Scanning Acoustic Microscopy (SAM) analysis is a non-destructive testing technique that uses high-frequency ultrasound to detect hidden internal defects such as delamination, cracks, and voids in electronic components and advanced packages. By converting reflected sound waves into depth-resolved images, SAM reveals failures that X-ray and optical inspection cannot detect, making it an essential tool for semiconductor failure analysis, reliability qualification, and quality control in accredited third-party laboratories. This article explains the working principles, sample types, testing items, methods, applicable standards, and practical applications of scanning acoustic microscopy analysis.
Technical Overview
Scanning Acoustic Microscopy (SAM), also referred to as ultrasonic microscopy or scanning ultrasonic inspection, is a non-destructive evaluation technique that uses focused high-frequency sound waves—typically in the range of 10 MHz to more than 1 GHz—to image the internal structures of opaque materials. When an ultrasonic pulse travels through a specimen, every material interface reflects a portion of the acoustic energy in proportion to the difference in acoustic impedance (the product of density and sound velocity) between adjacent layers. Because even a sub-micron air gap reflects nearly 100% of the incident sound, SAM is exceptionally sensitive to delamination, voids, and cracks that are difficult or impossible to resolve with X-ray or optical methods.
A typical SAM system consists of an ultrasonic transducer, a coupling medium (usually deionized water), a precision X-Y scanning stage, and signal-processing electronics. The transducer focuses acoustic energy into the sample and captures the returning echoes; by raster-scanning the transducer across the specimen and recording echo amplitude, phase, and time-of-flight at each point, the system reconstructs detailed planar, cross-sectional, or three-dimensional images of internal features. Since scanning is performed under immersion, samples must tolerate brief exposure to water unless inert-liquid or specialized coupling alternatives are used.
Compared with X-ray radiography, which produces projection images governed by density contrast, SAM provides depth-resolved information by electronically gating specific time windows of the returned signal. This allows an inspector to isolate a particular interface—such as the die-to-molding-compound interface—without interference from structures located above or below it, enabling precise localization of defects.
Testing Samples
SAM analysis is applicable to a wide range of samples across the electronics, semiconductor, and advanced materials industries. Common sample categories include:
Integrated Circuit Packages
Plastic-encapsulated microcircuits (PEMs) and other package formats such as QFP, QFN, BGA, LGA, CSP, SOP, DIP, and flip-chip devices are the most frequently examined samples. Typical concerns include mold-compound delamination, die cracking, and die-attach voiding that compromise long-term reliability.
Power Devices and Modules
IGBT modules, power MOSFETs, diodes, thyristors, and stacked power assemblies are inspected for solder-layer voids, substrate delamination, and bond degradation that directly affect thermal resistance and power cycling life.
Passive Components
Multilayer ceramic capacitors (MLCCs), tantalum capacitors, film capacitors, and inductors are examined for internal cracking, electrode delamination, and voids introduced by board-level reflow soldering or mechanical stress.
Printed Circuit Boards and Substrates
PCB laminates, HDI substrates, embedded-die panels, and ceramic hybrids are scanned to detect lamination defects, resin voids, copper-to-dielectric delamination, and interconnect anomalies.
Wafer-Level, MEMS, and Advanced Packaging
Bonded wafer pairs, through-silicon vias (TSV), interposers, wafer-level chip-scale packages, MEMS sensors, and hermetically sealed cavities are evaluated for bonding uniformity, un-bonded regions, and seal integrity.
Other Engineering Materials
Ceramics, sintered metals, composite laminates, adhesive and brazed joints, and polymer parts can also be assessed for porosity, unbonded areas, and structural consistency.
Testing Items
SAM analysis identifies and characterizes internal anomalies that affect mechanical integrity, thermal performance, and electrical reliability. Principal testing items include:
- Delamination at the die-to-mold-compound, die-pad, lead-frame, and lid-seal interfaces
- Die cracks, package cracks (popcorning), and substrate fractures
- Voids in die-attach solder, sintered silver, and conductive adhesive layers
- Flip-chip bump defects, underfill voids, and incomplete underfill coverage
- Bond line thickness, die-tilt, and layer thickness measurement
- Molding compound anomalies including porosity and foreign particles
- Moisture-induced defect screening supporting moisture sensitivity level classification
- Bonding quality of wafer-to-wafer, glass-to-silicon, and eutectic bonds
- Counterfeit screening through verification of internal die size, die count, and construction
Testing Methods
Appropriate technique selection depends on the sample geometry, materials, defect type of interest, and required throughput. The principal methods are described below.
Reflection (Pulse-Echo) Mode
A single transducer functions as both emitter and receiver. Echoes returning from internal interfaces provide depth information, and electronic gates isolate specific depth ranges so that each interface can be imaged independently. Phase inversion of the echo signal is a reliable indicator of an air gap such as a delamination or crack.
Transmission (Through-Scan) Mode
Two aligned transducers are positioned on opposite sides of the sample. Sound that passes through the full thickness indicates a defect-free path, while any delamination or void blocks the transmission and appears as a dark region. This mode offers rapid full-volume screening and is ideal for high-throughput lot acceptance, although it does not provide defect depth information.
