X-ray Inspection of PCBA Boards: Complete Guide
X-ray inspection of PCBA boards is used to see solder joints and internal structures that cannot be checked by eye or by an ordinary optical system. X-rays have a very short wavelength and very high energy, which allows them to pass through materials and create an image based on the density and thickness of the object.
When X-rays strike a circuit board, the amount of radiation that passes through depends on the material. Dense metals absorb more X-rays, while lighter plastics, silicon, and empty spaces allow more X-rays to reach the detector.
This guide explains how X-ray technology works, how it affects PCBA quality control, and the inspection workflow used to find hidden solder defects.
How X-rays Create a PCB Image
An X-ray inspection system sends radiation through the assembled board. The transmitted intensity is displayed as a grayscale image, where bright and dark areas show differences in material density and thickness.
A material with low density and thin cross-section allows more X-rays to pass. A material with high density absorbs more radiation and appears darker in the image. The contrast between materials allows the inspector to identify solder, leads, package material, and defects.
X-ray systems are calibrated so that the operator can interpret the image correctly. Voltage, power, and contrast settings are adjusted to make solder joints, component bodies, and holes clearly visible.
Without this calibration, a dense material can appear too bright or too dark, causing the operator to miss a defect.
Material Density Classes in an X-ray Image
Materials in a PCBA can be divided into four general density classes for X-ray interpretation. The first class is solder, which may be tin, lead, or a tin-lead alloy. Solder has high density and absorbs most of the X-ray energy.
The second class includes metal and ceramic package bodies, gold wire, and wafer bond materials. These materials are dense but not as dense as solder, so their image brightness falls between solder and laminate material.
The third class includes molding compound and silicon, which are much lighter than metal. X-rays pass through them more easily and create a lower grayscale value in the image.
The fourth class includes defects and empty spaces such as voids, cracks, and PCB through-holes. X-rays pass completely through these areas, so they appear as bright regions in the image.
Understanding these four classes helps the inspector distinguish a real solder joint from a package body, a void, or an empty via.
Why X-ray Inspection Is Important for PCBA
Optical inspection can only see the surface of the board. Components such as BGAs have all of their connections hidden under the package body, so a standard camera cannot verify the solder balls.
X-ray inspection sees through the component and reveals the actual solder connection. It can identify missing balls, bridges, insufficient solder, excess solder, voids, and cracks before the board is tested.
The technology is also useful for inspecting connectors, QFN packages, and through-hole joints where solder may not fill the barrel completely.
Finding these defects early reduces rework cost and prevents failures in the field. X-ray inspection is therefore an important part of PCBA quality control for complex and high-reliability products.
Can X-ray Damage the PCB
X-ray inspection is generally considered non-destructive. The radiation does not remove material, change the solder composition, or erase programmed components in normal inspection use.
Electronic components are exposed to X-rays during inspection, but the dose used in a typical 2D inspection system is low. The system is designed to create a clear image without damaging standard semiconductor devices.
Special products such as certain memory devices, image sensors, and radiation-sensitive components should be evaluated before X-ray inspection if the manufacturer specifies a limit. Process controls can then be set to use the lowest practical dose.
For most PCBA products, X-ray inspection is safe and provides quality information that cannot be obtained any other way.
2D X-ray Inspection Workflow
Two-dimensional X-ray inspection can be performed with a detector that tilts up to about plus or minus 70 degrees. The tilting ability gives the operator different views of the same joint so that hidden problems become visible.
Before inspection begins, the operator adjusts the X-ray voltage and detector power so that the image quality is clear. The contrast and brightness should make the solder joints and board features easy to identify.
A structured workflow makes inspection repeatable. The operator starts with a global view, then focuses on smaller areas, then uses magnification and tilt to confirm suspicious joints.
Finally, all BGA packages are checked with angled views to look for solder defects that may not appear in a straight vertical image.
Step 1: Global Inspection
The first inspection step is a global scan of the whole PCB. The image includes the board outline, component placement, and large solder areas.
At this magnification, the operator can quickly identify missing components, major shifts, large bridges, and obvious damage. Global inspection gives an overview of the assembly before detailed analysis begins.
Any suspicious area seen in the global image should be marked for the next step. The operator should not try to confirm every defect at low magnification because small features may be hidden.
