PCB X-Ray Inspection reveals solder joints and internal structures that cameras cannot see. It is especially valuable for BGA, QFN, LGA, CSP, thermal pads, and other bottom-terminated components.
X-ray images can reveal bridges, missing solder balls, misalignment, excessive voiding, insufficient solder, and some open-joint indications. However, X-ray inspection does not prove complete electrical performance or long-term reliability.
A reliable quality plan combines X-ray with SPI, AOI, process control, electrical testing, and functional verification.
What Is PCB X-Ray Inspection?
PCB X-ray inspection is a non-destructive imaging method used to examine internal PCBA structures.
X-rays pass through the circuit board and its components. Materials with different densities absorb different amounts of radiation.
Dense metals such as solder, copper, and component lead frames create strong image contrast. Air gaps and lower-density materials appear differently.
This contrast allows inspectors to examine hidden features without removing components or cutting the board.
Depending on the equipment, the system may inspect:
- BGA solder balls
- QFN thermal pads
- LGA joints
- CSP packages
- Bottom-terminated components
- Through-hole solder fill
- Press-fit connections
- PCB vias
- Component lead frames
- Bond wires
- Internal package structures
- Connector solder joints
Why Is X-Ray Inspection Important In PCB Assembly?
Modern electronic products use increasingly dense component packages.
Many solder joints remain completely hidden after assembly.
AOI and manual inspection cannot directly view joints beneath:
- BGA packages
- QFN components
- LGA packages
- CSP devices
- Power modules
- Shielded components
- Large thermal pads
A board may look perfect from the outside while containing a hidden bridge or open connection.
X-ray adds internal evidence to the quality-control process.
It helps manufacturers detect defects before they create:
- Electrical shorts
- Intermittent connections
- Thermal problems
- Product test failures
- Early field failures
- Expensive BGA rework
- Customer returns
How Does PCB X-Ray Inspection Work?
An X-ray inspection system includes an X-ray source, detector, motion system, shielding, imaging software, and operator controls.
X-Ray Generation
The source produces X-rays and directs them through the PCBA.
Material Absorption
Different materials absorb the energy at different rates.
Thick solder areas usually create stronger contrast than air, plastic, or thin laminate.
Image Detection
The detector captures the radiation that passes through the assembly.
Software converts the signal into a visible image.
Image Analysis
The inspector reviews:
- Shape
- Size
- Position
- Symmetry
- Density
- Void area
- Joint consistency
- Alignment
- Possible bridges
- Missing features
Advanced software may calculate measurements and compare them with programmed acceptance rules.
Which PCB Components Need X-Ray Inspection?
X-ray is most useful when solder joints are not visible.
Ball Grid Array Components
BGA packages use solder balls arranged beneath the component.
Visual inspection can examine package position but cannot directly verify the internal ball array.
Fine-Pitch BGA
Fine-pitch BGA packages contain smaller balls and tighter spacing.
They require accurate paste printing, placement, reflow, and inspection.
Chip-Scale Packages
CSP packages have compact dimensions and hidden connections.
Their small joints need high-resolution imaging.
QFN Components
QFN packages contain perimeter terminations and often a large center thermal pad.
X-ray can evaluate solder distribution and voiding beneath the center pad.
LGA Components
LGA packages use flat contact lands instead of solder balls.
Inspection can be difficult because the finished joint remains thin and hidden.
Bottom-Terminated Components
Bottom-terminated devices include power components, sensors, modules, and other leadless packages.
The current IPC-7093B provides guidance for the design, assembly, inspection, repair, and reliability of bottom-termination components. It appears in the Global Electronics Association’s current electronics assembly standards catalogue.
Through-Hole Components
X-ray may also evaluate solder distribution inside plated through-holes.
However, joint geometry and overlapping features can make interpretation difficult.
What Defects Can X-Ray Inspection Detect?
The exact detection capability depends on equipment resolution, viewing angle, package geometry, image quality, and operator skill.
Solder Bridges
A bridge connects adjacent solder joints.
It may cause a short circuit or abnormal electrical behavior.
X-ray images can show unintended solder connections beneath hidden packages.
