Scanning Acoustic Microscopy (SAM) is a non-destructive imaging technique widely used in semiconductor failure analysis to inspect structures and interfaces hidden inside IC packages. Unlike optical microscopy, which relies on visible light, or X-ray inspection, which relies primarily on differences in material density and X-ray absorption, SAM uses high-frequency acoustic waves. This makes acoustic microscopy particularly sensitive to defects involving separation between materials.
Typical applications include detecting:
Because SAM can inspect these structures without cutting or opening the package, it is often used early in the semiconductor failure-analysis process.
Scanning Acoustic Microscopy uses a focused ultrasonic transducer to send high-frequency acoustic pulses into a semiconductor device or package. When the acoustic wave reaches an interface between two different materials, part of the acoustic energy is reflected while the remainder may continue deeper into the structure. The amount and characteristics of the reflected signal depend on the materials at that interface. By scanning the transducer across the sample and measuring these reflections, the system builds an image of internal package structures.
A simplified process is:
Acoustic pulse generated
↓
Pulse enters semiconductor package
↓
Wave reaches internal material interface
↓
Part of signal reflects
↓
Reflected signal returns to transducer
↓
Signal amplitude, polarity and timing are analyzed
↓
Internal image is created
This ability to image internal interfaces makes SAM extremely useful for semiconductor package failure analysis.
The key to acoustic microscopy is the difference in acoustic impedance between materials.
Acoustic impedance depends on properties such as:
When an acoustic wave crosses an interface between two materials with different acoustic impedances, part of the wave is reflected. If two solid materials are properly bonded, part of the acoustic energy usually passes through the interface.
But if a very small air gap develops between them because of delamination, the situation changes dramatically. The acoustic impedance difference between a solid and air is extremely large. As a result, most of the acoustic signal is reflected.
This produces a strong indication of:
Even an extremely thin separation may therefore produce significant acoustic contrast. This is one reason SAM can detect defects that may be difficult to identify using X-ray inspection.
The term C-SAM is commonly used to describe C-mode Scanning Acoustic Microscopy. A C-scan creates an image of a selected horizontal plane inside the semiconductor package. The system selects acoustic signals arriving from a particular time window corresponding approximately to a particular depth. The transducer then scans across the X-Y area of the device. The resulting signal is converted into an image.
This allows the analyst to examine specific interfaces such as:
C-scan imaging is one of the most commonly used SAM modes in semiconductor failure-analysis reports.
Acoustic microscopy can display information in several different ways.
An A-scan shows the reflected acoustic signal at one X-Y location as a function of time.
The horizontal axis represents the return time of the signal.
The vertical axis represents signal amplitude.
An A-scan can help the analyst determine:
If an unexpected feature appears in a C-SAM image, comparing the A-scan over the suspect feature with an A-scan from a known-good region can provide valuable information.
A B-scan provides a side-view representation through the sample.
It can be considered a type of virtual cross-section.
A series of A-scans is collected along a line across the device and displayed with:
This allows the analyst to visualize how internal layers change with depth.
B-scans can be useful for:
The C-scan creates the familiar top-down acoustic image.
A specific depth range is selected from the A-scan and the transducer scans the X-Y area of the package.
The result ideally represents one internal plane of the package.
Together:
A-scan → signal information
B-scan → depth / virtual cross-section
C-scan → planar internal image
The three modes provide complementary information.
One of the most common acoustic microscopy configurations is pulse-echo SAM, sometimes abbreviated PE-SAM.
The same transducer:
This provides several advantages:
For many semiconductor-package investigations, pulse-echo SAM is the first acoustic inspection method considered.
Another method is through-transmission SAM, sometimes abbreviated TT-SAM.
In this configuration:
Transmitter
↓
Sample
↓
Receiver
The acoustic wave passes through the complete package rather than reflecting back to the same transducer. Through-transmission can be useful when complicated package structures create interfering echoes in pulse-echo mode.
This can be particularly relevant for:
BGA packages
Multilayer substrates
Lidded packages
Complex stacked structures
However, through-transmission generally provides information from all interfaces along the acoustic path rather than isolating one particular depth. This makes interpretation different from pulse-echo analysis.
One particularly important feature of pulse-echo SAM is signal polarity analysis. The direction of the acoustic-impedance change at an interface affects the polarity of the reflected signal. If an originally bonded interface becomes separated by air, the reflected signal can undergo a polarity reversal. This provides powerful information for identifying delamination. Importantly, not every delamination creates an obvious change in image brightness.
Looking only at reflected amplitude may therefore miss some defects.
Analyzing both:
Amplitude
and
Signal polarity
provides a more reliable assessment of package interfaces.
