Another powerful feature of the VRM is the ability to create Frequency Domain Images. To create an FDI the VRM data is first opened to the A-scan data, just like with TDI. Contained within the digitized data of the A-scans is frequency specific datum with respect to how the acoustic pulse was reflected from a feature or defect. Since an individual frequency will often interact uniquely with a given material-thickness combination, unique information is often revealed with FDI.
The images below demonstrate some of the unique information revealed with FDI. The time domain image shows the standard acoustic image that would be generated for the chip to bump-underfill level for this flip chip. When the data is analyzed with FDI, it is observed that selected frequencies reveal additional information about the component. A comparison of the bump quality, metallization and defects between the time domain image and the Frequency Domain Images illustrates the power of FDI. Certain metallization layers become quite clear as do subtle differences and defects within the solder bumps. Even the underfill defects are revealed in greater contrast for this sample in the FDI mode.