Missing Solder Balls
A missing BGA ball may create an open circuit.
The image may show an empty location or unusual contrast compared with surrounding joints.
BGA Misalignment
A shifted or rotated package changes the relationship between solder balls and PCB pads.
X-ray can reveal:
- Ball offset
- Package rotation
- Uneven array alignment
- Partial pad contact
- Possible bridging risk
Excessive Solder Voids
Voids are gas-filled spaces inside solder.
X-ray can reveal their area, location, distribution, and shape.
The effect depends on joint type and application.
Insufficient Solder
A joint may show smaller volume or weaker contrast than surrounding connections.
However, overlapping structures can complicate evaluation.
Excessive Solder
Too much solder may cause bridging, component floating, or abnormal joint shape.
Non-Uniform BGA Collapse
BGA solder balls should normally show reasonably consistent shape and collapse.
Irregular joints may indicate temperature, warpage, paste, or placement problems.
Possible Open Joints
Some open connections appear as visible separation or abnormal geometry.
However, conventional 2D X-ray may not detect every open joint.
Head-In-Pillow Indications
Head-in-pillow occurs when the BGA ball and solder paste fail to merge correctly.
Angled X-ray or 3D CT may reveal certain signs. Nevertheless, some cases remain difficult to confirm without additional analysis.
Through-Hole Fill Problems
X-ray may show:
- Incomplete barrel fill
- Internal voids
- Insufficient solder
- Pin misalignment
The inspection method must account for copper and component overlap.
What Are Solder Voids?
Solder voids are cavities or gas pockets inside a solder joint.
They may form when flux gases, moisture, or other volatiles become trapped during solidification.
Possible causes include:
- Solder paste chemistry
- Stencil aperture design
- Excessive paste
- Reflow profile
- Via-in-pad design
- Surface contamination
- PCB finish
- Component finish
- Thermal-pad geometry
- Outgassing
Not every void has the same effect.
Location can matter as much as total area. A void near a critical thermal path or high-stress region may create more risk than several small distributed voids.
How Is BGA Void Percentage Calculated?
Two-dimensional software often estimates voiding by comparing the projected void area with the projected solder-joint area.
A simplified calculation is:
Void Area Percentage = Total Detected Void Area ÷ Joint Area × 100
This calculation requires consistent:
- Image magnification
- Contrast
- Threshold settings
- Joint boundary selection
- Measurement method
- Viewing angle
A 2D area percentage is not the same as true three-dimensional void volume.
CT systems can provide volumetric information, but reconstruction and segmentation settings still affect results.
Inspection reports should identify the measurement method.
Is There A Universal Acceptable Void Percentage?
No single void limit applies to every component and application.
Acceptance depends on:
- Package type
- Joint function
- Thermal requirements
- Electrical current
- Mechanical stress
- Product environment
- Customer specification
- Component manufacturer guidance
- Applicable workmanship standard
- Reliability testing
A thermal pad may use different criteria from a signal BGA ball.
Therefore, buyers should not apply one general percentage to every hidden solder joint.
The customer and manufacturer should agree on acceptance criteria before production begins.
PCB X-Ray Inspection For BGA Components
BGA inspection evaluates the complete solder-ball pattern.
Inspectors may review:
- Missing balls
- Bridges
- Alignment
- Joint diameter
- Joint consistency
- Voiding
- Partial collapse
- Abnormal solder movement
- Possible opens
- Package warpage indications
The current IPC-7095E provides design and assembly guidance for BGA and fine-pitch BGA technology, including inspection, repair, reliability, and troubleshooting.
X-ray cannot replace a stable BGA assembly process.
Manufacturers should also control component storage, solder paste, stencil design, placement, reflow, and PCB pad quality.
PCB X-Ray Inspection For QFN Components
QFN packages often contain a center pad for electrical grounding and heat transfer.
X-ray can evaluate:
- Thermal-pad solder coverage
- Void distribution
- Component alignment
- Perimeter solder
- Excessive solder
- Solder bridging
- Via-related solder loss
Thermal Pad Voiding
Excessive voiding can increase thermal resistance.
However, the effect depends on void position, distribution, device power, PCB design, and operating conditions.
