In situ elastic property characterization of flip-chip underfills

The elastic properties of processing related inhomogeneities, such as filler settling and voids, are characterized using the acoustic microscope. A procedure to calculate the acoustic impedance of materials, and hence the elastic properties, at internal interfaces is proposed. The acoustic impedance of the silica filled polymer material under the die is measured at different locations corresponding to areas of different brightness in the acoustic image. Correlating the acoustic impedance measurements with the HS-model indicated that a) darker areas are regions where the underfill has a homogeneous distribution of filler; b) lighter areas are regions characterized by filler settling. Destructive cross sectioning and microscopy confirmed the above predictions. Further, the elastic properties of the different areas adjacent to the die are estimated using the model. The relatively quick, nondestructive technique presented in this paper could be useful in advanced process control, rapid yield management and in providing input into package reliability studies (such as finite element analysis).