Scan Types and Data Presentation
- A-scan: raw echo waveform (amplitude versus time) at a single point, used for precise depth and phase analysis
- B-scan: cross-sectional profile along a selected line, showing the vertical distribution of features
- C-scan: planar image of a chosen depth gate, the standard output for defect mapping
- T-scan: through-transmission image for rapid whole-part screening
- Tray scan: automated screening of multiple components in JEDEC trays for lot-level quality control
- Multi-gate and 3D imaging: layered or volumetric reconstruction for stacked and complex geometries
Frequency and Transducer Selection
Operating frequency is chosen to balance resolution against penetration depth. Frequencies of 15–30 MHz suit thick packages and power modules, 50–100 MHz is standard for most IC packages, and 100–230 MHz or higher resolves fine features such as thin die-attach layers, flip-chip bumps, and bonded wafer interfaces. Higher frequencies provide finer lateral resolution but attenuate more rapidly in polymers and thick materials.
Gating, Phase Analysis, and Result Interpretation
Electronic gates are positioned on the A-scan to isolate the interface under evaluation. Echo polarity, amplitude, and time-of-flight are then analyzed to distinguish genuine defects from normal material transitions. Defect area is quantified as a percentage of the interface or package area and compared against customer or industry acceptance criteria.
Applicable Standards
- IPC/JEDEC J-STD-035, Acoustic Microscopy for Nonhermetic Packages
- MIL-STD-883 Method 2030, Ultrasonic Inspection of Die Attach
- JEDEC JESD22-A104, Temperature Cycling, commonly paired with SAM before and after stress
- IPC/JEDEC J-STD-020 and J-STD-033 for moisture sensitivity classification and handling
- AEC-Q100 and AEC-Q101 qualification guidelines referencing acoustic inspection for automotive-grade components
Applications
SAM analysis supports the complete product lifecycle, from process development through qualification, production screening, and failure analysis, and is routinely performed in accredited third-party laboratories and in-house reliability facilities.
Failure Analysis
When electrical failures occur, SAM non-destructively locates internal anomalies before destructive physical analysis, guiding cross-sectioning and decapsulation to the exact failure site and preserving critical evidence.
Reliability and Qualification Testing
SAM is performed before and after environmental stresses such as temperature cycling, thermal shock, high-temperature storage, HAST, and moisture sensitivity preconditioning with reflow, revealing stress-induced delamination growth, crack propagation, and new void formation required by qualification reports.
Incoming Inspection and Quality Control
Tray-scan and through-scan modes enable rapid lot-level screening of incoming components, outgoing batches, and suspect shipments, ensuring that marginal or damaged parts are removed before assembly.
Process Development and Monitoring
Die-attach voiding, bond line thickness, underfill coverage, and molding quality are monitored during process optimization and ongoing production, providing quantitative feedback for assembly parameter refinement.
Counterfeit Component Detection
Internal construction revealed by SAM—die dimensions, die count, and packaging architecture—is compared against authentic reference devices to identify remarked, reconditioned, or substitute parts.
Industry-Specific Uses
Automotive electronics, aerospace and defense systems, medical implants, telecommunications infrastructure, and consumer devices all rely on SAM verification where internal integrity directly determines safety, durability, and warranty performance in demanding operating environments.
FAQ
Is SAM a destructive test?
No. SAM is fully non-destructive. The sample is immersed in deionized water or an inert coupling fluid during scanning, but it remains physically intact and can be returned to service or subjected to subsequent electrical testing, environmental stress, or cross-sectional analysis.
How does SAM differ from X-ray inspection?
X-ray imaging is governed by density contrast and produces a projection of the whole thickness, making it well suited to solder bridges and dense structures. SAM detects differences in acoustic impedance and provides depth-resolved images, and because air gaps block ultrasound almost completely, it is far more sensitive to delamination, fine cracks, and unbonded interfaces that X-ray frequently misses.
What sample preparation is required?
Preparation is minimal. Samples should be clean, free of loose contamination, and able to withstand brief immersion. Components are typically scanned in their as-received state, and trays, fixtures, or customized holders are used to position parts for optimal acoustic coupling.
What are the limitations of SAM?
Ultrasound cannot pass through air gaps, so a delamination near the surface can shadow features beneath it in reflection mode. Very thick, highly attenuating, or porous materials reduce penetration, and rough or highly curved surfaces complicate focusing. Water-sensitive devices may require special handling or alternative coupling media.
How large a sample can be inspected?
Modern systems accommodate items ranging from individual MLCCs and IC packages to full wafers, large PCB panels, and populated board sections, subject to tank dimensions and the ability to achieve adequate coupling across the area of interest.
How are results reported?
A typical report includes A-scan waveforms, C-scan or T-scan images at the relevant interfaces, defect maps with annotated locations, quantitative defect area percentages, and a conclusion against the applicable acceptance criteria, providing clear evidence for engineering decisions and qualification records.