SAT Technology
SAT (Scanning Acoustic Tomography)Technology Guide
SAT (scanning acoustic tomography) is a non-destructive inspection technique that uses ultrasound to find voids, delamination and cracks inside semiconductor wafers, packages and substrates. In Western usage the same class of instrument is called SAM (scanning acoustic microscopy) or C-SAM; the physics is identical. This page summarizes the terminology, principle, applications and equipment types of SAT from the viewpoint of semiconductor volume-production inspection.
1. Terminology
| Language | Production-inspection term | Academic / Western term |
|---|---|---|
| English | scanning acoustic tomography (SAT), SAT system | scanning acoustic microscopy (SAM), C-SAM |
| Korean | 초음파 검사(SAT), SAT 장비, 초음파 검사 장비 | 초음파 현미경, 주사 음향 현미경(SAM) |
| Chinese (Simplified) | 超声波扫描检测（SAT） | 超声波扫描显微镜（SAM） |
| Chinese (Traditional) | SAT 檢測、超音波掃描檢測（SAT） | 超音波掃描顯微鏡、超音波掃瞄顯微鏡（SAM、C-SAM；亦常標示為 SAT） |
| Japanese | SAT、超音波映像装置 | 走査型超音波顕微鏡（SAM） |
SAM and acoustic microscopy are broad terms that include laboratory instruments for materials research and failure analysis. SAT is mainly used for the production-line systems that inspect wafers and packages. C-SAM began as one manufacturer's product name and became a generic term; SAT spread from Japanese equipment naming into the Korean and Japanese semiconductor industries.
2. Principle
A focused ultrasonic transducer (probe) emits a short pulse, and the echoes returned from each internal interface are collected to form an image (pulse-echo). Ultrasound is almost completely reflected when it meets air, so it cannot reach the sample across an air gap. The space between the transducer and the sample is therefore filled with water, which carries the ultrasound into the sample. This is called coupling.
The strength of an echo grows with the acoustic-impedance difference between the two materials at the interface. A void or delamination creates a solid-to-air boundary that reflects strongly and inverts the phase of the echo, so this phase inversion separates such defects from foreign particles. Since the echo return time is proportional to depth, a time window (gating) selects only the interface at the depth of interest. This is the origin of the word tomography.
- A-scan: echo amplitude versus time at one point
- B-scan: a vertical cross-section of the sample
- C-scan: a planar image at a chosen depth; the mode used most in production inspection
- T-scan (through-transmission scan): ultrasound transmitted through the sample is received on the far side, showing defects as shadows
3. Frequency selection — resolution versus penetration
Higher frequency improves resolution but increases attenuation (roughly with the square of frequency), so it penetrates less deeply. The transducer is chosen for the target.
| Frequency band | Typical targets | Notes |
|---|---|---|
| 15–50 MHz | Thick packages, power modules (DCB/AMB substrate joints), parts with thick mold | Deep penetration, larger defects |
| 50–120 MHz | Standard packages (BGA, QFN), substrates, die attach | Main band for production inspection |
| 120–250 MHz | Wafer bond interfaces, thin stacks such as HBM, fine bumps | Fine defects at shallow depth |
| 300 MHz and above | Near-surface imaging, laboratory failure analysis | Analysis rather than 100% production screening |
The MIT SFSAT series uses 25–250 MHz transducers, a band matched to through-package and through-stack inspection on production lines.
4. Detectable defects
| Defect | Where it occurs | Appearance in SAT |
|---|---|---|
| Void | Die attach, solder, underfill, mold compound, wafer bond interface | Strong echo with phase inversion; bright spot in the C-scan |
| Delamination | Die-to-mold, lead frame-to-mold, substrate-to-adhesive, stacked interfaces | Area-shaped strong reflection with phase inversion |
| Crack | Die, mold, substrate, solder joints | Line-shaped reflection; depth confirmed by B-scan |
| Non-wet joint | Bump and solder joints | Abnormal reflection pattern at the joint plane |
| Inclusion / porosity | Mold, adhesive layers | Reflection without phase inversion (distinguished from voids) |
5. Applications in the semiconductor industry
For package reliability, the joint IPC/JEDEC standard J-STD-035 (acoustic microscopy for nonhermetic encapsulated electronic devices) defines the procedure; checking for delamination before and after moisture-sensitivity testing is the classic use. Inspection has recently extended from packages to the wafer level: as wafer stacking and bonding processes spread, bond-interface voids are inspected across full 300 mm wafers.
| Application | Inspection target | Main defects |
|---|---|---|
| HBM and stacked memory | Stacked bond interfaces, micro-bumps | Voids, non-wet joints, delamination |
| Advanced packages | FC-BGA, 2.5D/3D, chiplets | Underfill voids, bump cracks, delamination |
| Power semiconductor modules | DCB/AMB substrate–chip–base plate joints | Solder and sintered-joint voids (directly linked to thermal resistance and lifetime) |
| Wafer bonding | Wafer-to-wafer and die-to-wafer interfaces | Bond voids, particles |
| Substrates, MLCC, mini LED | Ceramic substrates, multilayer ceramics, LED packages | Internal cracks, delamination |
These application areas match the solution categories on the Ultrasonic Inspection business page.
