Advanced Guide to Scanning Acoustic Microscopes (SAM): Core Technical FAQs on C-SAM

Welcome to Part 3 of our Advanced SAM Guide—the deep end of ultrasonic non-destructive testing. This edition is engineered for professionals who want to unlock the absolute limits of C-SAM technology. We’re answering your most pressing technical FAQs on multi-layer IGBT structure analysis, RTAF real-time autofocus, and fully automated wafer inspection. No fluff, just the insights you need to achieve flawless, high-throughpUt Inspections.

Let's get technical. Click below to explore the FAQs and upgrade your SAM capabilities.

Q21: What layers of an IGBT module can SAM detect?

IGBT modules feature a multi-layer structure, and SAM can inspect them layer by layer:

Layer 1: Die Surface → Check for die cracks and passivation layer integrity.

Layer 2: Die Attach Solder Layer → Check for solder voids and delamination.

Layer 3: DBC Top Copper Layer → Check for delamination between the top copper and the ceramic substrate.

Layer 4: DBC Ceramic Layer → Check for ceramic cracks.

Layer 5: DBC Bottom Copper Layer → Check for delamination between the bottom copper and the ceramic substrate.

Layer 6: DBC-to-Baseplate Solder Layer → Check for solder voids.

By applying different time gates to focus on various depths, the C-scan image of each layer can be extracted separately.

This technique is known as X-Scan (simultaneous multi-layer scanning).

Q22: What is X-Scan, and how does it differ from C-Scan?

X-Scan = an upgraded version of C-Scan

C-Scan: Sets a single time gate to capture an image at only one specific depth.

X-Scan: Sets multiple time gates simultaneously to capture images across several depths at once.

Analogy:

C-Scan is like looking at only one floor of a building at a time.

X-Scan is like viewing every floor of the entire building simultaneously.

Advantages: A single scan yields multi-layer imaging with significantly higher efficiency. It is exceptionally well-suited for multi-layer structures such as IGBT modules.

Q23: How should samples be prepared? What are the requirements?

Preparation is straightforward, but there are a few key points to keep in mind:

✅ Must be waterproof (since the sample is immersed in water):

Standard plastic-encapsulated packages are fine.

Decapped samples or bare dies require waterproof treatment.

✅ Surface cleanliness → Prevent false reflections caused by surface contamination or debris.

✅ Oversized or irregular samples → Custom fixtures or jigs may be required to secure them.

✅ Thin samples → Ensure precautions are taken to prevent the sample from drifting or floating in the water bath.

What samples can not be tested?

❌ Water-absorbing materials (water can infiltrate the interior and compromise results).

❌ Porous materials (acoustic waves will be severely scattered).

❌ Samples that must remain completely dry (unless wrapped or sealed in a protective waterproof film).

Most semiconductor chips, packaged devices, and PCBs can be tested directly.

Q24: What is RTAF real-time autofocus, and what is its purpose?

RTAF (Real Time Auto Focus) is a proprietary dynamic focusing scanning technology independently developed by SBT ULTRASONIC. It overcomes the testing challenges that traditional Scanning Acoustic Microscopes (SAM) face with warped samples. By integrating the Transducer with a precision Z-axis fine-adjustment drive system, it automatically tracks surface undulations to keep the focal point locked onto the target interface throughout the scan, delivering more accurate, authentic internal C-scan images.

Why is autofocus necessary?

During ultrasonic scanning and imaging of chips, wafers, or high-precision materials, the sample surface often exhibits minor undulations or warpage. If the distance between the transducer and the sample surface varies, the resulting image becomes blurred (out of focus).

Benefits of RTAF:

✅ Captures reflected signals in real-time on a point-by-point basis simultaneously with high-speed X- and Y-axis scanning.

✅ Dynamically tracks irregular curved or wavy surfaces, as well as specific internal layers.

✅ Ensures complete image clarity without pausing, while maintaining maximum scanning speed.

Q25: What are the differences between C-scan and T-scan?

A C-scan is used to detect various internal defects within materials, such as delaminations, voids, and micro-cracks.

A T-scan is used to detect large-area defects, severe internal voids, and overall bonding failures across the entire thickness range of a component.

Q26: Which is better to use, SAM or CT?

Each has its own unique strengths, and neither is universally "better."

Best Practice: Many laboratories use them in combination:

First, use SAM for rapid screening of delamination and internal voids.

Then, use CT to examine detailed 3D structural features.

Q27: What are the common application scenarios for SAM?

SAM has a very wide range of applications, covering the entire semiconductor industry chain and beyond:

Semiconductor Packaging:

Inspection of packages such as SOT, SMA, SOP, QFP, QFN, BGA, IGBT, and TO.

Wafer bonding inspection.

Underfill defect inspection in advanced packaging (e.g., FCBGA, SiP).

Materials Inspection:

Delamination in composite materials.

Coating and electroplating quality.

Internal cracks in ceramics.

Solder joint quality.

In a nutshell: Whenever you need to inspect internal structures without damaging the sample, SAM is a go-to solution.

Q28: Why do shadows from another layer sometimes appear in the scanned image?

This phenomenon is known as interlayer projection (or acoustic shadowing), which is a normal occurrence in acoustic imaging.

When ultrasonic waves propagate through a multi-layer structure, if there is a major defect (such as a void or delamination) in an upper layer, the waves will be strongly reflected there, causing the signals from the layers beneath to be attenuated or obstructed.

How it appears in the image:

Bright or dark regions that do not belong to the current layer show up in its C-scan image.

Typically, these are the "shadows" cast by large defects in the upper layers.

How to distinguish between them:

Sharp outlines and regular shapes → Likely a real defect on the current layer.

Blurred edges and diffuse boundaries → Likely an interlayer projection/shadow.

Experienced operators can accurately differentiate them by analyzing the corresponding A-scan waveforms.

Q29: What does AutoWafer fully automated wafer inspection mean?

Full automation is reflected in the following features:

✅ Automatic wafer loading and unloading (eliminates manual handling).

✅ Automatic autofocusing.

✅ Automated scan path planning.

✅ Automatic defect recognition (ADR) and void ratio calculation.

✅ Automated test report generation.

Ideal Application Scenarios:

High-volume wafer inspection on mass-production lines.

24/7 unattended continuous operation.

Quality control in wafer-level packaging (WLP).

Automation minimizes human intervention, significantly boosting inspection efficiency and result consistency.

Q30: I want to test a sample—where should I start and what are the steps?

The standard SAM inspection workflow follows an 8-step process:

Step 1: Check water resistance → Ensure the sample can be immersed in water (apply a waterproof coating or seal if necessary).

Step 2: Select the transducer → Choose based on sample thickness and resolution requirements:

Thick samples requiring deep penetration → Low frequency (10–50 MHz).

Thin samples requiring high detail → High frequency (100–150 MHz+).

Step 3: Focus → Use the A-scan to find the optimal focal depth.

Step 4: Set the Gate → Configure the time window to target the specific layer of interest.

Step 5: Run the C-scan → Set the scanning area/boundaries and resolution.

Step 6: Analyze the results → Examine the image to identify defects.

Step 7: Quantify → Calculate void ratios and measure defect dimensions.

Step 8: Generate the report → Save images and data.

Ready to stop guessing and start seeing?

Whether you're troubleshooting complex IGBT modules, advanced packaging, or routine semiconductor components, SBT Ultrasonic’s expert engineers have your back. Let’s solve your toughest inspection challenges together. Contact our technical team to discuss your requirements or schedule a sample test today.