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58 SMT007 MAGAZINE I SEPTEMBER 2026 The dose rate varies according to the inverse- square law. For example, if the distance between the X-ray focal point and the sample is doubled, the dose rate decreases to approximately one-fourth of its original value. Solder Material Properties The elements used in tin-lead and lead-free solder alloys differ significantly in atomic number, mass, and density. These differences affect how the mate- rials absorb X-rays and how they appear within an X-ray image. Because lead has a higher atomic number and density than the primary elements used in most lead-free alloys, tin-lead solder typically produces stronger X-ray contrast. Lead-free solder joints may produce different grayscale values and may require adjustments to the inspection settings. Operators should avoid automatically compen- sating for reduced contrast by using unnecessarily high power, extended inspection times, or maximum magnification. These adjustments may improve the image but can also increase the radiation dose delivered to nearby semiconductor devices. Radiation Effects Radiation dose is the amount of energy deposited per unit mass of material. It is commonly expressed in Gray (Gy), where 1 Gy = 1 joule per kilogram = 100 rads. Dose rate refers to the amount of radiation deliv- ered over a given period and may be expressed as Gy per minute or rads per minute. The dose rate experienced by a device depends on its loca- tion relative to the X-ray source and the inspection settings being used. Extremely large radiation doses can deposit enough energy into a semiconductor die to cause direct physical damage and device failure. However, these gross-failure dose levels are generally much higher than those produced during normal shop-floor X-ray inspection. The more realistic concern is the potential for subtle or statistically occurring failure mecha- nisms. These can include bit flips, loss or corrup- tion of programmed data, changes in erase margin, increased leakage, and changes in device perfor- mance or reliability. K N O C K I N G D OW N T H E B O N E P I L E Element Lead Tin Silver Copper Symbol Pb Sn Ag Cu Atomic Number 82 50 47 29 Atomic Mass 207.2 118.71 107.87 63.55 Number of Protons 82 50 47 29 Approx. Neutrons 125 69 61 35 Density (g/cm³) 11.34 7.31 10.49 8.96 Table 1: Fundamental properties of elements used in tin-lead and lead-free solder alloys