Scanning Acoustic Microscopy (SAM), also known as C-SAM or ultrasonic microscopy, is a powerful non-destructive technique for revealing internal defects such as delamination, voids, and cracks inside electronic packages, components, and engineered materials. By analyzing reflected high-frequency sound waves, accredited third-party laboratories can pinpoint failure mechanisms at specific depths without destructive cross-sectioning. This article explains the principle, sample types, test items, methods, and applications of acoustic scanning internal failure analysis.
Technical Overview
Scanning Acoustic Microscopy (SAM) is a non-destructive failure analysis technique that uses a focused piezoelectric ultrasonic transducer to image the internal structure of optically opaque samples. During operation, the specimen is immersed in a bath of deionized water that acts as a coupling medium, while the transducer raster-scans across the surface and fires short acoustic pulses—typically in the range of 10 MHz to more than 300 MHz—into the material.
When an acoustic pulse encounters an interface between two materials with different acoustic impedances (Z, the product of density and sound velocity), a portion of the energy is reflected back to the transducer. Because air has an acoustic impedance close to zero, even an extremely thin air gap reflects virtually all of the incident energy. This makes SAM exceptionally sensitive to internal separations such as delamination, cracks, and voids, including gaps far thinner than the ultrasound wavelength. The time of flight of each echo encodes the depth of the reflecting interface, allowing both planar and depth-resolved defect imaging.
Standardized practice for acoustic inspection of electronic packages is guided by documents such as JEDEC J-STD-035, IPC/JEDEC J-STD-020, MIL-STD-883 Method 2030, and the IEC 60749 series.
- Fully non-destructive examination of internal interfaces and hidden layers
- Depth discrimination through time-of-flight echo gating
- High sensitivity to air gaps, delamination, cracks, and voids
- Compatibility with before-and-after stress comparison studies
Testing Samples
Plastic-Encapsulated IC Packages
Surface-mount and through-hole packages such as QFP, QFN, SOP, DIP, BGA, LGA, and CSP are the most common samples. Typical inspection targets include die-face delamination, lead frame and die pad delamination, die attach voids, die cracks, and moisture-induced popcorn cracking of the mold compound.
Power Devices and Modules
IGBT modules, MOSFETs, power diodes, and modules built on direct-bonded copper (DBC) substrates are examined for solder layer voids, sintered-silver bond quality, brazing defects, and ceramic substrate cracking. High current densities make even small bond voids a significant thermal and reliability risk, so quantitative void mapping is frequently required.
Advanced Packaging Structures
Flip-chip devices, wafer-level packages, fan-out packages, 2.5D/3D stacked configurations, and system-in-package assemblies are inspected for underfill voids, bump anomalies, through-silicon via delamination, and molding compound defects at very fine geometries.
Boards, Components, and Bulk Materials
Printed circuit board laminates, ceramic capacitors, LED packages, MEMS and sensor devices, sealed ceramic packages, composites, and metal-to-metal bonded or clad materials can also be evaluated for internal layer integrity and bonding uniformity.
Testing Items
Acoustic scanning internal failure analysis covers a comprehensive range of internal defect modes and material integrity checks, including:
- Die attach voids, missing die attach, and die tilt
- Delamination between the die, die pad, lead frame, or substrate and the molding compound
- Die cracking, substrate cracking, and package popcorn cracks
- Voids in solder layers, sintered layers, thermal interface materials, and underfill
- Brazed joint, weld, and diffusion bond integrity
- Seal and lid attachment quality in ceramic and metal packages
- Layer thickness measurement based on time-of-flight calculations
- Internal anomalies screening related to counterfeit or remarked components
Testing Methods
Sample Preparation and Coupling
Samples are cleaned, dried, and secured on fixtures inside a deionized water tank. Water provides the acoustic coupling path between the transducer and the part. For moisture-sensitive devices, immersion time is controlled and baking may be specified before and after testing to avoid altering the moisture sensitivity level of the component.
Transducer Frequency Selection
Frequency is chosen according to sample thickness, layer depth, and required resolution. Lower frequencies (approximately 10 to 30 MHz) penetrate thick or highly attenuating structures, while higher frequencies (100 MHz and above) resolve thin layers and fine features at the cost of penetration depth. Multiple frequencies are often combined for a complete evaluation.
Reflection Mode: A-Scan, B-Scan, and C-Scan
In pulse-echo reflection mode, echoes returning from different depths are displayed as an A-scan waveform at each point. An electronic gate isolates the interface of interest, and amplitude and phase data are compiled into images. A C-scan provides a planar view of a selected depth, a B-scan provides a cross-sectional profile along a line, and multi-gate scanning captures several interfaces in a single pass. Phase inversion of the reflected signal is a hallmark indicator of an air gap and is typically rendered as a distinct color contrast, such as red against a normal background.
Transmission Mode (THRU-Scan)
In through-transmission mode, the transmitter and receiver are positioned on opposite sides of the sample. Any air gap along the acoustic path blocks the transmitted signal and appears as a dark region. THRU-scan is a fast screening method for total-coverage delamination detection, although it does not provide depth information, so suspicious findings are normally followed up with reflection-mode imaging.
Integration with Reliability Stress Testing
A common workflow follows the sequence of initial inspection, environmental or reflow preconditioning, and post-stress re-inspection. Samples may be subjected to moisture sensitivity preconditioning and reflow simulation according to IPC/JEDEC J-STD-020, temperature cycling, or damp heat stress, after which the growth of delamination or cracking is quantified against the baseline image and relevant acceptance criteria.
Applications
Acoustic scanning internal failure analysis is widely applied across the electronics product life cycle, from development and qualification to production monitoring and field failure diagnosis:
- Package reliability qualification for automotive, industrial, aerospace, and medical electronics programs
- Root cause analysis of field returns, test failures, and rejected production lots
- Process optimization of die attach dispensing, molding, bonding, and underfill operations
- Incoming inspection and counterfeit component screening
- Material research, bonded structure evaluation, and academic packaging studies
- Evidence documentation for quality disputes and supplier quality audits
FAQ
Is acoustic scanning a destructive test?
No. SAM does not section, polish, or otherwise alter the sample, so the same part can subsequently undergo electrical testing or complementary analysis. The only sample contact is immersion in deionized water, which is managed with controlled immersion times and optional bake-out for moisture-sensitive devices.
What is the difference between reflection mode and transmission mode?
Reflection mode uses a single transducer to capture echoes from specific internal depths, enabling defect location in three dimensions. Transmission mode uses opposed transducers and detects whether any defect blocks the acoustic path, offering rapid full-volume screening without depth resolution.
How small a defect can be detected?
Lateral resolution depends on the transducer frequency, ranging from tens of micrometers at low frequencies down to a few micrometers at high frequencies. Because air gaps reflect almost all acoustic energy, separations thinner than one micrometer can often be detected when they extend laterally beyond the resolution limit.
Are there samples that cannot be inspected?
Yes. Highly attenuating materials, very rough or irregular topographies, extremely thick multi-layer structures, and assemblies with trapped air pockets above the region of interest can degrade image quality. In such cases, the laboratory selects alternative frequencies, angles, or complementary techniques such as X-ray inspection or cross-sectional analysis.
How are results reported?
Typical deliverables include C-scan and THRU-scan images at each interface of interest, B-scan cross sections, annotated defect maps, quantitative delamination or void percentages, and conclusions referenced against applicable acceptance criteria such as those in JEDEC J-STD-020 or customer-specific specifications.