. Oblique Angle Viewing Oblique-angle viewing is an important capability of X-ray inspection systems, particularly when evalu- ating BGAs, CSPs, and other area-array devices. In a direct top-down image, the body of the solder ball can obscure the interface between the ball, compo- nent termination, and PCB pad. Some older X-ray systems create an angled view by tilting the sample. Tilting generally requires moving the sample farther from the X-ray tube focal point, which reduces the available magnification. As package and solder-joint dimensions continue to decrease, higher magnification becomes impor- tant. For this reason, many modern systems main- tain the sample perpendicular to the X-ray focal point and instead create oblique views by moving the detector. Moving the detector preserves magnifica- tion and analytical detail. However, because the K N O C K I N G D OW N T H E B O N E P I L E Device Type Threshold (rads) Threshold (Gy) Linear 2,000–50,000 20–500 Mixed Signal 2,000–30,000 20–300 Flash Memory 5,000–15,000 50–150 DRAM 15,000–50,000 150–500 Microprocessors 15,000–70,000 150–700 Table 2: Approximate radiation-damage threshold ranges for selected commercial semiconductor devices