Boundary Reflection
Boundary behaviors at internal material junctions dictate the behavior of scanning acoustic microscopy when inspecting electronic packaging. An acoustic impedance mismatch occurs when ultrasonic waves encounter an interface between two materials with differing product values of density and acoustic velocity. This boundary condition causes a portion of the acoustic energy to reflect while the rest transmits deeper into the structure.
Material Property
Densities and elastic moduli of the constituent materials govern the acoustic resistance of each layer. Silicon, copper, epoxy mold compounds, and air gaps possess distinct acoustic impedances that dictate the magnitude of the mismatch. The transition from solid epoxy to an air-filled delamination creates an exceptionally large mismatch that returns a high-amplitude signal.
This discrepancy is particularly severe at the interface between the metal leadframe and the plastic molding compound where moisture ingress typically begins. Because the air gap reflects nearly all incoming acoustic energy, the resulting scan displays a bright white area indicating a completed separation of the material layers.
Inspection Output
Amplitude and polarity of the returned echo provide the raw data needed to locate internal structural defects. Instruments calculate the reflection coefficient directly from the difference in acoustic impedance between the adjacent layers. Analyzing these reflections allows inspectors to locate sub-micron gaps and voiding within the package assembly before board-level integration.
