In the field of acoustic microscopy, accurate interpretation of images and signals is essential to ensure the integrity of materials and components, especially in high-reliability industries such as aerospace, automotive, and semiconductors.
ALTER Technology, has extensive experience utilising acoustic microscopy to evaluate electronic components and critical materials. Thanks to this expertise, it is possible to clearly identify and explain phenomena such as false delaminations and false adhesions, which can lead to quality assessment errors if not properly interpreted.
This article explains these phenomena, their causes, and provides practical methods to differentiate them from real defects during acoustic inspections.
False delamination
This term refers to acoustic images or signals that simulate the presence of delaminations (separation lines between interfaces) in a sample, but which do not actually correspond to a real separation line
False adhesion
False adhesion occurs when acoustic microscopy imaging or signal interpretation suggests two surfaces or materials are bonded, but in reality, they are not. Factors such as trapped fluids, surface irregularities, or other anomalies during inspection can create this misleading appearance, potentially masking true defects.
Causes of False Delaminations
False delaminations arise due to various factors affecting acoustic wave behavior and image interpretation:
Acoustic Reflections
Differences in acoustic properties between materials (such as density and sound velocity) can cause reflections that are interpreted as delaminations, even when none exist.
Edge Effects or Surface Marking Dispersion
Abrupt changes in the material structure can create wave patterns that mimic delaminations.
Out-of-Focus Areas
This happens when the area being analyzed is not optimized, meaning it is out of focus or plane. This generates abnormal echoes or reflections, which may be interpreted as delaminations.
Bubbles Adhered to the Surface
This can occur during sample immersion, leading to confusing or false signals and causing an unexplained signal loss.
Frequency or Resolution Settings
Some internal structures may generate echoes or reflections similar to delaminations. Using optimal settings is essential to avoid false delaminations.
Causes of False Adhesions
False adhesion in acoustic microscopy can appear when fluids, such as water, get trapped between the layers of a material (for instance, during immersion), they can create a false appearance of adhesion. The trapped fluid can transmit acoustic signals in a way that suggests the layers are bonded when they are actually separated.
This factor can lead to inaccurate readings and may hinder the proper assessment of materials, potentially masking actual issues such as delamination or separation.
Differentiating Real from False Delaminations
To accurately distinguish false delaminations from real ones, several common verification techniques are employed:
Adjusting or Changing the Operating Frequency:
Optimizing the frequency can help improve detection accuracy.
Removal of Bubbles Are Adhering to the Surface
Ensure that no bubbles adhere to the surface, as their presence can lead to misinterpretations. If bubbles are detected, they should be removed prior to inspection.
Comparing with a Control Sample
Comparison with a control sample can help identify anomalies (delaminations or separation lines, cracks and voids).
Comparing with Other Inspection Methods
Techniques such as X-ray, cross-section analysis, etc., can provide additional confirmation.
This differentiation is crucial in critical applications, including the semiconductor, automotive, and aerospace industries, where real delaminations can compromise the structural integrity of the material or device.
Detecting False Adhesions
1. Quick Inspection in Transmission Mode:
Conduct an initial quick inspection in transmission mode. This mode allows for the early and precise detection of possible delaminations before water can enter the material. The rapid real-time inspection will help identify structural issues before water causes any interference.
2. Early Detection of Delaminations:
3. If Water Ingress is Suspected:
If water ingress is suspected in the delamination areas, it is recommended to adjust the system parameters (resolution, sensitivity, and focus) to enhance the detection of delaminations that may be masked by the presence of water between the layers.
4. Drying the Sample:
After the initial inspection, if water is confirmed to have entered the delamination areas, it is crucial to allow the sample to dry completely. This step will remove any trapped water between the layers of the material, preventing it from continuing to interfere with the acoustic measurements.
5. Reinspection with Optimized Approach:
Once the sample is fully dry, perform a reinspection using the optimized approach with the adjusted acoustic microscopy system. This will provide clearer, more reliable results without the interference from water.
6. Comparison of Results:
Compare the results from the initial inspection (with water present) with those from the reinspection (after drying the sample). If the acoustic signals change significantly after drying, it indicates that water ingress was causing false adhesion, allowing for more accurate conclusions.
Summary
Performing a quick initial inspection in transmission mode allows for the detection of delaminations before water causes interference. If water ingress is suspected, dry the sample and then perform an optimized inspection, which provides a more accurate approach to identifying and correcting false adhesion caused by water infiltration.
This approach ensures the results are as precise as possible and minimizes external interference during the inspection.
Specific case of false delamination and adhesion
False delamination due to differences in acoustic properties between materials.
One of the most typical false delaminations found in construction occurs in PEM components with a polymer coating on the die. It is so common that it is specifically addressed in the inspection method “PEM-INST-001 Section 3.”
This false delamination happens due to the low acoustic impedance of this polymer compared to the acoustic impedance of silicon die. This causes the ultrasonic waves to reflect more intensely at the interfaces where there is a significant difference in acoustic impedance, generating high acoustic contrast. As a result, the ultrasonic waves reflected at these interfaces may appear as a phase inversion, simulating delamination. Additionally, areas where reflection is intense (due to a high impedance contrast) will appear as dark regions in the transmission image. This is because very little acoustic energy manages to pass through the sample in these areas.
In summary, an interface with low acoustic impedance that reflects the waves intensely will produce dark regions in the transmission image, as the energy does not pass through the sample.
False adhesion due to water ingress
In the specific context of “fluid filling in delamination areas,” this phenomenon occurs when a fluid (e.g., water) enters areas where the material is delaminated, meaning where the layers of the material have separated. This phenomenon is very common and is addressed in the JEDEC standard J-STD-035, specifically in Annex B, under the section “False indication of adhesion: Fluid filling delamination areas.”
The fluid can fill the spaces between the layers, creating a false impression of adhesion between them. This may lead to acoustic images or data that appear to indicate the layers are bonded, when in fact they are separated due to delamination.
This type of “false adhesion” can hinder the accurate identification of defects, such as actual delamination, and may interfere with inspection results, especially if this phenomenon is not taken into account.
To illustrate this behavior the following image compares the C-mode inspection (circuit side) registered on the “fresh” samples with the one acquired after prolonged water immersion.
Conclusion
False delaminations and adhesions can significantly impact the reliability of acoustic inspections. Understanding their causes and applying rigorous verification methods, such as frequency adjustment, bubble removal, complementary inspections, and sample drying, helps prevent misinterpretation and ensures accurate defect detection.
ALTER Technology, with its deep knowledge and proven track record in acoustic inspection of electronic components, offers its expertise to ensure reliable evaluations, especially in industries where precision and quality are not optional, but critical requirements.