Component Floating
Excessive center-pad solder can lift the package.
This movement may reduce perimeter-joint quality.
Via-In-Pad Solder Loss
Open vias in thermal pads can draw molten solder away from the joint.
Filled and capped vias or optimized stencil apertures may reduce this risk.
PCB X-Ray Inspection For LGA Components
LGA joints can be more difficult to interpret than BGA joints.
They have limited stand-off height and may produce overlapping images.
Inspection may evaluate:
- Alignment
- Solder coverage
- Bridges
- Voiding
- Missing solder
- Joint consistency
Angled views or 3D reconstruction may provide better separation than a single top-down image.
Electrical testing remains important because image interpretation alone may not prove connection quality.
2D X-Ray Inspection
Two-dimensional X-ray produces a projection image through the complete assembly.
All structures along the X-ray path can overlap.
Advantages Of 2D X-Ray
- Fast inspection
- Non-destructive
- Suitable for production screening
- Useful for BGA alignment
- Useful for bridge detection
- Supports void-area measurement
- Lower cost than CT
Limitations Of 2D X-Ray
- Overlapping structures
- Limited depth information
- Difficult multilayer interpretation
- Some open joints remain hidden
- Limited separation of top and bottom features
- Area measurement instead of true volume
For many standard BGA inspections, 2D imaging provides useful production information.
2.5D Or Oblique X-Ray Inspection
Oblique X-ray captures the assembly from an angle.
The operator tilts the detector, source, or board to separate overlapping structures.
This method can improve visibility of:
- Outer BGA rows
- Joint shape
- Component stand-off
- Package alignment
- Possible separation
- Through-hole fill
It provides more perspective than a straight top-down image.
However, it does not create a complete three-dimensional reconstruction.
3D X-Ray And Computed Tomography
3D CT captures multiple projections and reconstructs an internal volume.
The software can display slices through selected depths.
Advantages Of 3D CT
- Separates overlapping structures
- Shows internal layers
- Supports volumetric void analysis
- Improves complex-joint interpretation
- Assists failure analysis
- Supports multilayer package inspection
Limitations Of 3D CT
- Longer scan time
- Higher equipment cost
- Larger data files
- Reconstruction artifacts
- More complex programming
- Greater operator training requirements
- Lower production throughput
CT may suit failure analysis, process validation, complex assemblies, and high-risk products.
It may not be necessary for every production board.
What Is Automated X-Ray Inspection?
Automated X-ray inspection, or AXI, uses programmed image capture and analysis.
The system may automatically:
- Locate components
- Capture target images
- Identify joint boundaries
- Measure voiding
- Compare joint size
- Detect missing features
- Flag possible bridges
- Record results
- Route failed boards
AXI can improve consistency and production speed.
However, programming and validation remain important.
Incorrect thresholds may create false failures or allow real defects to escape.
Borderline results may require engineering review.
X-Ray Inspection Vs AOI
AOI and X-ray inspect different structures.
| Inspection Feature | AOI | X-Ray |
|---|---|---|
| Visible Components | Strong | Limited detail |
| Component Presence | Strong | Possible |
| Polarity Markings | Strong | Usually limited |
| Visible Solder Joints | Strong | Possible |
| Hidden BGA Joints | No | Yes |
| QFN Thermal Pad | No | Yes |
| Internal Voids | No | Yes |
| Labels And Silkscreen | Strong | Limited |
| Color And Surface Condition | Strong | No |
| Internal Package Structure | No | Yes |
A strong inspection plan uses AOI for visible assembly conditions and X-ray for hidden joints.
Our SMT PCB assembly guide explains how these inspections fit the complete SMT process.
X-Ray Inspection Vs Solder Paste Inspection
SPI occurs before component placement and reflow.
It measures solder paste deposits on PCB pads.
X-ray usually occurs after reflow.
| Factor | SPI | X-Ray |
|---|---|---|
| Production Stage | Before placement | After reflow |
| Main Target | Solder paste | Finished hidden joints |
| Measures Volume | Paste volume | Joint or void image analysis |
| Finds Printing Defects | Yes | Indirectly |
| Finds Hidden Bridges | No | Yes |
| Prevents Defects Early | Yes | Detects completed defects |
Solder paste inspection can identify process problems before they become hidden solder defects.