Step 2: Local Inspection
The second step magnifies specific areas of the board for local inspection. The operator zooms into a BGA, connector, or dense component region to look for potential defects.
Local inspection reveals solder ball shape, alignment, and obvious voids. It also helps the operator determine whether a suspicious feature is a real defect or simply normal material contrast.
The magnified image should be clear enough to see individual balls or pads. If the image is not clear, the operator adjusts the voltage, current, or magnification before continuing.
Step 3: Confirm Suspicious Defects
When a possible defect is found, the operator should not reject the board immediately. The image should be magnified further and the detector should be tilted to inspect the joint from another angle.
Tilting is especially useful for a BGA ball that appears dark or missing in the vertical view. The second angle can show whether solder is present but hidden by another structure.
This confirm step separates false alarms from real defects. It also provides a clearer picture of void size, bridge height, and crack location.
Step 4: Inspect All BGA Devices
BGA packages are inspected more carefully than standard components because their joints are completely hidden. Every BGA on the board should be imaged with tilting views in the initial production phase.
The operator checks that each ball has enough solder, is round enough, and connects properly to both the package pad and the board pad. Void size should be compared with the product specification.
If a large number of boards show the same BGA defect, the reflow profile, stencil, or component quality should be reviewed. The X-ray image can be used as evidence when discussing a process change with the factory.
Advantages of This X-ray Method
The structured X-ray workflow makes the image clear and defects easier to recognize. The inspector does not depend on one fixed camera angle but can tilt the detector to confirm the shape of the joint.
It covers the common hidden defects found in PCBA, including solder balls, bridges, voids, opens, and misalignment. The workflow can be standardized so that different operators inspect the board in the same way.
Standardization improves inspection efficiency and reduces the chance of missing a defect. Each board is checked with the same level of detail, even when the operator changes.
X-ray inspection results can also guide PCB design and soldering process improvement. When the same defect appears repeatedly, the image data points directly to the area that needs correction.
Using X-ray Data for Process Improvement
X-ray inspection should not only reject bad boards. The recorded images should be used to monitor the process and identify the root cause of a defect.
A void that appears inside a BGA ball may be caused by outgassing from solder paste, an incorrect reflow profile, or moisture in the components. The X-ray image helps the engineer choose the correct correction.
A bridge between two fine-pitch leads may indicate too much solder paste or a stencil aperture that is too large. The pattern of defects across the board shows whether the problem is concentrated in one location or spread across the whole product.
Process improvement should be verified with additional X-ray inspection after the change. The data provides objective evidence that the defect rate has decreased.
Combining X-ray With Other PCBA Quality Controls
X-ray inspection works best when combined with AOI, ICT, and functional test. AOI finds visible defects, X-ray finds hidden solder defects, and electrical test verifies that the board operates correctly.
A well-designed PCBA production line should place inspection at the correct process step. SPI checks solder paste, placement inspection checks component position, and post-reflow X-ray checks the finished joint.
Quality records should include the X-ray images of critical components. If a field failure occurs, the image can be compared with the production data to determine whether the defect was present at manufacturing.
For high-reliability products, the customer and factory should agree on X-ray acceptance criteria before production. The criteria should specify the maximum void size, allowable bridge length, and sampling plan.
Working With an Experienced PCBA Supplier
An experienced PCBA testing and inspection partner should provide X-ray inspection for BGA and fine-pitch assemblies. The service should include both 2D inspection and angled views when the board design requires them.
The factory should be able to inspect the board during SMT PCB assembly so that defects are found before the assembly moves to conformal coating or final packaging.
For a turnkey project, X-ray inspection should be coordinated with PCB assembly, quality management, and the customer DFM review. The supplier should report defects and explain the corrective action.
X-ray inspection gives manufacturers the ability to deliver quality management evidence for hidden joints and to improve the soldering process based on real data.
Conclusion
X-ray inspection of PCBA boards is a safe, non-destructive quality method that makes hidden solder connections visible. It helps manufacturers detect defects before electrical testing and improve their process over time.
The four-step workflow of global, local, confirm, and BGA inspection is practical and repeatable. With clear images and structured operating procedures, X-ray inspection increases quality and production efficiency.
When X-ray data is combined with design review and process control, it becomes one of the most powerful tools for delivering reliable PCBA products.