6. Equipment types — laboratory versus fully automated in-line
| Item | Laboratory (manual / semi-automatic) | Fully automated in-line |
|---|---|---|
| Purpose | Failure analysis, research, sample checks | 100% or sampled inspection on the production line |
| Sample loading | Placed by an operator | Loader and handler feed wafers or trays automatically |
| Judgment | Operator reads the image | Automatic judgment (including deep learning), sorting, data link via SECS/GEM |
| Key performance | Resolution, image quality | Throughput (units per hour), judgment reliability, line integration |
| Probe configuration | Usually one probe | Multiple probes scanning in parallel (multi-probe) for throughput |
Throughput depends on scan pitch and configuration (number of probes and inspection zones), so systems must be compared at the same pitch and the same configuration.
The MIT SFSAT series is a fully automated in-line platform in which multiple probes scan in parallel in each inspection zone and two or more zones can be configured, with deep-learning-based automatic judgment, real-time warpage compensation (±3 mm), a maximum scan speed of 1,200 mm/s and in-house SECS/GEM software.
Related page: SFSAT series product page
7. Coupling — immersion and non-immersion
Conventional immersion systems submerge the sample in water. For moisture-sensitive advanced processes, non-immersion methods such as waterfall and bubble-trap coupling flow water only at the inspection area, so the sample is not submerged and water contact is minimised. MIT systems use non-immersion coupling.
8. Comparison with other methods
| Item | SAT (ultrasound) | X-ray / CT | Infrared (IR / SWIR) |
|---|---|---|---|
| What it sees | Interface condition, elastic properties | Density differences, geometry | Optical image through silicon |
| Strengths | Very sensitive to thin delamination and air gaps (sub-micron); suited to in-line production | Wiring and bump geometry; 3D structure with CT | Non-contact; fast screening of large bond voids right after bonding |
| Limits | Needs water coupling; upper defects shadow lower layers | Easily misses thin delamination and air gaps; CT is slow | Limited resolution for fine deep defects; opaque materials not possible |
In practice the methods are used to complement one another.
Frequently Asked Questions
- Are SAT and SAM (acoustic microscopy) different technologies?
- No. Both use the same physics: a focused ultrasonic transducer sends a short pulse and images the echoes returned from internal interfaces. The terms differ in lineage and usage. SAT usually refers to volume-production inspection systems for semiconductor wafers and packages, while SAM or acoustic microscopy is the broader term that also covers laboratory and failure-analysis instruments. C-SAM is a product name that became a generic term.
- What defects does SAT detect?
- Internal interface defects such as voids, delamination, cracks and non-wet joints. Ultrasound is almost totally reflected at a solid-to-air boundary and the reflected signal inverts its phase, so even very thin delaminations and air gaps are detected and distinguished from foreign particles.
- How does SAT differ from X-ray inspection?
- X-ray images density differences and geometry, while SAT responds to interface condition and elastic properties. Thin delaminations and air gaps are best found with SAT; wiring and bump geometry are better seen with X-ray. In production the two methods are used to complement each other.
- Which transducer frequency should be used?
- Higher frequency gives finer resolution but shallower penetration because attenuation rises roughly with the square of frequency. As a general guide, 15 to 50 MHz for thick packages and power modules, 50 to 120 MHz for standard packages and substrates, and 120 to 250 MHz for wafer bond interfaces and thin stacks. The SFSAT series covers 25 to 250 MHz.
- Does SAT inspection require water?
- Yes, water is the coupling medium that carries ultrasound into the sample, because air reflects almost all of the energy. Immersion-type systems submerge the sample; non-immersion systems such as waterfall and bubble-trap coupling flow water only at the inspection area and minimise water contact, which matters for moisture-sensitive advanced processes.
- What is the difference between a laboratory SAT and an in-line SAT system?
- An in-line system loads wafers or trays automatically, inspects, judges and sorts them without an operator and connects to the factory through SECS/GEM, so throughput and judgment reliability become the key figures. A laboratory system is loaded and interpreted by a person and is optimized for image quality. Throughput comparisons are only meaningful at the same scan pitch and the same probe configuration.