X-Ray Inspection Vs Electrical Testing
X-ray examines physical structures. Electrical testing verifies electrical behavior.
A solder joint may look unusual while still conducting electricity.
Another joint may look acceptable in a projection image but fail under load or movement.
Electrical methods include:
- Flying probe
- In-circuit testing
- Boundary scan
- Functional testing
- Continuity testing
- Resistance measurement
The manufacturer should use both physical and electrical evidence when product risk requires it.
Review our guide on how to test a PCB for additional testing methods.
What X-Ray Inspection Cannot Reliably Detect
X-ray provides valuable internal images, but it has limitations.
It may not reliably identify:
- Component electrical performance
- Incorrect firmware
- Every head-in-pillow defect
- Surface contamination
- Flux chemistry problems
- Weak metallurgical bonding
- Every microcrack
- Incorrect resistor values
- Incorrect capacitor values
- Color markings
- Complete long-term reliability
- All intermittent faults
Some defects require:
- Electrical testing
- Functional testing
- Cross-section analysis
- Dye-and-pry testing
- Microscopy
- Material analysis
- Thermal cycling
- Failure analysis
X-ray should form part of a layered quality plan.
How To Read A BGA X-Ray Image
Inspectors usually compare the complete ball array before examining individual joints.
Pattern Alignment
The ball pattern should align with the expected package geometry.
Joint Consistency
Large differences in ball size or shape may indicate irregular collapse, solder volume, or alignment.
Missing Locations
An unexpectedly light or empty position may indicate a missing ball or insufficient solder.
Connected Features
Solder joining two adjacent balls may indicate bridging.
Internal Light Areas
Depending on display settings, voids may appear lighter than the surrounding solder.
Edge And Center Comparison
Thermal conditions may differ between the center and perimeter of a large BGA.
Inspectors should consider package and PCB warpage.
Image interpretation should follow approved reference examples and defect criteria.
X-Ray Image Quality Factors
An inspection result depends on the quality of the captured image.
Important variables include:
- Tube voltage
- Tube current
- Exposure time
- Focal spot
- Magnification
- Detector resolution
- Board thickness
- Viewing angle
- Image contrast
- Noise reduction
- Software filtering
- Mechanical stability
Higher magnification does not automatically create better results.
Excessive magnification may reduce the field of view and increase sensitivity to movement.
The operator should optimize settings for the target feature.
X-Ray Inspection Sampling Vs 100% Inspection
Not every project needs the same inspection quantity.
Sample Inspection
Sampling may suit:
- Stable processes
- Low-risk products
- Large production volumes
- Process audits
- First-article approval
- Periodic verification
100% Inspection
Every relevant assembly may need inspection when:
- The product has high failure consequences
- BGA quality is critical
- The process is new
- The package is difficult
- Customer requirements specify it
- Previous defects have occurred
- Production volume is manageable
The customer and manufacturer should agree on:
- Inspection quantity
- Target components
- Viewing method
- Acceptance criteria
- Required records
- Failure response
First Article X-Ray Inspection
The factory should inspect early production units before completing the batch.
First-article X-ray inspection can verify:
- BGA alignment
- QFN coverage
- Reflow quality
- Solder voiding
- Stencil performance
- Placement accuracy
- Process settings
If the inspection reveals a systematic problem, production should pause.
The factory should correct the stencil, placement program, reflow process, PCB design, or material condition before continuing.
Repairing one first article does not correct an unchanged production process.
X-Ray Inspection During Process Validation
Process validation determines whether the assembly method can repeatedly create acceptable hidden joints.
The manufacturer may compare X-ray results across:
- Multiple boards
- Different panel positions
- Different oven loads
- Different material lots
- Production shifts
- Reflow profiles
- Stencil revisions
The data can reveal patterns.
For example, excessive voiding in the same panel position may indicate support, printing, heating, or panel-design variation.
