Many important semiconductor package defects are completely hidden from optical inspection. A solder ball can contain a void. A bond wire can be broken inside an encapsulated package. A die-attach layer may contain large areas of voiding. A package substrate can contain an open trace, and an advanced package may have a defective microbump buried beneath multiple dies and interconnect layers. Opening the package immediately is not always the best approach. Destructive preparation can remove evidence, introduce artifacts or make it impossible to determine whether a defect existed before the analysis began. This is where X-ray inspection becomes extremely valuable. X-ray imaging allows failure analysts to examine internal structures of semiconductor devices, packages and electronic assemblies with little or no physical modification of the sample.
Typical semiconductor X-ray applications include:
X-ray inspection uses high-energy electromagnetic radiation to create images of structures hidden inside a semiconductor device or electronic package.
In a conventional projection system:
X-ray source
↓
Semiconductor device or package
↓
X-rays pass through the sample
↓
Detector records transmitted X-rays
↓
Internal structures become visible
Different materials absorb X-rays differently. Dense or high-atomic-number materials generally attenuate the X-ray beam more strongly than lighter materials. This difference creates contrast between structures such as metals, silicon, solder and organic package materials.
One of the biggest advantages of X-ray inspection is that it can provide information before the package is destructively opened.
X-rays can penetrate relatively thick semiconductor and package structures, allowing complete packages and circuit boards to be examined internally.
The device normally requires little sample preparation or modification.
This allows X-ray analysis to be performed early in the failure-analysis sequence.
Unlike electron-beam analysis, X-ray imaging does not suffer from the electrical charging artifacts that can affect insulating materials during SEM inspection.
When combined with computed tomography, X-ray imaging can reconstruct internal structures in three dimensions.
Different materials can often be distinguished according to their X-ray attenuation.
These characteristics make X-ray particularly attractive for package failure analysis, where the defect is frequently hidden inside an opaque structure.
A good semiconductor failure-analysis sequence generally moves from the least invasive methods toward progressively more destructive techniques. A typical package investigation might follow:
Failure history review
↓
External optical inspection
↓
Electrical verification
↓
X-ray inspection
↓
Scanning Acoustic Microscopy if required
↓
Electrical fault localization
↓
Controlled package opening
↓
SEM / FIB / physical analysis
↓
Root cause
Non-destructive investigations should ideally be performed on the as-received device before destructive analysis begins. This preserves a record of the original sample condition and reduces the risk of mistaking preparation damage for the original failure.
Most conventional electronic-device X-ray systems use projection imaging. The sample is positioned between the X-ray source and detector. The detector records the transmitted X-ray intensity after the beam has passed through the sample.
The resulting image is effectively a shadow image representing variations in X-ray absorption. Image performance is influenced by factors including:
Unlike an optical microscope, a projection X-ray system can obtain magnification by changing the physical position of the sample. Moving the device closer to the X-ray source increases geometric magnification.
A simplified geometry is:
X-ray source → sample → detector
If the sample is moved closer to the source while the detector remains farther away, the projected image becomes larger. However, magnification alone does not guarantee greater useful resolution.
The source spot size and detector characteristics also determine how much fine detail can actually be resolved. A highly magnified image is therefore not necessarily a high-resolution image.
2D X-ray imaging is one of the most widely used forms of semiconductor package inspection. The device is placed in the beam and a projection image is collected.
The sample can often be:
This allows the failure analyst to examine the same internal structures from different directions. 2D projection systems can provide relatively fast inspection, making them particularly useful when the approximate defect location is already known.
X-rays can reveal areas where the die-attach material is absent or nonuniform.
Void size, distribution and overall coverage may often be evaluated.
High-magnification X-ray images can reveal damaged or disconnected bond wires inside encapsulated packages.
This is particularly useful because optical inspection cannot see through the molding compound.
Angled or oblique X-ray imaging can help identify through-hole connections with incomplete soldering or insufficient fill.
X-ray imaging can reveal insufficient or completely absent solder at an interconnect.
Unexpected conductive material connecting neighboring structures may be visible as an abnormal high-density feature.
Wire-bond failures are a common application for package X-ray inspection.
Possible problems include:
Because the wire is surrounded by package material, these defects may not be visible from outside the device. X-ray provides a way to check bond-wire geometry without decapsulation.
However, X-ray may show that a wire is physically abnormal without determining why the wire failed. Further analysis may still be required to determine whether the mechanism was:
X-ray helps locate and document the defect. Root-cause analysis still requires correlation with other evidence.
