Probe configuration and mode considerations
The selection of ultrasonic thickness gauges needs to be comprehensively evaluated based on the properties of the material to be tested, geometry and testing environment. Probe configuration and measurement mode directly affect data reliability. The following is a technical description of factors such as material sound velocity, coupling conditions, temperature range and surface state.
Probe frequency selection
The probe frequency (in MHz) determines the beam's focusing capability and penetration depth. High-frequency probes (5 MHz and above) are suitable for thin-walled materials (e.g., metal foils, plastic tubes) for better resolution and attenuation control; Low-frequency probes (2 MHz and below) are suitable for coarse-grained materials, rusted surfaces, or thick-walled components with less acoustic attenuation but reduced resolution. When choosing, balance the thickness of the blind zone with the lower limit of measurement.
Chip size and near-field area
The diameter of the wafer affects the divergence angle and near-field length of the sound field. Small diameter probes (e.g., 6 mm) are suitable for small curvatures or narrow areas; Larger diameter probes (e.g., 12 mm) are more stable for coupling to rough surfaces. The length of the near field area is determined by the formula N = (D²) / (4λ) Estimate, in it D is the diameter of the wafer,λ is the wavelength. The sound pressure fluctuates greatly in the near field, and it is recommended to avoid this area at the measurement point or use a software compensation algorithm.
Differences between twin and single crystal probes
Single crystal probe (pulsed echo) is suitable for uniform materials, but the blind zone is large; The dual-element probe features a one-shot, one-retract structure that significantly reduces dead zones (down to 0.25 mm) and is suitable for thin, high-temperature, or corrosive surfaces. Twin probes need to pay attention to the position of the beam intersection, which corresponds to the optimal depth of focus, and too much deviation increases the measurement error.
Delay block vs. contact
The delay block probe adds a solid delay medium between the wafer and the material, which can enhance the surface echo separation capability and is suitable for high-precision thin-wall inspection. Direct contact probe coupling is more efficient but sensitive to surface roughness. The differences between the two configurations are shown in the table below:
| Configuration type | Applicable scenarios |
| Delay block probe | Thin layer, small curvature, high temperature (with high temperature delay block) |
| Direct contact with the probe | Rough surfaces, thick plates, fast scans |
Mode selection and algorithms
Common measurement modes include single echo, echo-echo, and interface wave-echo modes. The single echo mode is timed from the transmitted pulse to a bottom echo and is sensitive to coupling changes. Echo-echo mode measures the echo interval between adjacent undersides and eliminates the thickness of the coating or surface paint. The interface wave-echo mode is specially used for thin or multi-layer structures, and uses the time difference between the interface wave and the bottom echo.
The basic relationships involved in calculating mode selection:d = v × t / 2, among them d is the thickness,v is the sound velocity of the material,t It is a two-way propagation time. The multi-mode instrument allows algorithms to be switched based on the workpiece condition or enhanced signal-to-noise ratio with automatic gain control.
Coupling and temperature compensation
The type of couplant needs to match the frequency of the probe. Low-viscosity couplants are suitable for smooth surfaces and thin layer measurements, and high-viscosity agents fill pits on rough surfaces. For every 20 °C increase in temperature, the sound velocity of most metals decreases by about 1%, which is compensated by a built-in temperature sensor or a manual input factor. For high temperature measurement, temperature-resistant gaskets or delay blocks should be selected to avoid damage to the wafer.
References
1. The chapter on device performance requirements and test methods for ultrasonic thickness measurement in international standards.
2. Discussion on frequency and resolution in the material sound velocity manual and probe manufacturer's technical documentation.
3. The process scheme for minimizing the blind spot of the twin probe in the summary report of long-term field testing experience.