A non-destructive diagnostic method uses high-frequency ultrasound waves to detect sub-surface delamination, voids, and cracks within microelectronic packages. Within the semiconductor and electronics manufacturing industries, csam imaging serves as a primary inspection technique to evaluate the integrity of internal material interfaces without damaging the sample. The system utilizes a transducer that focuses high-frequency acoustic energy into a liquid immersion medium, which is typically deionized water, to transmit the sound waves into the electronic component.
These sound waves reflect off internal boundaries and defects, returning to the transducer where they are converted into electrical signals that are processed into two-dimensional or three-dimensional images. This method is particularly effective for identifying air-gap defects such as delamination between the mold compound and the silicon die, which are difficult to detect using X-ray inspection.
Selection of the appropriate acoustic frequency determines both the resolution and the penetration depth of the imaging process. Higher frequencies, ranging from one hundred megahertz to three hundred megahertz, provide high resolution and are capable of detecting sub-micron defects in thin-die packages. However, these high frequencies suffer from significant signal attenuation and cannot penetrate deeply into thick or highly attenuating materials such as epoxy molding compounds.
Lower frequencies, between fifteen megahertz and fifty megahertz, offer deeper penetration to inspect thick power modules or multi-chip packages, but with a corresponding loss of spatial resolution. The technician must balance these competing factors based on the physical dimensions and material composition of the package being evaluated.
Acoustic waves are highly sensitive to changes in material density and acoustic impedance, which makes them ideal for detecting air-filled voids. When the sound wave encounters an interface between a solid material and air, such as a delamination or a package crack, it undergoes complete reflection due to the extreme mismatch in acoustic impedance. This complete reflection appears in the scan as a bright, high-contrast region that clearly indicates the presence and shape of the defect.
In contrast, a well-bonded interface allows a portion of the acoustic energy to pass through, resulting in a lower reflection signal and a darker region in the image. By mapping these reflection intensities across the package, the system generates a visual representation of the internal bonding quality.
While acoustic microscopy is a powerful diagnostic tool, several technical and physical constraints limit its application in standard production environments. The requirement for a liquid coupling medium means that some sensitive assemblies or non-hermetic packages cannot be inspected without risking moisture ingress or contamination. Air bubbles trapped in the coupling water can block the ultrasound signal and create false indications of internal defects, which requires the use of deaerated water and careful specimen placement.
Highly irregular surface geometries scatter the acoustic waves and reduce the clarity of the underlying layers, which limits the technique to packages with flat outer surfaces. Additionally, the process is sequential, which makes it slower than high-throughput batch inspection methods. For these reasons, the technique is primarily used for sample-based quality audits, failure analysis, and new product qualification rather than inline, hundred percent inspection.
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