Boundary Interface
A physical boundary between two materials with differing densities determines the portion of an ultrasonic wave that returns to a transducer. During acoustic impedance reflection, the difference in acoustic impedance between the packaging mold compound and the silicon die generates a characteristic echo. This transition dictates how much energy enters the second medium.
Measurement Method
Ultrasonic transducers emit high-frequency pulses that travel through the coupled medium to the test specimen. The acoustic impedance reflection provides the primary mechanism for detecting voids or delamination at the leadframe interface. Since air has an acoustic impedance close to zero, any internal air gap reflects nearly the entire wave energy.
This total reflection produces a bright feature in the resulting scan, allowing automated inspection systems to flag defective boards before they proceed to subsequent assembly stages. The resulting voltage amplitude is compared against calibrated reference values to determine the physical integrity of the joint.
Discontinuity Analysis
Subsurface defects such as delamination at the die paddle are identified by analyzing the polarity of the returned signal. A phase inversion occurs when the acoustic impedance reflection happens at an interface where the second material has a lower impedance than the first. This phase shift distinguishes a true mechanical separation from a simple change in material density.