Common BGA X-Ray Defects And Causes
| X-Ray Finding | Possible Cause |
|---|---|
| Missing Ball | Component damage, handling, or package defect |
| Bridge | Excessive paste, placement offset, or solder movement |
| Misalignment | Placement error, pad mismatch, or component movement |
| Uneven Collapse | Thermal gradient, warpage, or paste inconsistency |
| Large Voids | Paste, reflow, contamination, or outgassing |
| Possible Open | Warpage, oxidation, insufficient heat, or head-in-pillow |
| Excessive Solder | Stencil aperture or paste-printing problem |
| Insufficient Solder | Poor paste transfer, clogged aperture, or solder loss |
A finding should lead to root-cause investigation.
Simply sorting failed boards does not stabilize the process.
How PCB Design Affects X-Ray Results
PCB design influences both solder-joint quality and inspectability.
Overlapping Components
Components mounted directly opposite each other can overlap in 2D images.
This overlap makes analysis more difficult.
Copper Planes
Large copper areas absorb X-rays and reduce contrast.
Via-In-Pad Design
Open vias can remove solder from thermal pads.
Filled and capped vias may create more controlled joints.
Pad Geometry
Incorrect pad dimensions affect solder volume, collapse, and wetting.
Thermal Balance
Unequal copper distribution creates temperature differences during reflow.
Test Access
Electrical test points support confirmation of questionable X-ray results.
A proper PCB DFM checklist should review BGA footprints, thermal pads, vias, stencil apertures, and inspection access.
How Stencil Design Affects Hidden Joints
Stencil thickness and aperture geometry determine solder paste volume.
Important factors include:
- Aperture size
- Aperture shape
- Area ratio
- Paste-release efficiency
- Window-pane patterns
- Stencil thickness
- Step-stencil design
For a QFN thermal pad, a window-pane aperture can divide one large deposit into smaller deposits.
This design may improve paste release and provide escape paths for flux gases.
However, the correct pattern depends on package design, via structure, paste, and thermal requirements.
How Reflow Soldering Affects X-Ray Defects
An unsuitable thermal profile can contribute to:
- Excessive voiding
- Incomplete BGA collapse
- Head-in-pillow
- Non-wetting
- Component warpage
- Solder bridging
- Package damage
The process must balance:
- Ramp rate
- Soak
- Time above liquidus
- Peak temperature
- Cooling
- Temperature uniformity
X-ray shows the result of the process. It does not control the process itself.
Can X-Ray Inspect PCB Vias And Internal Layers?
X-ray systems can inspect certain PCB fabrication features.
Possible applications include:
- Filled vias
- Via-in-pad structures
- Hole position
- Internal registration
- Foreign material
- Broken connections
- Embedded structures
However, standard 2D PCBA X-ray may not provide enough detail for every internal PCB defect.
Microsection analysis, electrical testing, ultrasonic inspection, or CT may provide better evidence.
The required method depends on feature size and failure type.
Can X-Ray Help Identify Counterfeit Components?
X-ray can compare internal component structures without opening the package.
Possible comparison features include:
- Die size
- Die position
- Bond-wire count
- Bond-wire pattern
- Lead-frame shape
- Internal cavities
- Package construction
Differences from an approved reference may indicate that further investigation is required.
However, X-ray alone does not prove authenticity.
A counterfeit-risk evaluation may also require traceability, visual inspection, marking analysis, XRF, decapsulation, or electrical testing.
X-Ray Inspection Safety
Industrial X-ray systems produce ionizing radiation.
Equipment must include suitable shielding, controls, warning indicators, and interlocks.
Operators should follow:
- Equipment instructions
- Local regulations
- Facility safety procedures
- Access controls
- Maintenance requirements
- Training requirements
- Radiation monitoring rules where applicable
The U.S. FDA explains that cabinet X-ray systems use shielded enclosures and interlocks to support safe industrial inspection.
Requirements vary by country and equipment type.
How X-Ray Inspection Affects PCB Assembly Cost
X-ray inspection cost depends on:
- Number of boards
- Number of target components
- Package type
- Inspection coverage
- 2D or 3D method
- Magnification
- Scan time
- Programming
- Reporting
- Engineering review
- Failure analysis
A simple 2D production scan costs less than detailed CT reconstruction.
The quotation should state whether X-ray covers:
- Every board
- Selected samples
- Every hidden component
- Only specified packages
- First articles only
- Failed products only
Review our PCB assembly cost guide for a broader explanation of inspection and production costs.
What Should An X-Ray Inspection Report Include?
A useful report should identify:
- Customer
- Project
- PCBA part number
- PCB revision
- Assembly revision
- Work order
- Serial number
- Inspection date
- Equipment
- Operator
- Target component
- Reference designator
- Viewing angle
- Magnification
- Acceptance criteria
- Measurement method
- Image
- Result
- Finding description
Void reports should also define:
- Joint boundary
- Detection threshold
- Area or volume calculation
- Maximum individual void
- Total void measurement
- Pass or fail status
Images without identification or criteria provide limited traceability.
How To Specify X-Ray Inspection In An RFQ
Tell the PCBA manufacturer:
- Which components require inspection
- Whether inspection covers every board
- Whether 2D, oblique, or CT imaging is required
- Applicable acceptance standard
- Customer-specific void limits
- Required viewing angles
- Required report format
- Required image retention
- First-article hold point
- Failure response
- Reinspection requirements
Also provide:
- Gerber files
- BOM
- Assembly drawing
- Component datasheets
- Package drawings
- Stencil requirements
- Product application
- Reliability requirements
The factory cannot provide accurate scope and pricing from the phrase “X-ray required” alone.
PCB X-Ray Inspection Checklist
Planning
- Hidden components identified
- Product risk assessed
- Inspection quantity defined
- Equipment method defined
- Acceptance criteria approved
- Reporting requirements defined
Process Control
- Component storage controlled
- Solder paste controlled
- Stencil revision confirmed
- SPI completed
- Placement verified
- Reflow profile validated
Image Review
- Correct reference designator selected
- Magnification suitable
- Image contrast suitable
- Required angles captured
- Voids measured consistently
- Bridges checked
- Alignment checked
- Joint consistency reviewed
Records And Response
- Images saved
- Results linked to work order
- Failed boards separated
- Root cause investigated
- Rework controlled
- Reinspection completed
- Corrective action documented
Frequently Asked Questions About PCB X-Ray Inspection
Is X-Ray Inspection Non-Destructive?
Yes. Standard PCBA X-ray creates internal images without cutting or disassembling the board.
Does Every PCB Need X-Ray Inspection?
No. It provides the greatest value for hidden joints, high-risk assemblies, process validation, and failure analysis.
Can AOI Inspect A BGA?
AOI can check BGA presence, position, and visible edges. It cannot directly inspect the hidden solder-ball array.
Can X-Ray Detect Every BGA Open?
No. Some opens and head-in-pillow defects remain difficult to detect with standard 2D imaging.
Is 3D CT Better Than 2D X-Ray?
CT provides depth separation and volumetric information. However, it costs more and takes longer. The best method depends on inspection risk.
Does X-Ray Measure Solder Void Volume?
Standard 2D systems usually estimate projected void area. CT can estimate three-dimensional volume.
Are All Solder Voids Defects?
No. Acceptance depends on joint type, size, location, product requirements, and applicable criteria.
Can X-Ray Inspect Through-Hole Solder Joints?
Yes, although overlapping copper and component structures can complicate interpretation.
Can X-Ray Damage A PCBA?
Normal industrial inspection settings are generally designed for non-destructive examination. Sensitive technologies may require a product-specific exposure assessment.
Does A Passed X-Ray Guarantee The Board Works?
No. Electrical and functional tests must confirm product performance.
Choose Haode PCBA For X-Ray Inspection Support
Haode PCBA supports PCB fabrication, component sourcing, solder paste printing, SMT placement, reflow soldering, inspection, programming, and testing.
Our team can review your BGA, QFN, LGA, CSP, thermal-pad, and hidden-joint inspection requirements before production.
Our PCBA full-process quality control guide explains how inspection methods work together throughout manufacturing.
For an accurate PCB X-Ray Inspection quotation, send your Gerber files, BOM, assembly drawing, component datasheets, order quantity, acceptance criteria, and reporting requirements.