The time required for an acoustic pulse to travel into the sample and return also contains useful information. Because acoustic velocity in a material is known or can be estimated, the return time can provide information about depth.
This is known as time-of-flight analysis.
Time-of-flight imaging can help reveal:
This makes SAM more than simply a defect-detection technique. It can also provide information about the internal construction of a semiconductor package.
Selecting the correct acoustic transducer is critical. One of the most important parameters is frequency.
In general:
Higher frequency → shorter wavelength → better resolution
But there is a trade-off:
Higher frequency → greater attenuation → lower penetration
This means that the highest-frequency transducer is not automatically the best choice. For a thick molded package, a lower-frequency transducer may be required to penetrate sufficiently deep into the package. For small structures such as flip-chip bumps, a much higher frequency may be necessary to obtain adequate resolution.
The appropriate frequency therefore depends on:
This trade-off is fundamental to acoustic microscopy. Consider two situations.
The defect is deep beneath several millimeters of package material. A very high-frequency acoustic wave may lose too much energy before reaching the target. A lower-frequency transducer may provide better penetration.
The target is a small bump or interconnect close to the exposed silicon backside. Here, much higher frequencies can be used because the required penetration depth is smaller.
The improved resolution can reveal much finer structures. The failure analyst must therefore balance:
Resolution
against
Penetration depth
rather than simply maximizing frequency.
Plastic mold compound can strongly attenuate acoustic energy.
The amount of attenuation depends on factors including:
Attenuation typically becomes stronger as frequency increases.
This can reduce both:
For deeply buried structures, the best achievable resolution may therefore be substantially worse than the theoretical resolution of the transducer.
Delamination detection is one of the strongest applications of acoustic microscopy. Delamination can occur at several package interfaces, including:
Possible causes include:
Because a delaminated region contains a small gap, often filled with air or moisture, it can produce a strong acoustic reflection.
Plastic semiconductor packages can absorb moisture from the environment. During high-temperature processing, rapid expansion of trapped moisture can place considerable stress on package interfaces.
Potential consequences include:
SAM is especially valuable because the same device can be inspected:
Before reliability stress
↓
During intermediate intervals
↓
After stress
The growth of a delaminated area can therefore be tracked without destroying the sample.
This is a major advantage during package-reliability investigations.
The die-attach layer connects the semiconductor die to the package structure.
Depending on the product, it may provide:
Problems in die attach can include:
Pulse-echo SAM can be highly effective for evaluating adhesion across the die-attach area. A well-bonded region allows more acoustic energy to pass into the next material.
A disbonded area containing an air gap reflects much more of the acoustic signal. This difference can make areas of poor adhesion clearly visible.
X-ray and SAM respond to different physical properties. X-ray imaging primarily depends on differences in:
SAM is highly sensitive to:
Consider an extremely thin delamination. The gap may be so thin that it produces almost no meaningful difference in X-ray absorption. But acoustically, replacing a bonded solid-solid interface with a solid-air interface produces a major change in reflection. The defect may therefore appear clearly in SAM while remaining difficult to detect with conventional X-ray.
The two techniques should usually be viewed as complementary.
| Technique | Particularly Useful For |
|---|---|
| SAM / C-SAM | Delamination, cracks, interface separation, die-attach adhesion, underfill |
| X-Ray / 3D CT | Solder geometry, bond wires, BGA joints, voids, metal structures, package geometry |
For example:
Suspected BGA solder void
→ X-ray may be the better first technique.
Suspected mold-compound delamination
→ SAM may be the better first technique.
For complex package failures, both techniques may be used before destructive analysis.
Ball Grid Array packages introduce additional acoustic-analysis challenges. Their substrates may contain many thin layers, which produce multiple closely spaced acoustic reflections.
These signals can overlap and interfere with each other. As a result, conventional pulse-echo analysis may become difficult to interpret.
Possible approaches include:
SAM can still provide valuable information about:
but the scan mode should be chosen according to package construction.
Flip-chip packages are another important acoustic-microscopy application. In flip-chip devices, the active side of the die faces the substrate and is connected using an array of bumps. Underfill is normally present between the die and substrate.
High-frequency acoustic inspection can investigate:
High-frequency pulse-echo SAM can be particularly powerful when inspecting from the silicon backside.
Flip-chip devices contain many very thin layers close together. The acoustic echoes from these structures may overlap.
Therefore, selecting the correct data gate becomes extremely important. An early part of the acoustic echo packet may contain information about die interconnect structures.
A later part may contain information about:
If the analyst uses too wide a gate, signals from several different layers can combine into one confusing image. Using narrower gates at different times can separate information from different internal structures.
For advanced flip-chip analysis, correct gate placement can determine whether a defect is visible at all.
