Scanning Acoustic Microscopy Echo Phase Inversion for Package Failure Analysis
Acoustic phase inversion identifies package delamination by tracking 180-degree echo phase shifts caused by air gap impedance drop at internal material boundaries.
Acoustic Focal Length
Acoustic Reach
Distance between the ultrasonic transducer lens and the point of highest energy concentration defines the path for subsurface imaging. In semiconductor inspection, acoustic focal length determines which internal interface is interrogated during a scan. Technicians adjust this value to align the peak signal with the die attach or flip chip bump layer.
Lens Convergence
Energy density decreases rapidly as the probe moves away from the ideal plane. The transducer uses a spherical or cylindrical lens to converge sound waves through a coupling medium into the component body. If the acoustic focal length is not matched to the package thickness, the resulting image appears blurred or lacks the contrast needed to identify delamination.
Higher frequency transducers typically possess a shorter reach, limiting their use to thinner packages or near-surface features.
Resolution Limit
Beam diameter at the focus point dictates the lateral resolution of the inspection. A tight focus enables the detection of micron scale voids in solder bumps (a common failure mode in high density interconnects). Because the speed of sound varies between the water couplant and the mold compound, the effective acoustic focal length shifts when the wave enters the solid.
Correcting for this refractive effect ensures that the scan captures the intended depth.