The die-attach layer provides mechanical attachment and, in many semiconductor products, an important thermal path between the die and package.
Voids within this layer may affect:
X-ray can reveal voids because the missing material changes the amount of X-ray attenuation. The result may appear as areas of different contrast underneath the die.
Ball Grid Array packages are particularly well suited to X-ray inspection because the solder joints are located underneath the component. These joints cannot normally be inspected directly using conventional optical microscopy.
X-ray may reveal defects such as:
For simple BGA structures, 2D X-ray may provide sufficient information. For complex packages, however, structures from different depths overlap in the same projection. This leads to one of the most important limitations of 2D X-ray.
A 2D X-ray image compresses a three-dimensional package into a single projection.
Consider a complex device containing:
All of these structures may appear on top of one another in the same image. A defect could therefore be hidden behind another high-density feature. In complex multilevel packages, 3D X-ray imaging can separate structures according to depth, making defect interpretation significantly easier.
Computed Tomography (CT) creates a three-dimensional representation of the internal structure of the sample. Rather than collecting only one projection, the system acquires many X-ray images while the sample is rotated.
The basic sequence is:
Acquire X-ray projection
↓
Rotate sample
↓
Acquire another projection
↓
Repeat over many angles
↓
Computer reconstruction
↓
3D dataset
The reconstructed data can then be examined in ways that are impossible using conventional 2D imaging.
One of the most useful features of X-ray CT is virtual cross-sectioning. Once a three-dimensional dataset has been generated, software can display slices through different positions inside the sample.
For example:
Complete package
↓
Select depth
↓
View BGA layer
↓
Select another depth
↓
View substrate traces
↓
Select another depth
↓
View die / microbumps
No physical cut is required. This provides a major advantage in failure analysis because the analyst can determine where a defect is located before committing to destructive cross-sectioning. In some investigations, virtual cross-sections may provide enough information that a physical cross-section is not required.
Consider two neighboring BGA joints inside a complex package. A 3D dataset may reveal that one solder joint is open even though the solder appears normal in a conventional top-down projection.
For example, solder may have wetted the package pad but failed to form a complete interface with the solder ball. Such defects can be difficult to identify reliably in 2D because surrounding structures obscure the interface.
This illustrates one of the strongest applications of 3D X-ray:
determining the actual interface condition between structures rather than simply observing their projected shape.
Virtual cross-sectioning can also expose a crack between a solder ball and an underlying via or interconnect structure. A small defect buried among several other package features may be difficult to separate in a conventional projection.
By examining the reconstructed CT data at the correct depth, the analyst can isolate the suspect interface. This type of capability becomes increasingly important as package structures become more complex.
Modern semiconductor packaging increasingly combines many different structures within a single device.
Examples include:
As complexity increases, the distinction between die-level, package-level and board-level failure analysis becomes less clear. Hidden defects in highly integrated packages may be buried beneath multiple material layers and interconnections, making non-destructive localization increasingly important before physical analysis begins.
3D X-ray microscopy can be especially valuable for advanced packaging because a reconstructed volume can reveal defects deep inside structures that are difficult to access physically.
Potential structures include:
A useful strategy can therefore be:
Electrical fault identified
↓
Non-destructive X-ray localization
↓
3D dataset generated
↓
Virtual cross-sections identify defect coordinates
↓
Targeted physical analysis only if needed
This can significantly reduce the amount of destructive preparation required.
Micro-CT is increasingly useful for complex semiconductor packages. Modern micro-CT workflows can automate:
Micro-CT is particularly suited to situations where the package contains multiple overlapping structures that make 2D analysis ambiguous.
For smaller structures, higher-resolution X-ray approaches can extend inspection toward features that are difficult to characterize using conventional projection systems.
These methods can be useful for:
The trade-off is typically that higher-resolution 3D analysis requires:
The correct X-ray technique should therefore be selected according to the scale of the suspected defect.
| X-Ray Technique | Main Advantage | Typical Use |
|---|---|---|
| 2D Projection X-Ray | Fast inspection and real-time manipulation | Bond wires, die attach, simple solder joints, obvious package defects |
| Oblique 2D X-Ray | Helps separate structures by changing viewing angle | Through-hole joints, wire bonds, solder structures |
| Micro-CT | Three-dimensional imaging and virtual cross-sections | BGA, flip-chip, package substrates, multilevel packages |
| High-Resolution 3D X-Ray | Analysis of smaller internal features | Microbumps, advanced packaging and fine interconnect structures |
The general principle is: Use 2D first when it provides the answer.