Acoustic microscopy can detect several bump-related defects.
These may include:
In some cases, characteristic acoustic patterns can distinguish defective bump interfaces from normal ones. However, SAM does not provide equally good sensitivity to every type of bump defect. For example, reduced solder volume may be difficult to detect because SAM primarily images the interface between the bump and die rather than the complete spherical bump geometry. This is an important limitation.
Advanced ICs may contain mechanically fragile low-k dielectric structures. Package assembly and thermal stresses can cause cracks or delamination within these layers. High-frequency SAM can sometimes reveal damage propagating from the die edge toward rows of flip-chip bumps. This can be particularly valuable because physical deprocessing itself might introduce additional cracks.
Non-destructive acoustic inspection therefore provides evidence that damage existed before destructive preparation.
Stacked-die packages create additional challenges.
They may contain:
Acoustically, very thin layers create echoes that occur extremely close together in time. The echoes may overlap to such an extent that individual interfaces cannot easily be separated.
This becomes particularly difficult because the velocity of sound in silicon is high. As a result, specialized processing techniques may be required for deeper layers within stacked packages.
Consider two thin silicon dies separated by a thin adhesive layer. The acoustic wave generates reflections from:
If these layers are very close together, the returned acoustic pulses may overlap. The result is not several clean peaks but a complex combined signal.
Possible techniques to improve interpretation include:
Advanced package inspection therefore requires more than simply performing a conventional C-scan.
Traditional SAM primarily analyzes the acoustic pulse in the time domain. Another approach is to examine its frequency content. This is known as frequency-domain imaging.
Different internal structures can modify the acoustic frequency spectrum in different ways. Spectral information may therefore help distinguish features that are difficult to separate using conventional amplitude imaging.
Potential benefits include improved analysis of:
Frequency-domain methods become increasingly useful as package dimensions shrink.
Conventional acoustic microscopy typically operates from relatively low MHz frequencies into several hundred MHz. For very small features, even these frequencies may not provide sufficient resolution.
GHz acoustic microscopy extends the technique into the low-gigahertz range. Because wavelength decreases as frequency increases, GHz acoustic microscopy can achieve much finer lateral resolution.
Potential applications include:
Resolution can approach the micrometer range under suitable conditions. However, the improvement comes with significant challenges.
Very-high-frequency acoustic waves are strongly attenuated. Even the water used as a coupling medium can absorb significant acoustic energy at GHz frequencies.
Therefore:
GHz SAM should therefore be viewed as a specialized high-resolution technique rather than a replacement for conventional package acoustic microscopy.
Acoustic waves do not transfer efficiently from the transducer through air into a solid sample. A liquid coupling medium, usually water, provides much better acoustic transmission. The semiconductor package is therefore typically immersed or acoustically coupled through water during scanning. This allows the acoustic pulse to travel from the transducer into the package with relatively low coupling loss. The use of water should nevertheless be considered when planning the overall failure-analysis sequence, especially for devices where moisture exposure may influence subsequent testing.
One of SAM’s biggest benefits is that it is generally non-destructive.
This is especially important when:
A useful sequence might be:
Electrical verification
↓
Optical inspection
↓
X-ray
↓
SAM / C-SAM
↓
Fault localization
↓
Targeted cross-section
↓
SEM / FIB analysis
By collecting acoustic evidence before cross-sectioning, the failure analyst can determine whether a crack or delamination existed before sample preparation.
Suppose a C-SAM image reveals a localized delamination near one corner of the die. Instead of mechanically sectioning the package without guidance, the analyst can use the acoustic image to determine:
The package can then be physically sectioned through the suspect region. This significantly improves the probability that the destructive cross-section will intersect the actual defect.
SAM and SEM provide very different information.
Provides:
Provides:
A common workflow is:
SAM identifies internal interface defect
↓
Cross-section through SAM location
↓
SEM confirms physical morphology
SAM finds where the internal separation is. SEM can help determine what the exposed defect looks like physically.
| Suspected Failure | SAM Application | Possible Finding |
|---|---|---|
| Mold-compound delamination | C-SAM | Interface separation |
| Die-attach failure | Pulse-echo SAM | Voids or loss of adhesion |
| Package crack | C-SAM / B-scan | Internal crack location |
| Underfill defect | High-frequency SAM | Delamination or void |
| Flip-chip failure | High-frequency PE-SAM | Missing/non-wet bump, bridging, cracking |
| BGA package anomaly | PE-SAM or TT-SAM | Interface or internal package defect |
| Stacked-die failure | Multi-mode SAM | Separation between die/adhesive interfaces |
| Low-k cracking | High-frequency SAM | Delamination near die edge/bump region |
| Moisture damage | C-SAM | Growth of package delamination |
| Reliability degradation | Repeated SAM scans | Change in defect size over stress cycles |
SAM is powerful, but it does not identify every semiconductor defect.
Thick mold compound may strongly attenuate high-frequency sound.
Higher frequency improves resolution but reduces penetration.
Closely spaced package layers can produce overlapping signals.
Structures close to the edge of a sample may be more difficult to image accurately.
Curved or irregular structures can reflect acoustic energy away from the transducer.
A bright acoustic feature does not automatically prove a particular root cause.
The analyst must understand the package structure and inspect the underlying A-scan.
SAM indicates acoustic changes but does not normally identify chemical composition.
Other techniques may be required for material analysis.
SAM images should not be interpreted purely by brightness.
Several factors can alter acoustic contrast:
This is why the underlying A-scan is important. When an unusual feature appears, the analyst should compare:
A-scan at defect
with
A-scan at known-good region
before concluding that a real package defect exists.
Comparing a failed package with a known-good device can significantly improve interpretation.
The analyst can compare:
Differences between equivalent areas can help distinguish actual defects from normal package construction. When submitting a device for SAM failure analysis, providing one or more known-good units can therefore be very useful.
A practical semiconductor acoustic-microscopy investigation may follow:
Understand:
Document the package before immersion or additional handling.
Choose frequency and focal characteristics according to:
Confirm:
Image the interface of interest.
Check for possible:
Examine the defect in depth.
Identify abnormal acoustic behavior.
Potentially use:
If required:
The objective is not simply to produce an acoustic image. It is to determine whether the acoustic feature explains the actual semiconductor failure.
SAM stands for Scanning Acoustic Microscopy.
It uses high-frequency acoustic waves to inspect internal semiconductor package structures non-destructively.
C-SAM generally refers to C-mode Scanning Acoustic Microscopy, where acoustic information from a selected depth is used to create a planar image of an internal package interface.
Typical defects include:
A delamination often creates an air gap.
The acoustic-impedance difference between a solid material and air causes a very strong reflection, making small interface separations highly visible acoustically.
SAM is generally considered a non-destructive technique.
This allows devices to undergo additional electrical or physical analysis after acoustic inspection.
Neither technique is universally better.
SAM is particularly sensitive to interfaces, delamination and air gaps.
X-ray is particularly useful for solder, metal structures, bond wires and package geometry.
They are frequently complementary.
Yes.
Die-attach inspection is one of the important applications of SAM because areas of poor adhesion or separation can produce strong acoustic contrast.
Yes.
High-frequency SAM can investigate flip-chip bumps, underfill, cracking and other interface defects.
Some solder defects can be detected, particularly interface problems such as incomplete wetting, broken bumps or major bridging.
However, SAM is not equally sensitive to all solder geometry problems, and X-ray may be preferable for certain solder inspections.
Yes, but stacked-die packages are challenging because many thin, closely spaced layers produce overlapping acoustic echoes.
Advanced signal-processing and multiple imaging modes may be required.
An A-scan shows acoustic amplitude versus return time at one physical location.
A C-scan uses signals from a selected time/depth range to create an X-Y image of an internal package interface.
A B-scan provides a virtual cross-sectional view showing position along the sample versus acoustic depth.
Not automatically.
SAM can reveal a physical anomaly such as delamination or a crack.
Determining the actual root cause requires correlation with the electrical failure, package design, assembly history, stress conditions and sometimes destructive physical analysis.
The strength of acoustic microscopy lies in its ability to identify hidden structural damage before the sample is destroyed.
A semiconductor package failure may begin as:
Electrical failure
↓
Possible package problem
↓
SAM identifies interface anomaly
↓
Defect location established
↓
Targeted cross-section
↓
SEM / FIB examination
↓
Failure mechanism
↓
Root cause
This is particularly valuable when investigating package delamination because destructive preparation itself can potentially create cracks or separation. By performing SAM first, the original location and extent of the defect can be documented.
As semiconductor packaging becomes increasingly three-dimensional and combines more dies, interfaces and interconnect technologies, selecting the correct acoustic frequency, imaging mode and signal-analysis method becomes increasingly important. SAM remains particularly powerful because it addresses something many other imaging techniques struggle with: very small separations between internal material interfaces.
If your semiconductor device has a suspected package delamination, die-attach defect, internal crack, underfill problem or other interface failure, AnySilicon can help identify failure analysis providers with the appropriate acoustic microscopy capabilities.
Typical requirements may include:
When requesting SAM analysis, provide information about the package type, reported electrical failure, reliability or environmental stress history, suspected defect location and whether known-good comparison samples are available.
The appropriate failure analysis provider can then be selected according to the required equipment, frequency range, package experience and analysis capability.
