Scanning Acoustic Microscopy Approach
Scanning Acoustic Microscopy (SAM) has revealed critical anomalies within the solder interfaces of a power module. These internal defects represent a serious reliability risk and are rejectable according to the J-STD-020E standard.
Background
The integrity of solder interfaces is essential to ensure both thermal dissipation and mechanical stability in power modules.
Delamination or void formation exceeding 50% of the intended contact area significantly degrades the thermal path and can lead to localised overheating, mechanical stress, and premature device failure.
Sample & Method
The inspected component corresponds to a plastic-encapsulated power module, designed for high-efficiency DC-DC conversion applications.
Inspection was performed using Confocal Scanning Acoustic Microscopy (C-SAM) in reflection mode at 50 MHz, with the objective of identifying solder and interface integrity issues.
To validate the acoustic results, a destructive cross-section was performed through the affected regions, followed by detailed microstructural examination using Scanning Electron Microscopy (SEM).
Findings & Observations
The acoustic inspection revealed major delamination and voiding across the solder interface:
- The affected area exceeds 50% of the intended contact zone.
Although partial physical contact remains, the loss of bonded area severely compromises the component’s thermal and mechanical reliability. - According to J-STD-020E, this type of defect is classified as critical and is grounds for rejection, as it compromises the functionality and reliability of the component.
The images obtained via C-SAM show clear contrast between bonded and delaminated regions, using phase inversion to highlight the affected areas.
To confirm the SAM results, additional destructive analyses were conducted, confirming a separation line between the solder and substrate.
Conclusion
The study demonstrates that Scanning Acoustic Microscopy (SAM) is a highly effective non-destructive technique for detecting critical solder delamination in power modules.
Correlation between SAM, cross-section, and SEM confirmed the presence of extensive solder delamination (>50% area), rendering the parts non-compliant with the J-STD-020E acceptance criteria.
This case highlights the importance of acoustic inspection for the early detection of internal solder defects, which may remain undetected by conventional radiographic methods, particularly when the solder material exhibits a homogeneous structure without significant mass loss or density variation.