Move to 3D when overlapping structures or defect complexity prevent reliable interpretation.
Resolution is one of the most important considerations when selecting an X-ray technique.
For projection systems, important factors include:
There is therefore no single X-ray resolution that applies to every semiconductor package. The achievable result depends on the complete imaging geometry.
High-resolution imaging often involves a trade-off. To examine a very small package feature in greater detail, the Region of Interest may occupy most of the detector.
This reduces the amount of the overall device visible in one scan. A practical failure-analysis workflow may therefore use:
Low magnification
to locate the region
followed by
High magnification
to examine the suspected defect.
3D X-ray CT requires many individual projection images. Higher-quality reconstruction may require:
As a result, high-resolution CT can take substantially longer than a simple 2D inspection. The failure analyst should therefore not automatically select the highest available resolution.
The more useful question is: What resolution is required to answer the failure-analysis question?
X-ray data can also be correlated with package or board CAD information. The X-ray image shows the physical device. The CAD data shows the intended design.
When the two are aligned, the analyst can correlate a suspicious feature with:
This can help connect X-ray evidence with electrical-test data and package design information.
X-ray inspection is also highly valuable at board level. Modern electronic boards may contain:
Optical inspection cannot access many of these structures. Board-level applications may include:
X-ray becomes much more effective when the electrical failure signature is already understood.
Possible X-ray targets include:
Possible targets include:
Possible targets include:
The electrical signature should therefore be verified before deciding which internal structures to inspect.
A major practical use of X-ray is to guide subsequent physical cross-sectioning. Suppose CT reveals a crack inside one solder joint. Rather than sectioning the package randomly, the analyst can determine:
The physical cut can then target the actual defect. This reduces the chance that a cross-section will simply miss the failure.
X-ray and Scanning Acoustic Microscopy (SAM) are both non-destructive package-analysis techniques, but they detect different physical characteristics.
Primarily sensitive to:
Particularly useful for:
Primarily detects changes at material interfaces through differences in acoustic properties. Particularly useful for:
The two techniques are therefore complementary. A defect that is difficult to detect with X-ray may be obvious in SAM, and vice versa.
Optical microscopy is normally the first inspection method because it is:
But it can only observe exposed or optically accessible structures. X-ray’s main advantage is its ability to see through opaque package material. A good workflow therefore begins with:
Optical inspection
then
X-ray if hidden structures must be examined.
X-ray and SEM address different parts of the investigation.
A common sequence is:
X-ray locates defect
↓
Cross-section exposes defect
↓
SEM characterizes defect
Focused Ion Beam is not normally used for broad non-destructive inspection.
Instead:
X-ray identifies the buried Region of Interest
followed by:
FIB precisely exposes the selected structure
This combination can be particularly useful for:
| Suspected Defect | Typical X-Ray Approach | What May Be Observed |
|---|---|---|
| Die-attach void | 2D X-ray | Void size, distribution and coverage |
| Broken bond wire | High-magnification 2D X-ray | Discontinuity or abnormal wire geometry |
| Wire sweep | 2D / angled X-ray | Displacement of wire loops |
| Missing solder | 2D X-ray | Lack of expected solder material |
| Solder bridge | 2D X-ray | Conductive connection between neighboring joints |
| BGA open | 3D CT | Separation at pad / solder-ball interface |
| Solder void | 2D or 3D X-ray | Internal low-density region |
| Package crack | 3D CT where suitable | Crack through internal package structure |
| Microbump defect | High-resolution 3D X-ray | Open, void or abnormal geometry |
| Substrate interconnect defect | 3D CT | Open, displacement or bridge |
| Through-hole defect | Angled 2D X-ray | Incomplete solder fill or connection |
X-ray is powerful, but not every semiconductor defect is visible.
If two materials have similar X-ray attenuation, distinguishing their interface can be difficult.
Multiple layers can obscure one another.
3D imaging may be required.
Some semiconductor-level defects remain below the practical resolution of package-level X-ray systems.
A transistor can fail electrically without creating any X-ray-visible physical difference.
An unusual image feature does not automatically prove root cause.
The X-ray evidence must be correlated with the electrical failure signature and other analysis.
X-ray inspection is generally considered much less invasive than physically opening or cross-sectioning a semiconductor package.
Most package-level X-ray inspections can be performed while preserving the device for subsequent analysis.
However, X-rays are ionizing radiation.
Exposure conditions should therefore be considered when analyzing particularly radiation-sensitive devices.
