10 C-SAM Insights Every Packaging Engineer Needs

Q11: How long does it take to inspect a single chip?

It depends on the scanning resolution and Sample size.

Typical Inspection Times:

SOT-23 / SOT Packages: 1–3 minutes per unit

SMA / DO-214: 1–2 minutes per unit

TO Packages: 3–10 minutes per unit

IGBT Modules: 5–20 minutes per unit

Full Wafers: Requires longer inspection times due to the large surface area.

Resolution vs. Speed: Higher resolutions require longer scan times because smaller step sizes result in more data acquisition points.

In practice, scanning speed and precision are balanced based on the specific defect detection requirements.

Q12: What is Probe Resolution versus Imaging Resolution, and what is the difference between them?

These two concepts are often confused, but they represent very different aspects of the system:

1. Probe Resolution (Physical Limit)

Definition: Determined entirely by the probe’s frequency and focusing characteristics.

Meaning: It represents the minimum object size the probe can physically resolve (the focal spot size).

Analogy: Think of it as a camera lens. If the lens quality is poor, adding more pixels won't make the photo any clearer.

2. Imaging Resolution (Software Setting)

Definition: The step size (the distance between sampling points) configured during a scan.

Meaning: Expressed in pixels. For example, a setting of 50 μm/pixel means the system captures one data point every 50 μm.

Analogy: Think of it as the pixel count of a photo. The higher the pixel count, the finer and more detailed the image appears.

The Critical Relationship:

Imaging resolution cannot exceed probe resolution. Even if you configure an ultra-fine step size of 1μm/pixel, if the probe can only physically resolve down to 10 μm, the extra pixels will not provide any additional clarity or real data.

Here is the professional and natural English translation, utilizing standard industry terminology for Scanning Acoustic Microscopy (SAM).

Q13: How deep can SAM penetrate inside a sample?

Penetration depth depends on the probe frequency and the material properties.

Frequency vs. Penetration Depth (General Reference):

10–30 MHz: Penetrates 10–30 mm (Ideal for IGBT modules, thick plastic encapsulants)

30–100 MHz: Penetrates 3–10 mm (Ideal for standard packages)

50–100 MHz: Penetrates 1–5 mm (Ideal for small packages like SOT, SMA, etc.)

100–300 MHz: Penetrates 0.3–1 mm (Ideal for wafers, thin films)

The Core Rule: The higher the frequency, the shallower the penetration depth. Choosing the right probe is always a trade-off between "how deep you need to see" (penetration) versus "how fine you need to see" (resolution).

Q14: What is the relationship between Scanning Acoustic Microscopy (SAM) and medical ultrasound (B-scan)?

The underlying physics and principles are identical, but their application scenarios and specifications differ drastically:

Medical Ultrasound (B-Scan): 1–5 MHz | Resolution: Centimeter to millimeter scale | Used to image human internal organs.

Industrial Non-Destructive Testing (NDT): 0.1–15 MHz | Resolution: Millimeter scale | Used to inspect metal welds and structural integrity.

Scanning Acoustic Microscopy (SAM): 5–2000 MHz | Resolution: 0.3–100 μm | Used to inspect internal semiconductor packaging.

The Core Difference: SAM utilizes significantly higher frequencies and shorter wavelengths, enabling it to resolve details at the micrometer level. Think of it like comparing a magnifying glass to a high-powered microscope—both capture images, but their precision levels are worlds apart.

Q15: What is acoustic impedance? Why do voids cause such strong reflections?

Acoustic Impedance (Z) = Material Density (ρ) X Acoustic Velocity (c)

The reflection intensity of an ultrasonic wave depends entirely on the difference in acoustic impedance between two interface materials:

Larger Difference: Stronger reflection → Brighter image

Smaller Difference: Weaker reflection → Darker image

Why are voids exceptionally bright?

Solder / Conductive Epoxy Impedance: ≈ 20-40MRayl

Air Impedance: ≈ 0.0004 MRayl

The mismatch is a factor of tens of thousands!

Consequently:

Voids: The extreme acoustic impedance mismatch results in nearly 100% reflection, appearing as bright, pure white in the image.

Good Bonding Areas: The acoustic impedance is well-matched, resulting in almost zero reflection, appearing dark on the image.

This massive contrast is precisely why SAM is exceptionally sensitive when detecting voids and delamination!

Q16: Is a defect completely invisible if it is smaller than the probe's focal spot size (the resolution limit)?

Not necessarily!

In practice, detection limits follow these general guidelines relative to the focal spot size:

Defect size > Focal spot size: The defect will be successfully detected, and its measured size will be highly accurate.

Defect size is 60% to 100% of the focal spot size: The defect will still be detected, but its apparent size in the image will appear larger than its actual size.

Defect size < 60% of the focal spot size: The defect drops below the detection threshold and will not be resolved.

Real-World Example:

Assuming a probe with a 50 μm focal spot diameter:

An 80 um defect → Detected accurately at 80 um. A 40 μm defect → Successfully detected, but will display as roughly 50 um. A 20 μm defect → Will likely be completely invisible.

Conclusion:

To ensure highly accurate sizing and measurement when selecting a probe, it is best if your target defect size is larger than the focal spot size. However, if you already know the expected size range of your target defects, you can adjust your probe selection accordingly.

Q17: What do the white and black areas on the image represent?

The interpretation depends entirely on the scanning mode being utilized:

1. C-Scan (Reflection Mode)

⬜ White / High Contrast: Indicates a strong reflection. This typically points to a defect, such as a void, delamination, or crack.

⬛ Black / Dark Tone: Indicates a weak reflection. This represents a normal, well-bonded interface.

Gray: Indicates a moderate reflection. The interface exists and is intact under normal conditions.

2. T-Scan (Thru-Scan Transmission Mode)

⬛ Black: Indicates the signal is blocked or severely attenuated. This suggests the presence of internal defects or voids blocking the acoustic path.

⬜ White / Bright Tone: Indicates the signal successfully penetrated the sample. This represents a normal, defect-free area.

Color Mapping Note: Software can apply pseudo-color (false color) to replace standard grayscale images, making defects immediately obvious at a glance. For example, a common scheme is Red = Severe Defect, Yellow = Minor Defect, and Green = Normal.

Q18: Can SAM calculate the voiding rate?

Yes, it can—and with high precision.

The SAM software can automatically calculate the voiding rate (the ratio of the defect area to the total inspected area) within a designated region:

Quantitative Analysis Capabilities:

1. Define the ROI: The user selects the specific Region of Interest (ROI) to be analyzed. 2. Automated Detection: The software automatically identifies and thresholds the defect pixels.

3. Calculation: The system calculates the ratio:

Voiding Rate = Defect Area / Total Area

4. Data Output: The software outputs the precise voiding rate for each analyzed region.

Real-World Example (IGBT Module):

Zone 1 Voiding Rate: 4.54%

Zone 2 Voiding Rate: 1.78%

Overall/Total Voiding Rate: 19.62%

Industry Standards & Requirements:

Industrial Grade: Voiding Rate < 15%–20%

Automotive Grade: Voiding Rate < 10%

High-Reliability Applications: Voiding Rate < 3%–5%

Q19: What does TOF scanning mean?

TOF (Time of Flight) refers to measuring the time it takes for an ultrasonic echo to return, rather than just measuring its "bright" or "dark" intensity. This time delay is then converted into precise thickness or height data.

What can TOF do?

Map Solder Layer Thickness: Inspect the thickness distribution and uniformity of the solder or die-attach layer.

Detect Warpage: Measure mechanical distortion—such as determining if a DBC (Direct Bonded Copper) substrate has warped relative to the heat sink.

3D Color Mapping: Display topographical variations using color gradients (e.g., Red = Thick/High, Blue = Thin/Low).

A Simple Analogy: A standard C-Scan acts like a camera (it shows if a defect exists), whereas a TOF scan acts like a laser rangefinder (it measures how high or how thick a feature is).

Q20: What is a DBC, and why does DBC delamination require SAM inspection?

DBC (Direct Bonded Copper) refers to a Direct Bonded Copper ceramic substrate. It features a sandwich structure consisting of a Copper Layer $\rightarrow$ Ceramic Core $\rightarrow$ Copper Layer.

Why is SAM Inspection Critical for DBC?

CTE Mismatch: Under high-temperature operating conditions, stress builds up because the copper and ceramic layers have significantly different Coefficients of Thermal Expansion (CTE).

Thermal Cycling Failure: After repeated thermal cycling, this stress easily causes the copper layer to delaminate from the ceramic core.

Thermal Runaway Risk: Delamination severely degrades heat dissipation and dramatically increases thermal resistance, which ultimately leads to catastrophic module burnout.

What SAM Can Detect:

⬜ Top Interface: Delamination between the top copper layer and the ceramic core.

⬜ Bottom Interface: Delamination between the bottom copper layer and the ceramic core.

⬜ Internal Faults: Cracks propagating inside the ceramic core itself.

This makes DBC inspection one of the most critical quality control procedures for IGBT modules!