Automated X-ray Inspection (AXI) is an inspection method used in electronics manufacturing to evaluate solder joints, component structures, and assembly conditions that cannot be reliably inspected with visible-light cameras alone. In SMT production, AXI is especially important for components with hidden or partially hidden solder connections.
Where AOI depends on what a camera can see from the surface, AXI uses X-rays to look through packages and inspect internal or concealed features. This makes it a valuable tool for verifying solder joint integrity on advanced assemblies.
What AXI does
AXI creates an X-ray image of the PCB assembly so the inspection system can evaluate internal structures. Depending on system capability and inspection strategy, AXI may assess:
- solder joint presence
- solder joint shape and distribution
- voiding indications
- bridges hidden beneath packages
- opens or insufficient connection areas
- component alignment relative to hidden pads
- structural anomalies under bottom-terminated packages
Because it can inspect beneath component bodies, AXI fills a major gap that optical inspection methods cannot always cover.
Why AXI is important in SMT assembly
As package styles become more compact and connection points move underneath the component body, direct visual access becomes limited. Manufacturers may be unable to confirm solder joint quality by external appearance alone.
AXI is important because it helps inspect assemblies containing:
- ball grid arrays (BGAs)
- chip-scale packages (CSPs)
- quad flat no-lead packages (QFNs)
- land grid arrays (LGAs)
- package-on-package structures
- other bottom-terminated or hidden-joint components
Without X-ray inspection, a board may appear acceptable externally while still containing soldering defects that are concealed from optical view.
How AXI works
An AXI system directs X-rays through the board and captures the resulting image based on how materials absorb or transmit the radiation. Dense materials, such as solder, appear differently than surrounding structures, allowing the system to analyze joint-related features.
AXI systems can be configured in different ways:
- 2D X-ray inspection: produces planar images useful for many hidden-joint checks
- Computed tomography or advanced 3D approaches: build layered or reconstructed views that improve inspection of complex overlap conditions
A typical AXI process includes:
1. the populated board enters the machine
2. the system captures X-ray images from selected angles or positions
3. software evaluates target features against programmed criteria
4. suspect defects are flagged for review
5. the results are used for acceptance, repair, or process improvement
The exact imaging method depends on board complexity, inspection coverage goals, and equipment capability.
What defects AXI can help detect
AXI is commonly used to identify:
- hidden solder bridges beneath packages
- missing or collapsed solder balls
- insufficient solder connection areas
- voiding patterns that require review
- opens or non-wetted connections
- misalignment of hidden-joint packages
- solder shorts not visible externally
Not every defect is equally easy to interpret, and some calls still require engineering judgment. However, AXI provides visibility that optical methods alone cannot offer.
AXI versus AOI
AXI and AOI are complementary rather than competing technologies.
AOI strengths
AOI is usually preferred for:
- visible component presence
- polarity and orientation checks
- lifted leads on visible packages
- obvious visible solder bridges
- high-speed inspection of standard visible assemblies
AXI strengths
AXI is preferred for:
- concealed solder joints
- bottom-terminated components
- BGA connection analysis
- internal solder distribution review
- hidden bridge and void investigation
In many factories, AOI covers most visible inspection needs while AXI is used selectively or comprehensively for assemblies where hidden joints are critical.
When do you need AXI?
AXI becomes especially valuable when the product or process creates inspection blind spots for optical systems. You are more likely to need AXI when:
- the assembly uses BGAs, QFNs, LGAs, CSPs, or similar hidden-joint packages
- the product has high reliability expectations
- defects have been escaping AOI or electrical test
- process qualification requires deeper solder-joint evidence
- new package types are being introduced
- rework validation is needed on hidden-joint components
If your critical solder joints cannot be seen directly, AXI should be considered seriously.
Situations where AXI may be essential
In some environments, AXI is not just helpful; it can be close to essential for risk control. Examples include:
- complex industrial control electronics
- medical electronics where documentation and confidence are important
- automotive modules with hidden-joint packages
- communications hardware with dense package layouts
- aerospace or mission-oriented assemblies requiring rigorous inspection evidence
The more expensive or difficult a field failure would be, the stronger the case for hidden-joint inspection becomes.
Situations where AXI may be selective rather than universal
Not every board requires full AXI coverage. Some manufacturers use AXI strategically, for example:
- first-article or new product introduction builds
- sampling on stable production lines
- targeted inspection of critical packages only
- failure analysis and process troubleshooting
- confirmation after process changes
This selective approach can make sense when the product mix is broad or when full inline AXI is unnecessary for every assembly.
Benefits of AXI
Key benefits of AXI include:
- visibility into hidden solder joints
- stronger defect detection for bottom-terminated packages
- improved root-cause analysis for reflow and print issues
- support for process validation and documentation
- reduced reliance on destructive analysis for routine checks
- better confidence in assemblies with limited optical access
AXI can also help engineering teams understand whether a defect originated in stencil printing, placement, reflow profile behavior, or package interaction.
Limitations of AXI
AXI is highly capable, but it is not a complete substitute for all other tests.
Potential limitations include:
- equipment cost and integration complexity
- longer inspection cycle times compared with some AOI use cases
- interpretation challenges on dense or overlapping structures
- need for skilled programming and review
- inability to guarantee full functional performance on its own
AXI shows structural evidence, not whether the product fully meets electrical or system-level requirements. Functional and in-circuit testing may still be necessary.
AXI in a broader quality strategy
The most effective use of AXI is typically within a layered inspection plan that may include:
- SPI for solder paste process control
- AOI for visible placement and soldering checks
- AXI for hidden-joint verification
- electrical test for circuit behavior
- functional test for product performance
This layered approach reduces the chance that one inspection blind spot becomes a customer escape.
How to decide whether AXI is justified
A practical AXI decision often depends on questions such as:
- Are important joints hidden from view?
- Would a hidden-joint failure be costly or safety-relevant?
- Are bottom-terminated packages central to the design?
- Has AOI or electrical test failed to provide sufficient confidence?
- Is the process mature enough to support consistent hidden-joint quality without added visibility?
If the answer to several of these questions is yes, AXI is often justified either inline or as part of a focused quality control strategy.
Key takeaway
AXI is an X-ray-based inspection method used to evaluate hidden solder joints and internal assembly conditions that optical systems may miss. You need AXI when your products include concealed interconnections, when reliability demands stronger evidence of solder-joint quality, or when optical inspection alone cannot provide enough confidence. In modern SMT manufacturing, AXI is often the inspection method that makes hidden defects visible before they become field problems.