One reason to perform X-ray early is that it can document the sample before destructive analysis creates artifacts.
Possible artifacts from later preparation can include:
If a defect is already visible in the initial X-ray data, the analyst has strong evidence that it existed before destructive sample preparation.
This can be extremely important in root-cause investigations.
Consider a semiconductor package with an intermittent open circuit.
Electrical testing confirms the open.
No external abnormality is found.
No obvious defect is visible because the package contains several overlapping interconnect layers.
Virtual cross-sections identify separation at one BGA interface.
The package is sectioned through the exact suspect joint.
The interface morphology is examined.
The X-ray, electrical and physical evidence are correlated with assembly history.
This illustrates the role of X-ray: Non-destructively locate first, destructively confirm only when required.
Consider a device failing with a resistive short between neighboring pins.
Confirms the short.
Inspection reveals unexpected dense material between neighboring interconnect structures.
Thermal or magnetic-current imaging may further confirm the location.
Physical analysis identifies the conductive defect.
The final root cause could potentially involve:
X-ray can reveal the physical abnormality but must still be combined with material and process evidence.
Consider a board showing an electrical short somewhere along a power rail containing many components. Thermal analysis can first narrow the suspected area to one component.
High-magnification X-ray analysis may then reveal a fine crack within that component. The X-ray information can subsequently be used to choose the optimum orientation for destructive cross-sectioning.
Physical analysis can then determine whether the crack and associated damage explain the electrical failure. This illustrates how multiple non-destructive techniques can work together before destructive physical analysis.
Modern semiconductor failure analysis should preserve the sample for as long as practical. This is especially important when only one failed device is available. A useful principle is:
Document before destroying.
X-ray can help determine:
This improves the probability of finding the true defect while reducing unnecessary sample damage.
Semiconductor X-ray inspection is a non-destructive technique that uses X-rays to visualize structures hidden inside IC packages, electronic assemblies and circuit boards.
Typical defects include:
The exact detectability depends on defect size, material contrast, sample geometry and X-ray system resolution.
Normally no.
One of its main advantages is that packaged devices can be inspected internally without opening or physically sectioning the package.
2D X-ray creates a projection through the complete device.
3D CT collects projections from many angles and reconstructs the internal structure as a volume.
3D imaging is particularly useful when multiple structures overlap in a conventional projection.
Micro-CT is X-ray computed tomography optimized for microscopic structures.
It is commonly used to inspect electronic packages where three-dimensional separation of internal features is required.
Virtual cross-sectioning uses a reconstructed 3D X-ray dataset to display slices through the interior of a package without physically cutting it.
This can help identify the correct location for subsequent destructive analysis.
Yes.
BGA inspection is an important X-ray application because the solder joints are hidden beneath the package.
Yes, provided the wire and defect can be resolved with sufficient contrast and imaging resolution.
Some voids or separations may produce X-ray contrast, but Scanning Acoustic Microscopy is often more sensitive to delamination and small gaps at package interfaces.
Sometimes.
3D X-ray and virtual cross-sectioning can provide enough information to avoid destructive preparation in some investigations.
However, SEM, FIB or materials analysis may still be required when the precise failure mechanism must be established.
Not by itself.
X-ray identifies internal structures and potential physical defects.
Root cause requires correlation with electrical testing, failure history, process information and, where necessary, additional physical or materials analysis.
The real value of X-ray inspection is not simply the ability to “see inside” an IC. Its importance comes from where it sits in the analytical sequence.
A semiconductor failure-analysis investigation can progressively narrow the problem from:
Complete device
↓
Package
↓
Internal structure
↓
Specific interconnect
↓
Physical defect
↓
Failure mechanism
↓
Root cause
X-ray is particularly effective in the first half of this process because it can identify internal abnormalities while the package remains largely intact.
For straightforward defects, 2D X-ray may provide the answer quickly.
For advanced packages containing many overlapping structures, 3D X-ray CT can provide virtual cross-sections and isolate defects that would be extremely difficult to identify in conventional projections.
As semiconductor packages become increasingly three-dimensional, the value of non-destructive internal imaging also increases.
The best failure-analysis strategy is therefore not:
Open the package and look for the defect.
It is:
Characterize the electrical failure → inspect non-destructively → localize the defect → then perform targeted physical analysis only where necessary.
If you have a failed semiconductor package, IC, BGA, WLCSP or advanced package that requires X-ray inspection, AnySilicon can help connect you with semiconductor failure-analysis providers with the appropriate capabilities.
Typical services may include:
