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Knocking Down the Bone Pile: Minimizing X-ray Dosage During PCB Inspection

The ability to non-destructively inspect PCB assemblies is extremely important for verifying product quality without compromising the assembly. X-ray inspection provides a non-destructive method for examining optically hidden features that cannot be evaluated using automated optical inspection (AOI) or microscopic inspection. This makes it particularly valuable for verifying solder joint interconnections beneath BGAs, chip-scale packages (CSPs), and other bottom-terminated components.

During X-ray inspection, electronic devices are exposed to ionizing radiation produced by high-energy photons and therefore receive a measurable radiation dose. Most PCB assemblies can be inspected safely using normal X-ray practices. However, certain semiconductor devices, including DRAM, flash memory, and other sensitive electronic components, may be susceptible to elevated or cumulative levels of ionizing radiation.

When sensitive devices are present, users may need to consider the total radiation dose delivered during the inspection process. This is particularly important when an assembly requires extended inspection times, repeated inspection cycles, or high-magnification imaging.

X-ray Fundamentals
An X-ray inspection system functions as an X-ray shadow microscope. X-rays produced by an X-ray tube pass through the sample and are received by an image-capture device or image intensifier.

Materials within the PCB absorb different amounts of X-ray radiation based primarily on their density, thickness, and atomic number. Materials that absorb more radiation create darker or more defined shadows on the detector, allowing internal features to be evaluated.

Magnification is achieved by moving the sample closer to the X-ray tube’s focal point. As the distance between the sample and the focal point decreases, the projected shadow increases in size. While this improves magnification, it can also significantly increase the radiation dose rate received by the sample.

Factors affecting the radiation dose delivered during X-ray inspection include:

• Distance between the sample and the X-ray source
• X-ray tube voltage, measured in kilovolts
• X-ray tube current and total tube power in watts
• Length of exposure
• Filtration placed between the X-ray source and sample
• Number of repeated inspection cycles

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 materials 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 compensating 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 delivered 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 location 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 mechanisms. These can include bit flips, loss or corruption of programmed data, changes in erase margin, increased leakage, and changes in device performance or reliability.

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 inspection. 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 reference values rather than universal acceptance limits. Device-specific information provided by the semiconductor 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 evaluating 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, component 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 important. For this reason, many modern systems maintain the sample perpendicular to the X-ray focal point and instead create oblique views by moving the detector.

Moving the detector preserves magnification and analytical detail. However, because the sample remains close to the X-ray source, the device does not receive the dose-rate reduction that would result from increasing its distance from the focal point. Dose estimates should therefore account for the actual sample position during oblique inspection.

Minimizing Radiation Dosage
A semiconductor device receives some radiation throughout the X-ray inspection sequence. However, the greatest portion of the total dose is typically received while the device is positioned close to the X-ray source for high-magnification inspection.

Practical methods for reducing radiation dose include:

• Increase the distance between the sample and the X-ray tube focal point whenever maximum magnification is unnecessary
• Inspect only the areas requiring examination
• Limit extended exposure over radiation-sensitive components
• Minimize inspection time through standardized or automated inspection routines
• Avoid unnecessary repeated inspection following rework or repair
• Track the number of inspection cycles performed on sensitive assemblies
• Measure actual dose using a chemiluminescent dosimeter when component sensitivity is a concern
• Evaluate appropriate X-ray filtration for the specific equipment and inspection requirement

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 generally safe method for evaluating hidden solder joints and internal features within printed circuit board assemblies. However, assemblies containing radiation-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-sensitive, 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 radiation exposure to semiconductor devices.

This column originally appeared in the September 2026 issue of SMT007 Magazine.